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Multi-Site Cation Control of Ultra-Broadband Near-Infrared Phosphors for Application in Light-Emitting Diodes Gabriel Nicolo A. De Guzman, Veeramani Rajendran, Zhen Bao, Mu-Huai Fang, Wei-Kong Pang, Sebastian Mahlik, Tadeusz Lesniewski, Marek Grinberg, Maxim S. Molokeev, Grzegorz Leniec, Slawomir M. Kaczmarek, Jumpei Ueda, Kuang-Mao Lu, Shu-Fen Hu,* Ho Chang,* and Ru-Shi Liu*
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ABSTRACT: Near-infrared (NIR) phosphors are fascinating materials that have numerous applications in diverse fields. In this study, a series of La3Ga5GeO14:Cr3+ phosphors, which was incorporated with Sn4+, Ba2+, and Sc3+, was successfully synthesized using solid-state reaction to explore every cationic site comprehensively. The crystal structures were well resolved by combining synchrotron X-ray diffraction and neutron powder diffraction through joint Rietveld refinements. The trapping of free electrons induced by charge unbalances and lattice vacancies changes the magnetic properties, which was well explained by a Dyson curve in electron paramagnetic resonance. Temperature and pressure-dependent photoluminescence spectra reveal various luminescent properties between strong and weak fields in different dopant centers. The phosphor-converted NIR lightemitting diode (pc-NIR LED) package demonstrates a superior broadband emission that covers the near-infrared (NIR) region of 650−1050 nm. This study can provide researchers with new insight into the control mechanism of multiplecation-site phosphors and reveal a potential phosphor candidate for practical NIR LED application.
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INTRODUCTION The lighting industry has played a significant role in the development of cutting-edge devices through the rapid advancement of modern technology. The use of light-emitting diodes (LEDs) has been the key to some of these devices, particularly with near-infrared (NIR) phosphors as the main component. NIR phosphors have been studied all over the world since their discovery due to their numerous applications in different fields which include spectroscopic characterizations in medicine, food sciences, and agriculture.1−4 These applications rely on the characteristic absorptions of chemical components in the NIR region of the electromagnetic spectrum. Therefore, a broad emitting NIR phosphor is required to maximize these applications. One particular phosphor that fits this category is the Cr 3+ doped La3Ga5GeO14 (LGGO) family with a superior broadband NIR emission and that has already been reported for applications, such as ultraviolet (UV) and NIR excitable persistent phosphorescence,5,6 upconversion luminescence,7 and UV excitable photoluminescence (PL).8 Rajendran et al.9 first studied this phosphor for blue-excited PL applications. The existence of Cr3+ sites in pure gallogermanates structures,10 especially in LGGO, was shown and discussed at the end of the 20th century.11 Nevertheless, the maximum potential benefits of the LGGO phosphor (e.g., the crystallographic sites of this compound, the affecting factors of PL, and © XXXX American Chemical Society
the effects of multiple site substitutions) have yet to be explored. Therefore, in this study, a series of LGGO phosphors, which was incorporated with Sn4+, Ba2+, and Sc3+, was synthesized using solid-state reaction to explore every cationic site comprehensively. The strategy included substitution of heteroatoms, generation of lattice vacancies, and size tuning of the polyhedral site.
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EXPERIMENTAL SECTION
Synthesis. La3(1−z)Ga5(1−w−y)Ge1−xO14:5wCr3+, xSn4+, 3zBa2+, 5ySc3+ phosphors were synthesized using the conventional solidstate reaction method. The stoichiometric amounts of the starting precursors, La2O3 (Merck, 99.9%), Ga2O3 (Gredmann, 99.99%), GeO2 (Aldrich, 99.9%), Cr2O3 (Merck, 99.9%), SnO2 (Aldrich, 99.9%), BaCO3 (J.T. Baker, 99.9%), and Sc2O3 (Gredmann, 99.99%) were stoichiometrically weighed using an electronic balance and ground in an agate mortar for at least 15 min or until a homogeneous mixture was observed. The subsequent powders were then placed in an aluminum oxide crucible which was later subjected to sintering in a Received: July 11, 2020
A
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muffled furnace at 1250 °C for 5 h in air atmosphere. After sintering, the resultant powders were cooled to room temperature. Caution! Cr2O3 may cause skin corrosion/irritation or eye damage and should be handled with extreme care! Characterizations. The phase and purity of the samples were characterized by X-ray diffraction (XRD) using a D2 PHASER diffractometer (Bruker) with a Cu Kα radiation source (λ = 1.5418 Å). The fine structure data, lattice parameters, and atom positions and bond lengths were obtained by synchrotron XRD analysis (λ = 0.77491 Å) of BL01C2 beamline with a Debye−Scherrer camera at the National Synchrotron Radiation Research Center, Taiwan, and Neutron Powder Diffraction analysis (λ = 1.621274 Å) with a step size of 0.2°. Rietveld refinements were performed using total pattern analysis solutions (TOPAS) 4.2 software. Electron paramagnetic resonance (EPR) spectra were measured by a conventional X-band Bruker ELEXSYS E 500 CW spectrometer at the measurement conditions of nitrogen temperature (80−300 K), magnetic field up to 1.4 mT, microwave P = 2 mW, and frequency of 9.46 GHz. The absorption spectrum of the powder samples was recorded as a function of the applied magnetic field with respect to the first derivative. EPR/NMR software was used to separate the EPR lines, determine the spin Hamiltonian parameters, and simulate the EPR spectra. The Curie−Weiss temperature for individual paramagnetic centers was determined based on fitting the simulation to the Curie− Weiss law. Thermoluminescence (TL) curves were obtained by mounting the sample on a 10033 L Linkam thermal stage to control the temperature from 100−550 K. After illumination by UV light (250−400 nm) obtained by a MAX-302 Asahi spectrum Xe lamp with a UV module at 100 K for 5 min, the sample was kept in the dark at the same temperature for another 5 min and then heated at the rate of 10 K/min to 550 K while monitoring the TL intensity using an R11041 Hamamatsu Photonics PMT equipped with a 450 nm longpass filter. Photoluminescence excitation spectra were acquired using a FluoroMax-4P spectrofluorometer. Steady-state luminescence spectra were excited with the He−Cd laser with a wavelength of 442 nm. Spectra were acquired with an Andor SR-750-D1 grating spectrometer equipped with a DU420A-OE CCD camera. Timeresolved emission spectroscopy measurements were carried out using a system consisting of a Bruker Optics 2501S spectrograph and a Hamamatsu Streak Camera model C4334. The measurements under high hydrostatic pressure were performed using a Merrill Bassett type diamond anvil cell (DAC). The quantum yield measurements were carried out using a Hamamatsu Quantaurus-QY Plus model C13534 equipped with a combination of a 150 W Xe lamp monochromator and a high-power Xe lamp unit L13685 with an A13686 series bandpass filter. Fabrication of the LED device. The phosphor-converted lightemitting diodes (pc-LED) were fabricated by using LGGO phosphors as the infrared components for the blue LED. The LGGO phosphors and provision glue are weighted in the ratio of 1:0.5 by using the electronic balance. The starting materials are carefully mixed and placed into the syringe to remove the small bubbles through a defoaming process with the use of a defoaming machine. After the process of defoaming, the dispenser machine is connected to the syringe for the dispersion of the mixed paste over the blue LED chip. It must be noted that control of the dispersion quantity is very important for a higher quality LED package. After the process of dispersion, the LED chips are stored in a dry oven to remove the moisture and congestion of the mixed paste at 80 °C for 1 h and 150 °C for 2 h, subsequently. The LED chips are mounted into an integrating sphere to observe the electroluminescence spectrum of the NIR phosphors under a direct current forward-bias at an operating current of 350 mA. The performance of the pc-NIR LED is evaluated in the units of radiant flux or radiant power (mW) which signifies the amount of radiant energy emerging from the source per unit time.
Article
a space group of P321. The trigonal structure can be categorized into 4 metal sites, namely A, B, C, and D, based on the coordination numbers with oxygen atoms (Figure 1).
Figure 1. Crystal structure of La3Ga5GeO14. Short Ga−O bond distance (2.66 Å) of the Ga3 site forms a distorted octahedron instead of a tetrahedron.
The A site, occupied by La atoms, has a coordination number (CN) of 8 and thus forms a square antiprism. The B site, occupied by Ga atoms (termed as Ga1), consists of a CN = 6 and thus forms an octahedron. The C (shared by Ga and Ge atoms, termed as Ga2 and Ge2, respectively) and D (dominated by Ga atoms, termed as Ga3) sites form a tetrahedron with a CN = 4. Although the three Ga sites complicate the structure, the atomic arrangement of this trigonal structure is generally accepted, except for the coordination of the Ga3 site, which is still disputable; some claim that is has a CN = 4,7,8,12 whereas others claim it has a CN = 6.6,9 In this work, the Cr3+, Sn4+, Ba2+, and Sc3+ were introduced into LGGO structure systematically to form La3Ga4.95GeO14:0.05Cr3+ (LGGO-Cr), La 3 Ga 4 . 9 5 Ge 0. 9 O 14 :0.05Cr 3 + , 0.1Sn 4+ (LGGO-CrSn), La2.97Ga4.95Ge0.9O14:0.05Cr3+, 0.1Sn4+, 0.03Ba2+ (LGGOCrSnBa), and La2.97Ga4.7Ge0.9O14:0.05Cr3+, 0.1Sn4+, 0.03Ba2+, 0.25Sc3+ (LGGO-CrSnBaSc), respectively (Figure 2a). Figure 2b shows the XRD characterizations of all LGGO samples where we serve the confirmed LGGO-Cr structure as control. The diffraction peaks of all samples are in good agreement with ICSD#20783. The Sn4+ (r = 0.55 Å), Ba2+ (r = 1.42 Å), and Sc3+ (r = 0.75 Å) ions were successful in being partially doped to Ge4+ (r = 0.39 Å), La3+ (r = 1.16 Å), Ga3+ (r = 0.62 Å) sites, respectively, given that they have closer ionic radii and based on the figure-of-merits during refinement processes. To investigate the structure of the LGGO series thoroughly, joint Rietveld refinements of synchrotron X-ray diffraction (XRD) and neutron powder diffraction (NPD) were conducted. For this purpose, NPD is necessary to compliment XRD analysis, because of the higher sensitivity toward oxygen
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RESULTS AND DISCUSSION Structural Characterizations. The pristine LGGO compound has a very complex trigonal crystal structure with B
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Figure 2. Structural characterizations of La3Ga5GeO14:Cr3+ phosphor. (a) Schematic of the cation-doping mechanism. (b) XRD patterns of La3Ga5GeO14:Cr3+ with additional dopants (Sn4+, Ba2+, and Sc3+). Joint Rietveld refinements between (c) XRD data (λ = 0.77491 Å) and (d) NPD data (λ = 1.621274 Å) of La3Ga5GeO14:Cr3+. (e) Lattice parameters of La3Ga5GeO14:Cr3+ samples with the additional dopants (Sn4+, Ba2+, and Sc3+).
Figure 3. (a) EPR spectrum registered for LGGO compound doped with chromium ions and its deconvolution to three different spectra attributed to three different chromium centers Cr1, Cr2, and Cr3. Temperature-dependent EPR susceptibility (integrated intensity) of LGGO phosphors. (b) LGGO-Cr, (c) LGGO-CrSn, (d) LGGO-CrSnBa, and (e) LGGO-CrSnBaSc.
replaced with an element with a larger ionic radius, we also found that the lattice parameters a and c, as well as the lattice volume, increased with the addition of the dopants Sn, Ba, and Sc (Figure 2e). Furthermore, the relationship between lattice parameters and the doping content can be found in Figures S1−S6 while the joint Rietveld refinements of the LGGO samples with different doping concentrations can be found in Figures S7−S12. The electron paramagnetic resonance (EPR) was measured to confirm the local environment of Cr3+ ions. EPR studies allow characterization of a local symmetry of a paramagnetic center and the structure of the LGGO compounds doped by Cr3+, Sn4+, Ba2+, and Sc3+ ions. Chromium ions have a spin S = 3/2; thus, one signal with three spectral lines exists for each (x,
atoms and the different scattering lengths for different elements (La, Ga, Ge, Cr, Sn, Ba, Sc, and O) to XRD. As shown in Figure 2c,d, in addition to the main phase, LGGO with trigonal symmetry (P321, ICSD#20783) with an impurity (Ga2O3, space group C2/m, ICSD#34243) is also detected. Although the existence of Ga2O3 may complicate the refinement, the joint Rietveld refinements were carefully performed using the TOPAS 4.2 software against NPD and synchrotron XRD data simultaneously. The refinement strategy is detailed in SI. The details of the refined crystallography of these LGGO samples are tabulated in Tables S1−S6. The refinements were shown to be stable with low R-factors which implies that the suggested doping strategy was close to the pristine sample. As each site was partially C
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Figure 4. Deconvoluted EPR spectrum (upper panel) and its temperature dependence (lower panel) in (a) LGGO-Cr, (b) LGGO-CrSn, (c) LGGO-CrSnBa, and (d) LGGO-CrSnBaSc.
where HZ is the Zeeman term, HZFS is the zero-field splitting term, μB is the Bohr magneton, B is the induction of magnetic field, g is the spectroscopic splitting factor, S is the electron spin, D is axial, and E is the rhombic distortion of an octahedral. As shown in Table S7, the EPR signals corresponding to Cr1, Cr2, and Cr3 are attributed to chromium ions with their corresponding SH parameters. These parameters are on the axial symmetry (C4) site of all three chromium centers and are typical for Cr1 and Cr2. Cr3 ions placed in highly distortive octahedral positions are rarely observed in gallium germanates. EPR spectra stay in superposition (Cr1 and Cr3), and octahedral centers cannot be distinguished in some cases. Co-doping of the LGGO-Cr crystal with the Sn4+ ion; Sn4+ and Ba2+ ions; and Sn4+, Ba2+, and Sc3+ ions leads to the same SH parameters as calculated previously for the LGGO-Cr crystal.9 This condition implies that the codopants, namely, Sn4+, Ba2+, and Sc3+ ions, do not affect the local environment of chromium ions. Figure S13 shows the EPR spectra of LGGO-Cr compounds and the ones codoped with Sn4+, Ba2+, and Sc3+ ions at two different temperatures. Figure S13 shows that the EPR spectra registered for different kinds of codoping differ only by the intensity of the EPR signal. The symmetry of the chromium ions remains unchanged. The intensity of the EPR signal increases with the increase in the amounts of codopants at temperatures below 160 K (right panel of Figure S13). The reverse situation occurs at temperatures above 200 K except for LGGO-CrSnBaSc (left panel of Figure S13). The highest intensity of the EPR signal is observed for LGGO-Cr in the high-temperature range, whereas
y, z) direction and each center. In the case of differently oriented molecules, such as in powders, the EPR spectrum is an envelope of these signals in different directions. The EPR signals in the entire magnetic induction up to 700 mT were recorded. Figure 3a shows the EPR spectrum registered for LGGO doped with chromium ions. In the entire range of magnetic induction, we observed three signals that originated from paramagnetic chromium centers with a spin of S = 3/2. Joint Rietveld refinement confirms that the Ga crystallographic site is substituted by Cr. The LGGO crystal structure (Figure 1) has two octahedral gallium sites (Ga1 and Ga3) and one tetrahedral gallium site (Ga2/Ge2). The EPR signal at a middle magnetic induction indicates that Cr3+ ions occupy the tetrahedral site (Cr2) with a weak crystal field. Conversely, in the case of EPR signals at low magnetic inductions, Cr3+ ions occupy two octahedral sites (Cr1 and Cr3) with intermediate crystal fields. The Cr1 and Cr3 signals consist of broad and asymmetric lines centered at a low magnetic induction with an effective g value of nearly 4.38 and 4.12, respectively. The Cr2 signal consists of broad asymmetric lines centered at a medium magnetic induction with an effective g value of around 1.99. An EPR simulation of the experimental EPR data13 can be used to extract values of spin Hamiltonian (SH) parameters and determine local symmetry of chromium ions. EPR spectrum of the chromium ions can be described by the following SH eq 1 i y 1 = μB B ·g ·S + DjjjSz2 − S(S + 1)zzz + E(Sx2 + Sy2) 3 k {
H = HZ + HZFS
(1) D
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Figure 5. (a) Room-temperature PL spectra of the LGGO samples excited at 442 nm. Streak images showing the time-resolved photoluminescence spectra of (b) LGGO-Cr, (c) LGGO-CrSn, (d) LGGO-CrSnBa, and (e) LGGO-CrSnBaSc obtained at 10 K.
axis in negative T values, which indicates antiferromagnetic interactions, and for Cr1 and Cr2 ions, crosses the abscissa axis in positive T values, which indicates ferromagnetic interactions. The inset of Figure S14 shows the product of χ*T which is proportional to the square of the magnetic moment. For Cr1 ions, we observe an increase in the magnetic moment, while for Cr3 ions, we observe a decrease in the magnetic moment, which indicates ferromagnetic and antiferromagnetic interactions, respectively. Unnoticed changes of the magnetic moment indicate weak interactions between Cr2 ions. The TCW value indicates weak ferromagnetic interactions. The additional admixture does not cause any significant changes in the environment of Cr2. Similar behavior is observed for LGGO-CrSnBaSc (Figure 4d). However, the compound weakens the ferromagnetic and antiferromagnetic interactions for Cr1 and Cr3, respectively, but increases interactions in Cr2 slightly. Another feature of the EPR spectra for the investigated compounds is the Dyson line, as shown in Figure 4 (pink). The presence of this line among deconvoluted lines clarifies the shape of the temperature dependence of integral intensity, as observed in Figure 3b,c, for LGGO-Cr and LGGO-CrSn compounds. The intensity of this signal increases above T > 160 K and begins to dominate in the EPR spectrum, as shown in Figure S15. Thus, the total EPR signal intensity increases with the increase in temperature for the two compounds. This behavior may suggest that the line originates from free electrons (conductive electrons). In LGGO-CrSnBa and LGGO-CrSnBaSc, Ba2+ ions are substituted for La3+ ions and charge compensation must occur. In the EPR experimental spectra of the investigated compounds codoped with Ba2+, the Dyson line has considerably low intensity (Figure S15). Thus, free electrons have a considerably small effect on the total EPR spectrum. A possible explanation of such observations is that charge compensation is achieved by free electrons that are trapped by lattice vacancies created by the codopants, namely, Ba2+ ions. The presence of free electrons in the conductive band confirms the occurrence of Ruderman−Kittel−Kasuya− Yosid (RKKY) interactions.14,15 The interactions between chromium ions occur through oxygen bridges. Strong ferromagnetic interactions of Cr1 are transported through conductive electrons to Cr3. Therefore, we observe the ferromagnetic interactions of Cr3 in LGGO-Cr and LGGOCrSn compounds. Introduction of the admixture of Ba2+ causes
the lowest one is observed at the lowest temperature. Adding the codopants amplifies the EPR signal below T < 160 K. To explain this observation, a study of the temperature dependence of the integral intensity of the EPR signal was performed (Figure 3b−e). The integral intensity of the entire EPR signal drops to temperatures approximately 160 K for LGGO-Cr and LGGO-CrSn (Figure 3b,c). Above this temperature, the intensity increases. For compounds LGGO-CrSnBa and LGGO-CrSnBaSc (Figure 3d,e, respectively), the intensity decreases with increasing temperature from 80 to 280 K. To explain this behavior, the total EPR signal must be separated into signals originating from the individual chromium centers. A total of 11 Voight lines and 1 Dyson line were used to fit the experimental data. Figure 4a shows the deconvoluted EPR fitting results for LGGO-Cr. The Voight lines were used to fit three chromium centers, whereas the Dyson line was used to optimize the simulated EPR spectrum due to the large line width of the unknown signal. The separation of the individual chromium centers allows determination of the number of chromium ions in a given site in the compound. For LGGO-Cr, a large number of chromium ions are located in octahedral sites (Cr1−44.7%, Cr3−53.6%) and only a small amount are at the tetrahedral site (Cr2−1.7%). Individual chromium centers satisfy the Curie−Weiss law above Curie temperature. The Curie−Weiss temperature (TCW) value characterizes the strength of the interactions between the Cr3+ ions, and its sign defines the type of these interactions. A positive TCW value indicates that interactions between chromium ions are ferromagnetic, such as the LGGO-Cr compound (bottom panels of Figure 4a). Similar temperature dependencies are obtained for the LGGO-CrSn compound (Figure 4b). The admixture of Sn 4+ causes a significant reduction in ferromagnetic interactions between Cr1 and Cr3. Minor differences are observed for the concentration of chromium dopants (Cr2) in the tetrahedral site. Significant variations are observed for chromium ions in octahedral locations for LGGO-CrSnBa (Figure 4c). Introducing the Ba2+ dopant increases the ferromagnetic interactions of Cr1. Meanwhile, ferromagnetic interactions between Cr3 ions change their character to antiferromagnetic. Figure S14 shows the inverse of the magnetic susceptibility of EPR and the product of χ*T for LGGO-CrSnBa. The linear adjustment of Cr3 ions to the inverse of magnetic susceptibility of EPR crosses the abscissa E
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C (3) R5 where C is a constant that can be determined from the experimental data. This equation suggests that the incorporation of Sc3+ (ionic radius equal to 0.745 Å20) into the Ga3+ (ionic radii equal to 0.62 Å20) sites expands the lattice and decreases the crystal field strength in the vicinity of Cr3+ ions. Consequently, the 4T2 state energy decreases with respect to the ground state. Thus, redshift occurs. Nevertheless, the position of the Cr2 emission band is observed to be the same. As shown in Figure S17, the photoluminescence excitation (PLE) spectra are observed at 700, 800, and 850 nm. All the PLE spectra consist of two bands centered at 600 and 450 nm, whereas the 450 nm band consists of two overlapping bands. Apparently, the PLE spectra at all observation wavelengths are dominated by the Cr1/Cr3 emission because the two distinct bands can be attributed to transitions from the ground state 4 A2 to quartet excited states 4T2 and 4T1 (600 and 450 nm bands, respectively). The PLE spectrum of Cr2 emission cannot be distinguished clearly, but 850 nm monitored PLE spectrum has a different proportion of the two bands and the shift of the peak of the 4A2 to 4T2 transition from 600 to 620 nm. This finding indicates that the Cr2 PLE spectrum overlaps with the Cr1/Cr3 PLE spectrum. All these results confirm that Cr2 is an independent optically active center from Cr1/Cr3. The streak images of the LGGO samples (Figure 5b−e) were obtained from time-resolved emission spectroscopy measurements at 10 K. Low-temperature measurements were carried out to investigate the effects of multiple site substitutions clearly without the influence of thermal broadening. These measurements uncover information that cannot be observed at room temperature.21 The images consist of two distinct features. A set of narrow lines from 700 to 730 nm possesses a decay time of milliseconds together with broadband on the 800−900 nm region. The long lifetime luminescence of the narrowband is clearly attributed to the 2E → 4A2 transition of Cr1/Cr3 with the thermally populated 4T2 → 4A2 broadband emission that is completely suppressed. The 4 T2 → 4T1 emission of Cr2 at low temperatures does not show any structure. In terms of the LGGO samples with additional dopants, the contribution coming from the 2E → 4A2 R-line gradually decreases with each additional dopant. This observation suggests that the crystal field strength regularly decreases with the addition of each dopant (Sn4+, Ba2+, and Sc3+). Further streak images measured at 300 K are presented in Figure S18. A fundamental aspect of phosphors is the internal quantum efficiency (IQE). This qualitative measurement refers to the ratio between the numbers of emitted and absorbed photons. The measured IQE values (Table 1) of the LGGO-Cr, LGGO-CrSn, and LGGO-CrSnBa samples are observed to be similar to one another. By contrast, LGGOCrSnBaSc exhibits a slight enhancement compared with the other samples.
the conductivity electrons to be trapped by the vacancies, and this condition leads to a decrease in the number of free electrons and the lack of RKKY interactions. The interactions between chromium ions are antiferromagnetic, such as for the LGGO-CrSnBa and LGGO-CrSnBaSc compounds. This mechanism concerns only ions placed in the octahedral site. Ions in the tetrahedral site are isolated, and magnetic properties do not vary with the additional dopants studied here. In order to investigate the lattice vacancies even further, the thermoluminescence (TL) glow curves were measured for the LGGO samples. Figure S16 shows the TL glow curves of the LGGO-CrSn and LGGO-CrSnBa samples. A characteristic peak was observed in 314 K in both samples. The similar TL glow peak in the range from 320−480 K was reported by Wu et al.6 in Cr3+-singly doped LGGO. Therefore, the main TL glow peak at 314 nm is ascribed to be carrier traps by Cr3+ itself. In relation to the Ba2+ doping, it can be seen that the TL intensity of the Ba2+-codoped sample appeared to be weaker as compared to the LGGO-CrSn sample. This signifies that there is definitely a contribution coming from the Ba2+ to the formation of vacancies in this system. However, the lack of difference between the TL glow curves, other than the intensity, implies that the trap depths of the vacancies formed by the Ba2+-doping are too deep such that the additional TL glow peaks cannot be observed. Photoluminescence Measurements. PL spectra for each sample were measured under 442 nm excitation, as shown in Figure 5a.16 The PL spectra show two distinguishable broadband emissions: the first broadband emission, which ranges from 670 to 820 nm and accompanied by narrow lines, corresponds to Cr1/Cr3; the second emission, which ranges from 800 to 1000 nm, is ascribed to Cr2. The Cr1/Cr3 emission is typical for octahedrally coordinated Cr3+ ions in the intermediate crystal field. However, the individual contribution of the Cr1 and Cr3 emissions is difficult to distinguish. The narrow lines correspond to the spin-forbidden transitions (2E → 4A2) with doubly split emission lines (R-lines) that are possibly accompanied by a sideband phonon structure. The broadband emission centered near 750 nm (790 nm for the LGGO-CrSnBaSc) corresponds to the spin-allowed transitions from the higher, thermally populated 4T2 state to the 4A2 ground state of Cr1/Cr3. The Cr2 emission can be ascribed to the 4T2 → 4T1 transition of Cr3+ in the tetrahedral coordination and corresponds to the 3d3 electronic configuration in tetrahedral coordination, which is equivalent to the 3d7 electronic configuration in octahedral coordination.9 The PL spectra of the LGGO-Cr, LGGO-CrSn, and LGGO-CrSnBa samples are identical despite a slight variation in the Cr1/Cr3 and Cr2 emissions. Conversely, the LGGO-CrSnBaSc sample shows a different kind of emission with the Cr1/Cr3 peak by exhibiting a redshift and less pronounced R-lines. Given the crystal field strength, Dq is greatly dependent on the interatomic bond distances and is given by2,11,17−19 Dq =
ze 2 < r 4> 6R5
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Dq =
Table 1. Internal Quantum Efficiency and Electroluminescent Power Output of LGGO Samples
(2)
where Dq is the crystal field strength, z is the charge of the anion, e is the charge of the electron, < r4> is the expected value of fourth power of radius operator with respect to 3d electronic wave function, and R is the bond distance. Since is usually considered as a parameter, the simplified expression can be used F
Sample
IQE [%]
Radiant Flux [mW]
LGGO-Cr LGGO-CrSn LGGO-CrSnBa LGGO-CrSnBaSc
20 21 19 23
19.9 17.6 17.9 23.8
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Figure 6. (a) Pressure-dependent PL of LGGO-CrSnBaSc sample. Pressure-dependent PL decay time of (b−c) Cr1/Cr3 emission and (d−e) Cr2 emission of LGGO-CrSnBaSc.
calculated values of decay times are consistently shorter than those of the other samples. The detailed discussion of the temperature dependence of the luminescence properties LGGO-Cr was presented in our previous work.9 The dependence of the Cr1/Cr3 luminescence decay is nearly the same for the LGGO-Cr, LGGO-CrSn, and LGGO-CrSnBa samples. The different behavior of LGGO-CrSnBaSc is related to the lower crystal field strength of its Cr1/Cr3 sites than those of other samples. Moreover, multiexponential decay indicates the existence of two sites. This occurrence induces the individuality of Cr1 and Cr3 sites that become evident with the substitution of Sc3+ into the Ga3 sites, given that these are the same crystallographic sites in which Cr3+ ions are incorporated. This condition causes the redshift and is responsible for the fast component of the luminescent decay (Figure S20d). Figure S22 presents the decay profiles of the short Cr2 emission collected in the microsecond range. The temperature-dependent decay curves in all samples are similar. The luminescence is stable up to the threshold of 200 K, above which the decay curves begin to shorten. The decay curves have two components: one is the Cr2 emission; the other, which is much longer and visible especially at high temperatures, is attributed to an overlap of the Cr1/Cr3 luminescence. A fitting procedure was carried out to analyze the temperature dependence of the decay time. In the procedure, the long component was approximated as a constant offset (justified by much longer decay time of Cr1/ Cr3 emission than that of Cr2) as follows
The detailed thermally induced transformation of emission spectra of the LGGO samples are shown in Figure S19. The low-temperature spectra show a single narrow line (R1-line) that is accompanied by the phonon sideband. As the temperature increases, another narrow line (R2-line) appears on the shortwave side of the R1-line due to the thermal population. Simultaneously, a broadband emission due to the 4 T2 → 4A2 transition commences and rapidly becomes dominant. The main reason for this situation is that the 4T2 → 4A2 is spin allowed unlike the 2E → 4A2 transition and is characterized by a considerably higher transition probability. Such a phenomenon affects the effective decay times of the luminescence. This condition decreases strongly with the increase in temperature, as shown in Figure S19. The temperature-dependent decay curves of samples were obtained for the Cr1/Cr3 emission (Figure S20). The lowtemperature decay curves are long because of the radiative lifetime of the 2E excited state and have no distortions caused by the 4T2 state. As the temperature increases, the thermal population of the 4T2 state strongly decreases the effective decay time, which approaches the natural decay time of the spin-allowed 4T2 state. The decay curves of LGGO-Cr and LGGO-CrSn are single exponential. By contrast, the decay curves of LGGO-CrSnBa and LGGO-CrSnBaSc exhibit slightly and strongly nonexponential decays, respectively. This condition may be due to the inherent inhomogeneous broadening of Cr1/Cr3 or contribution to the decay curve of the overlapping Cr2 emission band. Given the nonexponential decay of some of the samples, the average decay times were calculated to compare the behavior of the effective decay time with temperature using the following formula:
i ty I(t ) = I0expjjj− zzz + Ioffset k τ{
The results of the fitting are presented in the form of smooth lines in Figure S21b. The temperature-dependent decay time τ(T) for every sample was extracted from the fitting and plotted as a function of temperature. The data show that the Cr2 emission is stable up to 200 K and starts to decline above this threshold. The τ(T) values were fitted into the formula for a single activation barrier quenching process
t
τ̅ =
∫0 max tI(t )dt t
∫0 max I(t )dt
(5)
(4)
where I(t) is the intensity of luminescence at time t and tmax is the measurement time. Figure S21a presents the calculated decay times. The decay curves of the LGGO-Cr, LGGO-CrSn, and LGGO-CrSnBa samples have a stable luminescence first and then become short due to the thermal population of the 4 T2 state (Figure S20). For the LGGO-CrSnBaSc sample, the threshold for diminishing decay time is relatively lower and the
τ (T ) =
G
( E) E τ0−1 + sexp(− kT ) 1 + exp − kT
(6)
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where E and s are the activation energies for luminescence quenching and τ0 is the radiative lifetime of the excited state. The obtained values are presented in Table S8, in which the frequency factor s was set to 2 × 108 s−1. Obtained values of E are generally the same within the measurement uncertainty. The relatively long (10 μs) decay time indicates that the lowest emitting state contains the strong admixture of 2T1 state to the emitting state. The LGGO-CrSnBaSc sample exhibits a different luminescence from the other samples. Thus, pressure-dependent PL spectroscopy studies were carried out (Figure 6). With the increase in pressure, the gradual transition from low crystal field type, and the 4T2 broadband Cr1/Cr3 emission toward high crystal field type, narrow line emission from the 2E state is observed. The reason for such a situation is that the location of the 4T2 state is dependent on the crystal field strength and the energy of the state increases as the crystal field strength increases due to compression in the diamond anvil cell.22 This finding supports the conclusion that the distinct emission spectrum of LGGO-CrSnBaSc is due to the low value of the crystal field. The Cr2 emission is visible in the low-pressure range (up to 60 kbar) but declines as the pressure increases further. We do not observe the pressure-induced spectral shift of the Cr2 luminescence. The lack of the spectral shift is related to the fact that the crystal field strength for Cr2 is approximately 2.6 times of the Racah parameter B, that corresponds to the crossing point of the 4T2 state, in which the energy increases, and the 2E state, in which the energy decreases with pressure.23 The pressure dependence of PL spectra is consistent with the pressure-dependent decay time of Cr1/Cr3 presented in Figure 6b. The thermal population of the 4T2 is suppressed and the decay times of Cr1/Cr3 luminescence are increased markedly due to the increase in the energy distance between the 4T2 and 2E states. The pressure dependence of decay time values, which was calculated using eq 4, shows a steady increase in decay time up to 100 kbar, above which the decay time stabilizes. Figure 6d presents the pressure-dependent decay times of Cr2 emission. Again, a strong deviation from a single exponential shape may be partly due to the overlapping of the strong Cr1/Cr3 emission. The pressure dependence is difficult to follow directly from the graph, but the values of the average decay times calculated by eq 4 and plotted in Figure 6c show that the luminescence lifetime decreases up to the threshold of 100 kbar and increases thereafter. The initial decrease in the Cr2 decay time may indicate luminescence quenching, whereas the latter increase may be a spurious effect due to the interference of Cr1/Cr3 luminescence. However, the increase in the decay time may also be due to achieving the crossing point of 4T2 and 2E states of tetrahedrally coordinated Cr3+. Notably, the 100 kbar pressure threshold may be a result of pressure-induced phase transition in the langasite crystal structure.24,25 For practical applications, a phosphor-converted LED prototype was fabricated using a blue LED chip as the main excitation source with the LGGO phosphor as the lightconverting material. As shown in Figure 7, all the samples exhibited superior broadband electroluminescence that covered the region of 650−1050 nm. The spectral performance of the NIR LED devices is represented by the radiant power output (radiant flux).3,4,9,26 Table 1 shows the obtained radiant flux values from the LGGO samples. Similar results are obtained with the IQE values. Specifically, the LGGO-
Article
Figure 7. Electroluminescence spectra of LGGO samples excited at 450 nm. The inset figures are the LED device and the lightening LED device with LGGO samples.
CrSnBaSc sample exhibits the highest performance among all samples with a radiant flux of 23.8 mW at an injection current of 350 mA. This finding signifies that the addition of Sc into the LGGO-CrSnBa phosphor not only causes a redshift in the emission spectrum but also enhances the performance of phosphor as a photoluminescent material. The results of this work display the effects of the cationic substitution in all crystallographic sites in the LGGO phosphor. The joint Rietveld refinements prove the existence of two octahedral sites (Ga1 and Ga3) and one tetrahedral site (Ga2/Ge2) for Ga3+. The EPR studies reveal the presence of three different sites of chromium ions in the LGGO compound with two octahedral sites (Cr1 and Cr3) and one tetrahedral site (Cr2). Therefore, the Cr3+ ions are successfully incorporated into the Ga3+ sites. The Cr concentration is similar in the two octahedral sites, whereas it is less than approximately 2% in the tetrahedral site. The additional codopants of Sn4+, Ba2+, and Sc3+ were systematically incorporated into the LGGO compound to investigate the different effects that each would bring. First, Sn4+, which was partially incorporated into Ge4+, has minimal effects on the decrease in crystal field with a slight lattice expansion. Ba2+ was incorporated into the La3+ sites. For this substitution, a charge unbalance mechanism will be introduced into the lattice, thereby leading to the formation of oxygen vacancies. With the shape of the EPR signal comprising the Dyson line assigned to conductive electrons (which significantly influence the transport of interactions between chromium ions in octahedral sites), the occurrence of defects due to Ba2+ ions affects the trapping of conductivity electrons and their localization in vacancies of the crystal lattice. However, the TL glow curve studies show that these vacancies may be too deep to be investigated further. The addition of Sc3+ mainly affects the crystal field strength, which becomes evident in the PL measurements by exhibiting a redshift in the PL and electroluminescence spectra. The incorporation of Sc3+ also enhances the IQE and radiant flux values of the LGGO phosphor. Overall, this study provides other researchers with a unique insight into multiple site substitutions in phosphors. H
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Maxim S. Molokeev − Laboratory of Crystal Physics, Kirensky Institute of Physics, Federal Research Center KSC SB RAS, Krasnoyarsk 660036, Russia; Siberian Federal University, Krasnoyarsk 60041, Russia; Department of Physics, Far Eastern State Transport University, Khabarovsk 680021, Russia Grzegorz Leniec − Institute of Physics, Department of Mechanical Engineering and Mechatronics, West Pomeranian University of Technology, 70-311 Szczecin, Poland Slawomir M. Kaczmarek − Institute of Physics, Department of Mechanical Engineering and Mechatronics, West Pomeranian University of Technology, 70-311 Szczecin, Poland Jumpei Ueda − Graduate School of Human and Environmental Studies, Kyoto University, Kyoto 606-8501, Japan; orcid.org/0000-0002-7013-9708 Kuang-Mao Lu − Everlight Electronics Co., Ltd., New Taipei City 238, Taiwan Complete contact information is available at: https://pubs.acs.org/10.1021/acs.inorgchem.0c02055
CONCLUSIONS A series of LGGO phosphors were synthesized via solid-state reaction. All structures were well resolved by XRD and NPD with joint refinements. The effects of multiple site substitutions were resolved using EPR and temperature- and pressuredependent PL. The heteroatom substitution with Sn4+ exhibits no significant effect. The Ba2+ substitution generates lattice vacancies via a charge unbalance mechanism. As a result, the magnetic property is changed. The Sc3+ substitution exhibits the greatest effects with a significant reduction in crystal field strength. This condition leads to a redshift in the emission of the LGGO phosphor with a notable enhancement in the quantum efficiency. The LED packaging demonstrates that the LGGO phosphor codoped with Sn3+, Ba2+, and Sc3+ is a promising candidate for NIR LED applications.
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ASSOCIATED CONTENT
* Supporting Information sı
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02055. Detailed Rietveld refinement results, X-ray diffraction characterizations, temperature-dependent photoluminescence, and pressure-dependent photoluminescence (PDF)
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Article
Author Contributions
The manuscript was written through the contributions of all authors. All authors have approved the final version of the manuscript. Notes
The authors declare no competing financial interest.
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AUTHOR INFORMATION
ACKNOWLEDGMENTS The authors would like to acknowledge the support of the Ministry of Science and Technology of Taiwan (Contract Nos. MOST 109-2112-M-003-011, MOST 109-2113-M-002-020MY3, MOST 107-2113-M-002-008-MY3, MOST 107-2923M-002-004-MY3, and MOST 106-2112-M-003-007-MY3), the National Science Center Poland Grant Opus (No. 2016/23/ B/ST3/03911), and the National Center for Research and Development Poland Grant (No. PL-TW/V/1/2018). T. Lesniewski would like to acknowledge the support of the National Science Center Poland, Grant Preludium 13 (No. 2017/25/N/ST3/02412).
Corresponding Authors
Shu-Fen Hu − Department of Physics, National Taiwan Normal University, Taipei 116, Taiwan; orcid.org/0000-00019561-8206; Email: [email protected] Ho Chang − Department of Mechanical Engineering and Graduate Institute of Manufacturing Technology, National Taipei University of Technology, Taipei 106, Taiwan; Email: [email protected] Ru-Shi Liu − Department of Chemistry, National Taiwan University, Taipei 106, Taiwan; orcid.org/0000-00021291-9052; Email: [email protected]
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Authors
Gabriel Nicolo A. De Guzman − Department of Physics, National Taiwan Normal University, Taipei 116, Taiwan Veeramani Rajendran − Department of Mechanical Engineering and Graduate Institute of Manufacturing Technology, National Taipei University of Technology, Taipei 106, Taiwan; orcid.org/0000-0003-1479-8829 Zhen Bao − Department of Chemistry, National Taiwan University, Taipei 106, Taiwan Mu-Huai Fang − Department of Chemistry, National Taiwan University, Taipei 106, Taiwan; orcid.org/0000-00031475-0200 Wei-Kong Pang − Institute for Superconducting & Electronic Materials, Faculty of Engineering, University of Wollongong, Wollongong 2522, Australia Sebastian Mahlik − Institute of Experimental Physics, Faculty of Mathematic, Physics and Informatics, University of Gdańsk, 80308 Gdańsk, Poland; orcid.org/0000-0002-9514-049X Tadeusz Lesniewski − Institute of Experimental Physics, Faculty of Mathematic, Physics and Informatics, University of Gdańsk, 80-308 Gdańsk, Poland; orcid.org/0000-0003-2451-7760 Marek Grinberg − Institute of Experimental Physics, Faculty of Mathematic, Physics and Informatics, University of Gdańsk, 80308 Gdańsk, Poland
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(1) Wang, Z. Y. Near-Infrared Organic Materials and Emerging Applications; CRC Press: Boca Raton, 2013. (2) Bai, Q.; Zhao, S.; Guan, L.; Wang, Z.; Li, P.; Xu, Z. Design and Control of the Luminescence of Cr3+-Doped Phosphors in the NearInfrared I Region by Fitting the Crystal Field. Cryst. Growth Des. 2018, 18, 3178−3186. (3) Shao, Q.; Ding, H.; Yao, L.; Xu, J.; Liang, C.; Li, Z.; Dong, Y.; Jiang, J. Broadband Near-Infrared Light Source Derived from Cr3+Doped Phosphors and a Blue LED Chip. Opt. Lett. 2018, 43, 5251− 5254. (4) Hayashi, D.; van Dongen, A. M.; Boerekamp, J.; Spoor, S.; Lucassen, G.; Schleipen, J. Broadband LED Source in Visible to Short-Wave-Infrared for Spectral Tumor Diagnostics. Appl. Phys. Lett. 2017, 110, 233701−233705. (5) Yan, W.; Liu, F.; Lu, Y.; Wang, X. J.; Yin, M.; Pan, Z. NearInfrared Long Persistent Phosphoresence in La3Ga5GeO14:Cr3+ Phosphor. Opt. Express 2010, 18, 20215−20220. (6) Wu, Y.; Li, Y.; Qin, X.; Chen, R.; Wu, D.; Liu, S.; Qiu, J. Dual Mode NIR Long Persistent Phosphorescence and NIR-to-NIR Stokes Luminescence in La3Ga5GeO14:Cr3+, Nd3+ Phosphor. J. Alloys Compd. 2015, 649, 62−66. (7) Yi, X.; Chen, Z.; Ye, S.; Li, Y.; Song, E.; Zhang, Q. Multifunctionalities of Near-Infrared Upconversion Luminescence, Optical Temperature Sensing and Long Persistent Luminescence in I
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La3Ga5GeO14:Cr3+, Yb3+, Er3+ and Their Potential Coupling. RSC Adv. 2015, 5, 49680−49687. (8) Lu, J.; Mu, Z.; Zhu, D.; Wang, Q.; Wu, F. Luminescence Properties of Eu3+ Doped La3Ga5GeO14 and Effect of Bi3+ CoDoping. J. Lumin. 2018, 196, 50−56. (9) Rajendran, V.; Fang, M. H.; De Guzman, G. N.; Lesniewski, T.; Mahlik, S.; Grinberg, M.; Leniec, G.; Kaczmarek, S. M.; Lin, Y. S.; Lu, K. M.; Lin, C. M.; Chang, H.; Hu, S. F.; Liu, R. S. Super Broadband Near-Infrared Phosphors with High Radiant Flux as Future Light Sources for Spectroscopy Applications. ACS Energy Lett. 2018, 3, 2679−2684. (10) Grinberg, M.; Macfarlane, P. I.; Henderson, B.; Holliday, K. Inhomogeneous Broadening of Optical Transitions Dominated by Low-Symmetry Crystal-Field Components in Cr3+-Doped Gallogermanates. Phys. Rev. B: Condens. Matter Mater. Phys. 1995, 52, 3917− 3929. (11) Macfarlane, P. I.; Henderson, B.; Holliday, K.; Grinberg, M. Substitutional Disorder and the Optical Spectroscopy of Gallogermanate Crystals. J. Phys.: Condens. Matter 1996, 8, 3933−3946. (12) Dudka, A. P. Multicell Model of La3Ga5GeO14 Crystal: A New Approach to the Description of the Short-Range Order of Atoms. Crystallogr. Rep. 2017, 62, 374−381. (13) Mombourquette, M.; Weil, J.; McGavin, D. EPR-NMR User’s Manual. Department of Chemistry, University of Saskatchewan, Saskatoon, SK, Canada, 1996. (14) Ruderman, M. A.; Kittel, C. Indirect Exchange Coupling of Nuclear Magnetic Moments by Conduction Electrons. Phys. Rev. 1954, 96, 99−102. (15) Kasuya, T. A Theory of Metallic Ferro- and Antiferromagnetism on Zener’s Model. Prog. Theor. Phys. 1956, 16, 45−57. (16) Even though the Cr3+ can dope into the Ga2O3 impurity phase, the concentration of the impurity is less than 10% according to the Rietveld refinement results. Moreover, the luminescence intensity of Ga2O3:Cr3+ obviously decreases after 750 nm, which differs from that of LGGO phosphor. As a result, the luminescence spectra of LGGO are credible. See for example, Fang, M. H.; De Guzman, G. N. A.; Bao, Z.; Majewska, N.; Mahlik, S.; Grinberg, M.; Leniec, G. L.; Kaczmarek, S. M.; Yang, C. W.; Lu, K. M.; Sheu, H. S.; Hu, S. F.; Liu, R. S. Ultra-High-Efficiency Near-Infrared Ga2O3:Cr3+ Phosphor and Controlling of Phytochrome. J. Mater. Chem. C 2020, 8, 11013− 11017. (17) Meng, X.; Wang, Z.; Qiu, K.; Li, Y.; Liu, J.; Wang, Z.; Liu, S.; Li, X.; Yang, Z.; Li, P. Design of a Novel Near-Infrared Phosphor by Controlling Cationic Coordination Environment. Cryst. Growth Des. 2018, 18, 4691−4700. (18) Kamimura, H.; Sugano, S.; Tanabe, Y. Ligand Field Theory and Its Applications; Syokabo, Tokyo. 1969. (19) De Guzman, G. N. A.; Fang, M. H.; Liang, C. H.; Bao, Z.; Hu, S. F.; Liu, R. S. Near-Infrared Phosphors and Their Full Potential: A Review on Practical Applications and Future Perspectives. J. Lumin. 2020, 219, 116944−116953. (20) Shannon, R. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32, 751−767. (21) Malysa, B.; Meijerink, A.; Jüstel, T. Temperature Dependent Cr3+ Photoluminescence in Garnets of the Type X3Sc2Ga3O12 (X = Lu, Y, Gd, La). J. Lumin. 2018, 202, 523−531. (22) Grinberg, M. Spectroscopic Characterization of Disordered Materials Doped with Chromium. Opt. Mater. 2002, 19, 37−45. (23) Wiśniewski, K.; Zorenko, Y. U.; Gorbenko, V.; Zorenko, T.; Kukliński, B.; Grinberg, M. High Pressure Spectroscopy Study of SCF Tb3Al5O12:Mn. J. Phys. Conf. Ser. 2010, 249, 24−29. (24) Mill, B. V.; Maksimov, B. A.; Pisarevsky, Y. V.; Danilova, N. P.; Markina, M. P.; Pavlovska, A.; Werner, S.; Schneider, J. In Phase Transitions in Langasite Family Crystals, Proceedings of the 2004 IEEE International Frequency Control Symposium and Exposition, Aug. 23− 27, 2004; pp 52−60.
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(25) Pavlovska, A.; Werner, S.; Maximov, B.; Mill, B. PressureInduced Phase Transitions of Piezoelectric Single Crystals from the Langasite Family: La3Nb0.5Ga5.5O14 and La3Ta0.5Ga5.5O14. Acta Crystallogr., Sect. B: Struct. Sci. 2002, 58, 939−947. (26) Shao, Q.; Ding, H.; Yao, L.; Xu, J.; Liang, C.; Jiang, J. Photoluminescence Properties of a ScBO3:Cr3+ Phosphor and Its Applications for Broadband Near-Infrared LEDs. RSC Adv. 2018, 8, 12035−12042.
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Multi-Site Cation Control of Ultra-Broadband Near-Infrared Phosphors for Application in Light-Emitting Diodes Gabriel Nicolo A. De Guzman, Veeramani Rajendran, Zhen Bao, Mu-Huai Fang, Wei-Kong Pang, Sebastian Mahlik, Tadeusz Lesniewski, Marek Grinberg, Maxim S. Molokeev, Grzegorz Leniec, Slawomir M. Kaczmarek, Jumpei Ueda, Kuang-Mao Lu, Shu-Fen Hu,* Ho Chang,* and Ru-Shi Liu*
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sı Supporting Information *
ABSTRACT: Near-infrared (NIR) phosphors are fascinating materials that have numerous applications in diverse fields. In this study, a series of La3Ga5GeO14:Cr3+ phosphors, which was incorporated with Sn4+, Ba2+, and Sc3+, was successfully synthesized using solid-state reaction to explore every cationic site comprehensively. The crystal structures were well resolved by combining synchrotron X-ray diffraction and neutron powder diffraction through joint Rietveld refinements. The trapping of free electrons induced by charge unbalances and lattice vacancies changes the magnetic properties, which was well explained by a Dyson curve in electron paramagnetic resonance. Temperature and pressure-dependent photoluminescence spectra reveal various luminescent properties between strong and weak fields in different dopant centers. The phosphor-converted NIR lightemitting diode (pc-NIR LED) package demonstrates a superior broadband emission that covers the near-infrared (NIR) region of 650−1050 nm. This study can provide researchers with new insight into the control mechanism of multiplecation-site phosphors and reveal a potential phosphor candidate for practical NIR LED application.
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INTRODUCTION The lighting industry has played a significant role in the development of cutting-edge devices through the rapid advancement of modern technology. The use of light-emitting diodes (LEDs) has been the key to some of these devices, particularly with near-infrared (NIR) phosphors as the main component. NIR phosphors have been studied all over the world since their discovery due to their numerous applications in different fields which include spectroscopic characterizations in medicine, food sciences, and agriculture.1−4 These applications rely on the characteristic absorptions of chemical components in the NIR region of the electromagnetic spectrum. Therefore, a broad emitting NIR phosphor is required to maximize these applications. One particular phosphor that fits this category is the Cr 3+ doped La3Ga5GeO14 (LGGO) family with a superior broadband NIR emission and that has already been reported for applications, such as ultraviolet (UV) and NIR excitable persistent phosphorescence,5,6 upconversion luminescence,7 and UV excitable photoluminescence (PL).8 Rajendran et al.9 first studied this phosphor for blue-excited PL applications. The existence of Cr3+ sites in pure gallogermanates structures,10 especially in LGGO, was shown and discussed at the end of the 20th century.11 Nevertheless, the maximum potential benefits of the LGGO phosphor (e.g., the crystallographic sites of this compound, the affecting factors of PL, and © XXXX American Chemical Society
the effects of multiple site substitutions) have yet to be explored. Therefore, in this study, a series of LGGO phosphors, which was incorporated with Sn4+, Ba2+, and Sc3+, was synthesized using solid-state reaction to explore every cationic site comprehensively. The strategy included substitution of heteroatoms, generation of lattice vacancies, and size tuning of the polyhedral site.
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EXPERIMENTAL SECTION
Synthesis. La3(1−z)Ga5(1−w−y)Ge1−xO14:5wCr3+, xSn4+, 3zBa2+, 5ySc3+ phosphors were synthesized using the conventional solidstate reaction method. The stoichiometric amounts of the starting precursors, La2O3 (Merck, 99.9%), Ga2O3 (Gredmann, 99.99%), GeO2 (Aldrich, 99.9%), Cr2O3 (Merck, 99.9%), SnO2 (Aldrich, 99.9%), BaCO3 (J.T. Baker, 99.9%), and Sc2O3 (Gredmann, 99.99%) were stoichiometrically weighed using an electronic balance and ground in an agate mortar for at least 15 min or until a homogeneous mixture was observed. The subsequent powders were then placed in an aluminum oxide crucible which was later subjected to sintering in a Received: July 11, 2020
A
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muffled furnace at 1250 °C for 5 h in air atmosphere. After sintering, the resultant powders were cooled to room temperature. Caution! Cr2O3 may cause skin corrosion/irritation or eye damage and should be handled with extreme care! Characterizations. The phase and purity of the samples were characterized by X-ray diffraction (XRD) using a D2 PHASER diffractometer (Bruker) with a Cu Kα radiation source (λ = 1.5418 Å). The fine structure data, lattice parameters, and atom positions and bond lengths were obtained by synchrotron XRD analysis (λ = 0.77491 Å) of BL01C2 beamline with a Debye−Scherrer camera at the National Synchrotron Radiation Research Center, Taiwan, and Neutron Powder Diffraction analysis (λ = 1.621274 Å) with a step size of 0.2°. Rietveld refinements were performed using total pattern analysis solutions (TOPAS) 4.2 software. Electron paramagnetic resonance (EPR) spectra were measured by a conventional X-band Bruker ELEXSYS E 500 CW spectrometer at the measurement conditions of nitrogen temperature (80−300 K), magnetic field up to 1.4 mT, microwave P = 2 mW, and frequency of 9.46 GHz. The absorption spectrum of the powder samples was recorded as a function of the applied magnetic field with respect to the first derivative. EPR/NMR software was used to separate the EPR lines, determine the spin Hamiltonian parameters, and simulate the EPR spectra. The Curie−Weiss temperature for individual paramagnetic centers was determined based on fitting the simulation to the Curie− Weiss law. Thermoluminescence (TL) curves were obtained by mounting the sample on a 10033 L Linkam thermal stage to control the temperature from 100−550 K. After illumination by UV light (250−400 nm) obtained by a MAX-302 Asahi spectrum Xe lamp with a UV module at 100 K for 5 min, the sample was kept in the dark at the same temperature for another 5 min and then heated at the rate of 10 K/min to 550 K while monitoring the TL intensity using an R11041 Hamamatsu Photonics PMT equipped with a 450 nm longpass filter. Photoluminescence excitation spectra were acquired using a FluoroMax-4P spectrofluorometer. Steady-state luminescence spectra were excited with the He−Cd laser with a wavelength of 442 nm. Spectra were acquired with an Andor SR-750-D1 grating spectrometer equipped with a DU420A-OE CCD camera. Timeresolved emission spectroscopy measurements were carried out using a system consisting of a Bruker Optics 2501S spectrograph and a Hamamatsu Streak Camera model C4334. The measurements under high hydrostatic pressure were performed using a Merrill Bassett type diamond anvil cell (DAC). The quantum yield measurements were carried out using a Hamamatsu Quantaurus-QY Plus model C13534 equipped with a combination of a 150 W Xe lamp monochromator and a high-power Xe lamp unit L13685 with an A13686 series bandpass filter. Fabrication of the LED device. The phosphor-converted lightemitting diodes (pc-LED) were fabricated by using LGGO phosphors as the infrared components for the blue LED. The LGGO phosphors and provision glue are weighted in the ratio of 1:0.5 by using the electronic balance. The starting materials are carefully mixed and placed into the syringe to remove the small bubbles through a defoaming process with the use of a defoaming machine. After the process of defoaming, the dispenser machine is connected to the syringe for the dispersion of the mixed paste over the blue LED chip. It must be noted that control of the dispersion quantity is very important for a higher quality LED package. After the process of dispersion, the LED chips are stored in a dry oven to remove the moisture and congestion of the mixed paste at 80 °C for 1 h and 150 °C for 2 h, subsequently. The LED chips are mounted into an integrating sphere to observe the electroluminescence spectrum of the NIR phosphors under a direct current forward-bias at an operating current of 350 mA. The performance of the pc-NIR LED is evaluated in the units of radiant flux or radiant power (mW) which signifies the amount of radiant energy emerging from the source per unit time.
Article
a space group of P321. The trigonal structure can be categorized into 4 metal sites, namely A, B, C, and D, based on the coordination numbers with oxygen atoms (Figure 1).
Figure 1. Crystal structure of La3Ga5GeO14. Short Ga−O bond distance (2.66 Å) of the Ga3 site forms a distorted octahedron instead of a tetrahedron.
The A site, occupied by La atoms, has a coordination number (CN) of 8 and thus forms a square antiprism. The B site, occupied by Ga atoms (termed as Ga1), consists of a CN = 6 and thus forms an octahedron. The C (shared by Ga and Ge atoms, termed as Ga2 and Ge2, respectively) and D (dominated by Ga atoms, termed as Ga3) sites form a tetrahedron with a CN = 4. Although the three Ga sites complicate the structure, the atomic arrangement of this trigonal structure is generally accepted, except for the coordination of the Ga3 site, which is still disputable; some claim that is has a CN = 4,7,8,12 whereas others claim it has a CN = 6.6,9 In this work, the Cr3+, Sn4+, Ba2+, and Sc3+ were introduced into LGGO structure systematically to form La3Ga4.95GeO14:0.05Cr3+ (LGGO-Cr), La 3 Ga 4 . 9 5 Ge 0. 9 O 14 :0.05Cr 3 + , 0.1Sn 4+ (LGGO-CrSn), La2.97Ga4.95Ge0.9O14:0.05Cr3+, 0.1Sn4+, 0.03Ba2+ (LGGOCrSnBa), and La2.97Ga4.7Ge0.9O14:0.05Cr3+, 0.1Sn4+, 0.03Ba2+, 0.25Sc3+ (LGGO-CrSnBaSc), respectively (Figure 2a). Figure 2b shows the XRD characterizations of all LGGO samples where we serve the confirmed LGGO-Cr structure as control. The diffraction peaks of all samples are in good agreement with ICSD#20783. The Sn4+ (r = 0.55 Å), Ba2+ (r = 1.42 Å), and Sc3+ (r = 0.75 Å) ions were successful in being partially doped to Ge4+ (r = 0.39 Å), La3+ (r = 1.16 Å), Ga3+ (r = 0.62 Å) sites, respectively, given that they have closer ionic radii and based on the figure-of-merits during refinement processes. To investigate the structure of the LGGO series thoroughly, joint Rietveld refinements of synchrotron X-ray diffraction (XRD) and neutron powder diffraction (NPD) were conducted. For this purpose, NPD is necessary to compliment XRD analysis, because of the higher sensitivity toward oxygen
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RESULTS AND DISCUSSION Structural Characterizations. The pristine LGGO compound has a very complex trigonal crystal structure with B
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Figure 2. Structural characterizations of La3Ga5GeO14:Cr3+ phosphor. (a) Schematic of the cation-doping mechanism. (b) XRD patterns of La3Ga5GeO14:Cr3+ with additional dopants (Sn4+, Ba2+, and Sc3+). Joint Rietveld refinements between (c) XRD data (λ = 0.77491 Å) and (d) NPD data (λ = 1.621274 Å) of La3Ga5GeO14:Cr3+. (e) Lattice parameters of La3Ga5GeO14:Cr3+ samples with the additional dopants (Sn4+, Ba2+, and Sc3+).
Figure 3. (a) EPR spectrum registered for LGGO compound doped with chromium ions and its deconvolution to three different spectra attributed to three different chromium centers Cr1, Cr2, and Cr3. Temperature-dependent EPR susceptibility (integrated intensity) of LGGO phosphors. (b) LGGO-Cr, (c) LGGO-CrSn, (d) LGGO-CrSnBa, and (e) LGGO-CrSnBaSc.
replaced with an element with a larger ionic radius, we also found that the lattice parameters a and c, as well as the lattice volume, increased with the addition of the dopants Sn, Ba, and Sc (Figure 2e). Furthermore, the relationship between lattice parameters and the doping content can be found in Figures S1−S6 while the joint Rietveld refinements of the LGGO samples with different doping concentrations can be found in Figures S7−S12. The electron paramagnetic resonance (EPR) was measured to confirm the local environment of Cr3+ ions. EPR studies allow characterization of a local symmetry of a paramagnetic center and the structure of the LGGO compounds doped by Cr3+, Sn4+, Ba2+, and Sc3+ ions. Chromium ions have a spin S = 3/2; thus, one signal with three spectral lines exists for each (x,
atoms and the different scattering lengths for different elements (La, Ga, Ge, Cr, Sn, Ba, Sc, and O) to XRD. As shown in Figure 2c,d, in addition to the main phase, LGGO with trigonal symmetry (P321, ICSD#20783) with an impurity (Ga2O3, space group C2/m, ICSD#34243) is also detected. Although the existence of Ga2O3 may complicate the refinement, the joint Rietveld refinements were carefully performed using the TOPAS 4.2 software against NPD and synchrotron XRD data simultaneously. The refinement strategy is detailed in SI. The details of the refined crystallography of these LGGO samples are tabulated in Tables S1−S6. The refinements were shown to be stable with low R-factors which implies that the suggested doping strategy was close to the pristine sample. As each site was partially C
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Figure 4. Deconvoluted EPR spectrum (upper panel) and its temperature dependence (lower panel) in (a) LGGO-Cr, (b) LGGO-CrSn, (c) LGGO-CrSnBa, and (d) LGGO-CrSnBaSc.
where HZ is the Zeeman term, HZFS is the zero-field splitting term, μB is the Bohr magneton, B is the induction of magnetic field, g is the spectroscopic splitting factor, S is the electron spin, D is axial, and E is the rhombic distortion of an octahedral. As shown in Table S7, the EPR signals corresponding to Cr1, Cr2, and Cr3 are attributed to chromium ions with their corresponding SH parameters. These parameters are on the axial symmetry (C4) site of all three chromium centers and are typical for Cr1 and Cr2. Cr3 ions placed in highly distortive octahedral positions are rarely observed in gallium germanates. EPR spectra stay in superposition (Cr1 and Cr3), and octahedral centers cannot be distinguished in some cases. Co-doping of the LGGO-Cr crystal with the Sn4+ ion; Sn4+ and Ba2+ ions; and Sn4+, Ba2+, and Sc3+ ions leads to the same SH parameters as calculated previously for the LGGO-Cr crystal.9 This condition implies that the codopants, namely, Sn4+, Ba2+, and Sc3+ ions, do not affect the local environment of chromium ions. Figure S13 shows the EPR spectra of LGGO-Cr compounds and the ones codoped with Sn4+, Ba2+, and Sc3+ ions at two different temperatures. Figure S13 shows that the EPR spectra registered for different kinds of codoping differ only by the intensity of the EPR signal. The symmetry of the chromium ions remains unchanged. The intensity of the EPR signal increases with the increase in the amounts of codopants at temperatures below 160 K (right panel of Figure S13). The reverse situation occurs at temperatures above 200 K except for LGGO-CrSnBaSc (left panel of Figure S13). The highest intensity of the EPR signal is observed for LGGO-Cr in the high-temperature range, whereas
y, z) direction and each center. In the case of differently oriented molecules, such as in powders, the EPR spectrum is an envelope of these signals in different directions. The EPR signals in the entire magnetic induction up to 700 mT were recorded. Figure 3a shows the EPR spectrum registered for LGGO doped with chromium ions. In the entire range of magnetic induction, we observed three signals that originated from paramagnetic chromium centers with a spin of S = 3/2. Joint Rietveld refinement confirms that the Ga crystallographic site is substituted by Cr. The LGGO crystal structure (Figure 1) has two octahedral gallium sites (Ga1 and Ga3) and one tetrahedral gallium site (Ga2/Ge2). The EPR signal at a middle magnetic induction indicates that Cr3+ ions occupy the tetrahedral site (Cr2) with a weak crystal field. Conversely, in the case of EPR signals at low magnetic inductions, Cr3+ ions occupy two octahedral sites (Cr1 and Cr3) with intermediate crystal fields. The Cr1 and Cr3 signals consist of broad and asymmetric lines centered at a low magnetic induction with an effective g value of nearly 4.38 and 4.12, respectively. The Cr2 signal consists of broad asymmetric lines centered at a medium magnetic induction with an effective g value of around 1.99. An EPR simulation of the experimental EPR data13 can be used to extract values of spin Hamiltonian (SH) parameters and determine local symmetry of chromium ions. EPR spectrum of the chromium ions can be described by the following SH eq 1 i y 1 = μB B ·g ·S + DjjjSz2 − S(S + 1)zzz + E(Sx2 + Sy2) 3 k {
H = HZ + HZFS
(1) D
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Figure 5. (a) Room-temperature PL spectra of the LGGO samples excited at 442 nm. Streak images showing the time-resolved photoluminescence spectra of (b) LGGO-Cr, (c) LGGO-CrSn, (d) LGGO-CrSnBa, and (e) LGGO-CrSnBaSc obtained at 10 K.
axis in negative T values, which indicates antiferromagnetic interactions, and for Cr1 and Cr2 ions, crosses the abscissa axis in positive T values, which indicates ferromagnetic interactions. The inset of Figure S14 shows the product of χ*T which is proportional to the square of the magnetic moment. For Cr1 ions, we observe an increase in the magnetic moment, while for Cr3 ions, we observe a decrease in the magnetic moment, which indicates ferromagnetic and antiferromagnetic interactions, respectively. Unnoticed changes of the magnetic moment indicate weak interactions between Cr2 ions. The TCW value indicates weak ferromagnetic interactions. The additional admixture does not cause any significant changes in the environment of Cr2. Similar behavior is observed for LGGO-CrSnBaSc (Figure 4d). However, the compound weakens the ferromagnetic and antiferromagnetic interactions for Cr1 and Cr3, respectively, but increases interactions in Cr2 slightly. Another feature of the EPR spectra for the investigated compounds is the Dyson line, as shown in Figure 4 (pink). The presence of this line among deconvoluted lines clarifies the shape of the temperature dependence of integral intensity, as observed in Figure 3b,c, for LGGO-Cr and LGGO-CrSn compounds. The intensity of this signal increases above T > 160 K and begins to dominate in the EPR spectrum, as shown in Figure S15. Thus, the total EPR signal intensity increases with the increase in temperature for the two compounds. This behavior may suggest that the line originates from free electrons (conductive electrons). In LGGO-CrSnBa and LGGO-CrSnBaSc, Ba2+ ions are substituted for La3+ ions and charge compensation must occur. In the EPR experimental spectra of the investigated compounds codoped with Ba2+, the Dyson line has considerably low intensity (Figure S15). Thus, free electrons have a considerably small effect on the total EPR spectrum. A possible explanation of such observations is that charge compensation is achieved by free electrons that are trapped by lattice vacancies created by the codopants, namely, Ba2+ ions. The presence of free electrons in the conductive band confirms the occurrence of Ruderman−Kittel−Kasuya− Yosid (RKKY) interactions.14,15 The interactions between chromium ions occur through oxygen bridges. Strong ferromagnetic interactions of Cr1 are transported through conductive electrons to Cr3. Therefore, we observe the ferromagnetic interactions of Cr3 in LGGO-Cr and LGGOCrSn compounds. Introduction of the admixture of Ba2+ causes
the lowest one is observed at the lowest temperature. Adding the codopants amplifies the EPR signal below T < 160 K. To explain this observation, a study of the temperature dependence of the integral intensity of the EPR signal was performed (Figure 3b−e). The integral intensity of the entire EPR signal drops to temperatures approximately 160 K for LGGO-Cr and LGGO-CrSn (Figure 3b,c). Above this temperature, the intensity increases. For compounds LGGO-CrSnBa and LGGO-CrSnBaSc (Figure 3d,e, respectively), the intensity decreases with increasing temperature from 80 to 280 K. To explain this behavior, the total EPR signal must be separated into signals originating from the individual chromium centers. A total of 11 Voight lines and 1 Dyson line were used to fit the experimental data. Figure 4a shows the deconvoluted EPR fitting results for LGGO-Cr. The Voight lines were used to fit three chromium centers, whereas the Dyson line was used to optimize the simulated EPR spectrum due to the large line width of the unknown signal. The separation of the individual chromium centers allows determination of the number of chromium ions in a given site in the compound. For LGGO-Cr, a large number of chromium ions are located in octahedral sites (Cr1−44.7%, Cr3−53.6%) and only a small amount are at the tetrahedral site (Cr2−1.7%). Individual chromium centers satisfy the Curie−Weiss law above Curie temperature. The Curie−Weiss temperature (TCW) value characterizes the strength of the interactions between the Cr3+ ions, and its sign defines the type of these interactions. A positive TCW value indicates that interactions between chromium ions are ferromagnetic, such as the LGGO-Cr compound (bottom panels of Figure 4a). Similar temperature dependencies are obtained for the LGGO-CrSn compound (Figure 4b). The admixture of Sn 4+ causes a significant reduction in ferromagnetic interactions between Cr1 and Cr3. Minor differences are observed for the concentration of chromium dopants (Cr2) in the tetrahedral site. Significant variations are observed for chromium ions in octahedral locations for LGGO-CrSnBa (Figure 4c). Introducing the Ba2+ dopant increases the ferromagnetic interactions of Cr1. Meanwhile, ferromagnetic interactions between Cr3 ions change their character to antiferromagnetic. Figure S14 shows the inverse of the magnetic susceptibility of EPR and the product of χ*T for LGGO-CrSnBa. The linear adjustment of Cr3 ions to the inverse of magnetic susceptibility of EPR crosses the abscissa E
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C (3) R5 where C is a constant that can be determined from the experimental data. This equation suggests that the incorporation of Sc3+ (ionic radius equal to 0.745 Å20) into the Ga3+ (ionic radii equal to 0.62 Å20) sites expands the lattice and decreases the crystal field strength in the vicinity of Cr3+ ions. Consequently, the 4T2 state energy decreases with respect to the ground state. Thus, redshift occurs. Nevertheless, the position of the Cr2 emission band is observed to be the same. As shown in Figure S17, the photoluminescence excitation (PLE) spectra are observed at 700, 800, and 850 nm. All the PLE spectra consist of two bands centered at 600 and 450 nm, whereas the 450 nm band consists of two overlapping bands. Apparently, the PLE spectra at all observation wavelengths are dominated by the Cr1/Cr3 emission because the two distinct bands can be attributed to transitions from the ground state 4 A2 to quartet excited states 4T2 and 4T1 (600 and 450 nm bands, respectively). The PLE spectrum of Cr2 emission cannot be distinguished clearly, but 850 nm monitored PLE spectrum has a different proportion of the two bands and the shift of the peak of the 4A2 to 4T2 transition from 600 to 620 nm. This finding indicates that the Cr2 PLE spectrum overlaps with the Cr1/Cr3 PLE spectrum. All these results confirm that Cr2 is an independent optically active center from Cr1/Cr3. The streak images of the LGGO samples (Figure 5b−e) were obtained from time-resolved emission spectroscopy measurements at 10 K. Low-temperature measurements were carried out to investigate the effects of multiple site substitutions clearly without the influence of thermal broadening. These measurements uncover information that cannot be observed at room temperature.21 The images consist of two distinct features. A set of narrow lines from 700 to 730 nm possesses a decay time of milliseconds together with broadband on the 800−900 nm region. The long lifetime luminescence of the narrowband is clearly attributed to the 2E → 4A2 transition of Cr1/Cr3 with the thermally populated 4T2 → 4A2 broadband emission that is completely suppressed. The 4 T2 → 4T1 emission of Cr2 at low temperatures does not show any structure. In terms of the LGGO samples with additional dopants, the contribution coming from the 2E → 4A2 R-line gradually decreases with each additional dopant. This observation suggests that the crystal field strength regularly decreases with the addition of each dopant (Sn4+, Ba2+, and Sc3+). Further streak images measured at 300 K are presented in Figure S18. A fundamental aspect of phosphors is the internal quantum efficiency (IQE). This qualitative measurement refers to the ratio between the numbers of emitted and absorbed photons. The measured IQE values (Table 1) of the LGGO-Cr, LGGO-CrSn, and LGGO-CrSnBa samples are observed to be similar to one another. By contrast, LGGOCrSnBaSc exhibits a slight enhancement compared with the other samples.
the conductivity electrons to be trapped by the vacancies, and this condition leads to a decrease in the number of free electrons and the lack of RKKY interactions. The interactions between chromium ions are antiferromagnetic, such as for the LGGO-CrSnBa and LGGO-CrSnBaSc compounds. This mechanism concerns only ions placed in the octahedral site. Ions in the tetrahedral site are isolated, and magnetic properties do not vary with the additional dopants studied here. In order to investigate the lattice vacancies even further, the thermoluminescence (TL) glow curves were measured for the LGGO samples. Figure S16 shows the TL glow curves of the LGGO-CrSn and LGGO-CrSnBa samples. A characteristic peak was observed in 314 K in both samples. The similar TL glow peak in the range from 320−480 K was reported by Wu et al.6 in Cr3+-singly doped LGGO. Therefore, the main TL glow peak at 314 nm is ascribed to be carrier traps by Cr3+ itself. In relation to the Ba2+ doping, it can be seen that the TL intensity of the Ba2+-codoped sample appeared to be weaker as compared to the LGGO-CrSn sample. This signifies that there is definitely a contribution coming from the Ba2+ to the formation of vacancies in this system. However, the lack of difference between the TL glow curves, other than the intensity, implies that the trap depths of the vacancies formed by the Ba2+-doping are too deep such that the additional TL glow peaks cannot be observed. Photoluminescence Measurements. PL spectra for each sample were measured under 442 nm excitation, as shown in Figure 5a.16 The PL spectra show two distinguishable broadband emissions: the first broadband emission, which ranges from 670 to 820 nm and accompanied by narrow lines, corresponds to Cr1/Cr3; the second emission, which ranges from 800 to 1000 nm, is ascribed to Cr2. The Cr1/Cr3 emission is typical for octahedrally coordinated Cr3+ ions in the intermediate crystal field. However, the individual contribution of the Cr1 and Cr3 emissions is difficult to distinguish. The narrow lines correspond to the spin-forbidden transitions (2E → 4A2) with doubly split emission lines (R-lines) that are possibly accompanied by a sideband phonon structure. The broadband emission centered near 750 nm (790 nm for the LGGO-CrSnBaSc) corresponds to the spin-allowed transitions from the higher, thermally populated 4T2 state to the 4A2 ground state of Cr1/Cr3. The Cr2 emission can be ascribed to the 4T2 → 4T1 transition of Cr3+ in the tetrahedral coordination and corresponds to the 3d3 electronic configuration in tetrahedral coordination, which is equivalent to the 3d7 electronic configuration in octahedral coordination.9 The PL spectra of the LGGO-Cr, LGGO-CrSn, and LGGO-CrSnBa samples are identical despite a slight variation in the Cr1/Cr3 and Cr2 emissions. Conversely, the LGGO-CrSnBaSc sample shows a different kind of emission with the Cr1/Cr3 peak by exhibiting a redshift and less pronounced R-lines. Given the crystal field strength, Dq is greatly dependent on the interatomic bond distances and is given by2,11,17−19 Dq =
ze 2 < r 4> 6R5
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Dq =
Table 1. Internal Quantum Efficiency and Electroluminescent Power Output of LGGO Samples
(2)
where Dq is the crystal field strength, z is the charge of the anion, e is the charge of the electron, < r4> is the expected value of fourth power of radius operator with respect to 3d electronic wave function, and R is the bond distance. Since is usually considered as a parameter, the simplified expression can be used F
Sample
IQE [%]
Radiant Flux [mW]
LGGO-Cr LGGO-CrSn LGGO-CrSnBa LGGO-CrSnBaSc
20 21 19 23
19.9 17.6 17.9 23.8
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Figure 6. (a) Pressure-dependent PL of LGGO-CrSnBaSc sample. Pressure-dependent PL decay time of (b−c) Cr1/Cr3 emission and (d−e) Cr2 emission of LGGO-CrSnBaSc.
calculated values of decay times are consistently shorter than those of the other samples. The detailed discussion of the temperature dependence of the luminescence properties LGGO-Cr was presented in our previous work.9 The dependence of the Cr1/Cr3 luminescence decay is nearly the same for the LGGO-Cr, LGGO-CrSn, and LGGO-CrSnBa samples. The different behavior of LGGO-CrSnBaSc is related to the lower crystal field strength of its Cr1/Cr3 sites than those of other samples. Moreover, multiexponential decay indicates the existence of two sites. This occurrence induces the individuality of Cr1 and Cr3 sites that become evident with the substitution of Sc3+ into the Ga3 sites, given that these are the same crystallographic sites in which Cr3+ ions are incorporated. This condition causes the redshift and is responsible for the fast component of the luminescent decay (Figure S20d). Figure S22 presents the decay profiles of the short Cr2 emission collected in the microsecond range. The temperature-dependent decay curves in all samples are similar. The luminescence is stable up to the threshold of 200 K, above which the decay curves begin to shorten. The decay curves have two components: one is the Cr2 emission; the other, which is much longer and visible especially at high temperatures, is attributed to an overlap of the Cr1/Cr3 luminescence. A fitting procedure was carried out to analyze the temperature dependence of the decay time. In the procedure, the long component was approximated as a constant offset (justified by much longer decay time of Cr1/ Cr3 emission than that of Cr2) as follows
The detailed thermally induced transformation of emission spectra of the LGGO samples are shown in Figure S19. The low-temperature spectra show a single narrow line (R1-line) that is accompanied by the phonon sideband. As the temperature increases, another narrow line (R2-line) appears on the shortwave side of the R1-line due to the thermal population. Simultaneously, a broadband emission due to the 4 T2 → 4A2 transition commences and rapidly becomes dominant. The main reason for this situation is that the 4T2 → 4A2 is spin allowed unlike the 2E → 4A2 transition and is characterized by a considerably higher transition probability. Such a phenomenon affects the effective decay times of the luminescence. This condition decreases strongly with the increase in temperature, as shown in Figure S19. The temperature-dependent decay curves of samples were obtained for the Cr1/Cr3 emission (Figure S20). The lowtemperature decay curves are long because of the radiative lifetime of the 2E excited state and have no distortions caused by the 4T2 state. As the temperature increases, the thermal population of the 4T2 state strongly decreases the effective decay time, which approaches the natural decay time of the spin-allowed 4T2 state. The decay curves of LGGO-Cr and LGGO-CrSn are single exponential. By contrast, the decay curves of LGGO-CrSnBa and LGGO-CrSnBaSc exhibit slightly and strongly nonexponential decays, respectively. This condition may be due to the inherent inhomogeneous broadening of Cr1/Cr3 or contribution to the decay curve of the overlapping Cr2 emission band. Given the nonexponential decay of some of the samples, the average decay times were calculated to compare the behavior of the effective decay time with temperature using the following formula:
i ty I(t ) = I0expjjj− zzz + Ioffset k τ{
The results of the fitting are presented in the form of smooth lines in Figure S21b. The temperature-dependent decay time τ(T) for every sample was extracted from the fitting and plotted as a function of temperature. The data show that the Cr2 emission is stable up to 200 K and starts to decline above this threshold. The τ(T) values were fitted into the formula for a single activation barrier quenching process
t
τ̅ =
∫0 max tI(t )dt t
∫0 max I(t )dt
(5)
(4)
where I(t) is the intensity of luminescence at time t and tmax is the measurement time. Figure S21a presents the calculated decay times. The decay curves of the LGGO-Cr, LGGO-CrSn, and LGGO-CrSnBa samples have a stable luminescence first and then become short due to the thermal population of the 4 T2 state (Figure S20). For the LGGO-CrSnBaSc sample, the threshold for diminishing decay time is relatively lower and the
τ (T ) =
G
( E) E τ0−1 + sexp(− kT ) 1 + exp − kT
(6)
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where E and s are the activation energies for luminescence quenching and τ0 is the radiative lifetime of the excited state. The obtained values are presented in Table S8, in which the frequency factor s was set to 2 × 108 s−1. Obtained values of E are generally the same within the measurement uncertainty. The relatively long (10 μs) decay time indicates that the lowest emitting state contains the strong admixture of 2T1 state to the emitting state. The LGGO-CrSnBaSc sample exhibits a different luminescence from the other samples. Thus, pressure-dependent PL spectroscopy studies were carried out (Figure 6). With the increase in pressure, the gradual transition from low crystal field type, and the 4T2 broadband Cr1/Cr3 emission toward high crystal field type, narrow line emission from the 2E state is observed. The reason for such a situation is that the location of the 4T2 state is dependent on the crystal field strength and the energy of the state increases as the crystal field strength increases due to compression in the diamond anvil cell.22 This finding supports the conclusion that the distinct emission spectrum of LGGO-CrSnBaSc is due to the low value of the crystal field. The Cr2 emission is visible in the low-pressure range (up to 60 kbar) but declines as the pressure increases further. We do not observe the pressure-induced spectral shift of the Cr2 luminescence. The lack of the spectral shift is related to the fact that the crystal field strength for Cr2 is approximately 2.6 times of the Racah parameter B, that corresponds to the crossing point of the 4T2 state, in which the energy increases, and the 2E state, in which the energy decreases with pressure.23 The pressure dependence of PL spectra is consistent with the pressure-dependent decay time of Cr1/Cr3 presented in Figure 6b. The thermal population of the 4T2 is suppressed and the decay times of Cr1/Cr3 luminescence are increased markedly due to the increase in the energy distance between the 4T2 and 2E states. The pressure dependence of decay time values, which was calculated using eq 4, shows a steady increase in decay time up to 100 kbar, above which the decay time stabilizes. Figure 6d presents the pressure-dependent decay times of Cr2 emission. Again, a strong deviation from a single exponential shape may be partly due to the overlapping of the strong Cr1/Cr3 emission. The pressure dependence is difficult to follow directly from the graph, but the values of the average decay times calculated by eq 4 and plotted in Figure 6c show that the luminescence lifetime decreases up to the threshold of 100 kbar and increases thereafter. The initial decrease in the Cr2 decay time may indicate luminescence quenching, whereas the latter increase may be a spurious effect due to the interference of Cr1/Cr3 luminescence. However, the increase in the decay time may also be due to achieving the crossing point of 4T2 and 2E states of tetrahedrally coordinated Cr3+. Notably, the 100 kbar pressure threshold may be a result of pressure-induced phase transition in the langasite crystal structure.24,25 For practical applications, a phosphor-converted LED prototype was fabricated using a blue LED chip as the main excitation source with the LGGO phosphor as the lightconverting material. As shown in Figure 7, all the samples exhibited superior broadband electroluminescence that covered the region of 650−1050 nm. The spectral performance of the NIR LED devices is represented by the radiant power output (radiant flux).3,4,9,26 Table 1 shows the obtained radiant flux values from the LGGO samples. Similar results are obtained with the IQE values. Specifically, the LGGO-
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Figure 7. Electroluminescence spectra of LGGO samples excited at 450 nm. The inset figures are the LED device and the lightening LED device with LGGO samples.
CrSnBaSc sample exhibits the highest performance among all samples with a radiant flux of 23.8 mW at an injection current of 350 mA. This finding signifies that the addition of Sc into the LGGO-CrSnBa phosphor not only causes a redshift in the emission spectrum but also enhances the performance of phosphor as a photoluminescent material. The results of this work display the effects of the cationic substitution in all crystallographic sites in the LGGO phosphor. The joint Rietveld refinements prove the existence of two octahedral sites (Ga1 and Ga3) and one tetrahedral site (Ga2/Ge2) for Ga3+. The EPR studies reveal the presence of three different sites of chromium ions in the LGGO compound with two octahedral sites (Cr1 and Cr3) and one tetrahedral site (Cr2). Therefore, the Cr3+ ions are successfully incorporated into the Ga3+ sites. The Cr concentration is similar in the two octahedral sites, whereas it is less than approximately 2% in the tetrahedral site. The additional codopants of Sn4+, Ba2+, and Sc3+ were systematically incorporated into the LGGO compound to investigate the different effects that each would bring. First, Sn4+, which was partially incorporated into Ge4+, has minimal effects on the decrease in crystal field with a slight lattice expansion. Ba2+ was incorporated into the La3+ sites. For this substitution, a charge unbalance mechanism will be introduced into the lattice, thereby leading to the formation of oxygen vacancies. With the shape of the EPR signal comprising the Dyson line assigned to conductive electrons (which significantly influence the transport of interactions between chromium ions in octahedral sites), the occurrence of defects due to Ba2+ ions affects the trapping of conductivity electrons and their localization in vacancies of the crystal lattice. However, the TL glow curve studies show that these vacancies may be too deep to be investigated further. The addition of Sc3+ mainly affects the crystal field strength, which becomes evident in the PL measurements by exhibiting a redshift in the PL and electroluminescence spectra. The incorporation of Sc3+ also enhances the IQE and radiant flux values of the LGGO phosphor. Overall, this study provides other researchers with a unique insight into multiple site substitutions in phosphors. H
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Maxim S. Molokeev − Laboratory of Crystal Physics, Kirensky Institute of Physics, Federal Research Center KSC SB RAS, Krasnoyarsk 660036, Russia; Siberian Federal University, Krasnoyarsk 60041, Russia; Department of Physics, Far Eastern State Transport University, Khabarovsk 680021, Russia Grzegorz Leniec − Institute of Physics, Department of Mechanical Engineering and Mechatronics, West Pomeranian University of Technology, 70-311 Szczecin, Poland Slawomir M. Kaczmarek − Institute of Physics, Department of Mechanical Engineering and Mechatronics, West Pomeranian University of Technology, 70-311 Szczecin, Poland Jumpei Ueda − Graduate School of Human and Environmental Studies, Kyoto University, Kyoto 606-8501, Japan; orcid.org/0000-0002-7013-9708 Kuang-Mao Lu − Everlight Electronics Co., Ltd., New Taipei City 238, Taiwan Complete contact information is available at: https://pubs.acs.org/10.1021/acs.inorgchem.0c02055
CONCLUSIONS A series of LGGO phosphors were synthesized via solid-state reaction. All structures were well resolved by XRD and NPD with joint refinements. The effects of multiple site substitutions were resolved using EPR and temperature- and pressuredependent PL. The heteroatom substitution with Sn4+ exhibits no significant effect. The Ba2+ substitution generates lattice vacancies via a charge unbalance mechanism. As a result, the magnetic property is changed. The Sc3+ substitution exhibits the greatest effects with a significant reduction in crystal field strength. This condition leads to a redshift in the emission of the LGGO phosphor with a notable enhancement in the quantum efficiency. The LED packaging demonstrates that the LGGO phosphor codoped with Sn3+, Ba2+, and Sc3+ is a promising candidate for NIR LED applications.
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ASSOCIATED CONTENT
* Supporting Information sı
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02055. Detailed Rietveld refinement results, X-ray diffraction characterizations, temperature-dependent photoluminescence, and pressure-dependent photoluminescence (PDF)
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Author Contributions
The manuscript was written through the contributions of all authors. All authors have approved the final version of the manuscript. Notes
The authors declare no competing financial interest.
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AUTHOR INFORMATION
ACKNOWLEDGMENTS The authors would like to acknowledge the support of the Ministry of Science and Technology of Taiwan (Contract Nos. MOST 109-2112-M-003-011, MOST 109-2113-M-002-020MY3, MOST 107-2113-M-002-008-MY3, MOST 107-2923M-002-004-MY3, and MOST 106-2112-M-003-007-MY3), the National Science Center Poland Grant Opus (No. 2016/23/ B/ST3/03911), and the National Center for Research and Development Poland Grant (No. PL-TW/V/1/2018). T. Lesniewski would like to acknowledge the support of the National Science Center Poland, Grant Preludium 13 (No. 2017/25/N/ST3/02412).
Corresponding Authors
Shu-Fen Hu − Department of Physics, National Taiwan Normal University, Taipei 116, Taiwan; orcid.org/0000-00019561-8206; Email: [email protected] Ho Chang − Department of Mechanical Engineering and Graduate Institute of Manufacturing Technology, National Taipei University of Technology, Taipei 106, Taiwan; Email: [email protected] Ru-Shi Liu − Department of Chemistry, National Taiwan University, Taipei 106, Taiwan; orcid.org/0000-00021291-9052; Email: [email protected]
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Authors
Gabriel Nicolo A. De Guzman − Department of Physics, National Taiwan Normal University, Taipei 116, Taiwan Veeramani Rajendran − Department of Mechanical Engineering and Graduate Institute of Manufacturing Technology, National Taipei University of Technology, Taipei 106, Taiwan; orcid.org/0000-0003-1479-8829 Zhen Bao − Department of Chemistry, National Taiwan University, Taipei 106, Taiwan Mu-Huai Fang − Department of Chemistry, National Taiwan University, Taipei 106, Taiwan; orcid.org/0000-00031475-0200 Wei-Kong Pang − Institute for Superconducting & Electronic Materials, Faculty of Engineering, University of Wollongong, Wollongong 2522, Australia Sebastian Mahlik − Institute of Experimental Physics, Faculty of Mathematic, Physics and Informatics, University of Gdańsk, 80308 Gdańsk, Poland; orcid.org/0000-0002-9514-049X Tadeusz Lesniewski − Institute of Experimental Physics, Faculty of Mathematic, Physics and Informatics, University of Gdańsk, 80-308 Gdańsk, Poland; orcid.org/0000-0003-2451-7760 Marek Grinberg − Institute of Experimental Physics, Faculty of Mathematic, Physics and Informatics, University of Gdańsk, 80308 Gdańsk, Poland
REFERENCES
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https://dx.doi.org/10.1021/acs.inorgchem.0c02055 Inorg. Chem. XXXX, XXX, XXX−XXX
Inorganic Chemistry
pubs.acs.org/IC
La3Ga5GeO14:Cr3+, Yb3+, Er3+ and Their Potential Coupling. RSC Adv. 2015, 5, 49680−49687. (8) Lu, J.; Mu, Z.; Zhu, D.; Wang, Q.; Wu, F. Luminescence Properties of Eu3+ Doped La3Ga5GeO14 and Effect of Bi3+ CoDoping. J. Lumin. 2018, 196, 50−56. (9) Rajendran, V.; Fang, M. H.; De Guzman, G. N.; Lesniewski, T.; Mahlik, S.; Grinberg, M.; Leniec, G.; Kaczmarek, S. M.; Lin, Y. S.; Lu, K. M.; Lin, C. M.; Chang, H.; Hu, S. F.; Liu, R. S. Super Broadband Near-Infrared Phosphors with High Radiant Flux as Future Light Sources for Spectroscopy Applications. ACS Energy Lett. 2018, 3, 2679−2684. (10) Grinberg, M.; Macfarlane, P. I.; Henderson, B.; Holliday, K. Inhomogeneous Broadening of Optical Transitions Dominated by Low-Symmetry Crystal-Field Components in Cr3+-Doped Gallogermanates. Phys. Rev. B: Condens. Matter Mater. Phys. 1995, 52, 3917− 3929. (11) Macfarlane, P. I.; Henderson, B.; Holliday, K.; Grinberg, M. Substitutional Disorder and the Optical Spectroscopy of Gallogermanate Crystals. J. Phys.: Condens. Matter 1996, 8, 3933−3946. (12) Dudka, A. P. Multicell Model of La3Ga5GeO14 Crystal: A New Approach to the Description of the Short-Range Order of Atoms. Crystallogr. Rep. 2017, 62, 374−381. (13) Mombourquette, M.; Weil, J.; McGavin, D. EPR-NMR User’s Manual. Department of Chemistry, University of Saskatchewan, Saskatoon, SK, Canada, 1996. (14) Ruderman, M. A.; Kittel, C. Indirect Exchange Coupling of Nuclear Magnetic Moments by Conduction Electrons. Phys. Rev. 1954, 96, 99−102. (15) Kasuya, T. A Theory of Metallic Ferro- and Antiferromagnetism on Zener’s Model. Prog. Theor. Phys. 1956, 16, 45−57. (16) Even though the Cr3+ can dope into the Ga2O3 impurity phase, the concentration of the impurity is less than 10% according to the Rietveld refinement results. Moreover, the luminescence intensity of Ga2O3:Cr3+ obviously decreases after 750 nm, which differs from that of LGGO phosphor. As a result, the luminescence spectra of LGGO are credible. See for example, Fang, M. H.; De Guzman, G. N. A.; Bao, Z.; Majewska, N.; Mahlik, S.; Grinberg, M.; Leniec, G. L.; Kaczmarek, S. M.; Yang, C. W.; Lu, K. M.; Sheu, H. S.; Hu, S. F.; Liu, R. S. Ultra-High-Efficiency Near-Infrared Ga2O3:Cr3+ Phosphor and Controlling of Phytochrome. J. Mater. Chem. C 2020, 8, 11013− 11017. (17) Meng, X.; Wang, Z.; Qiu, K.; Li, Y.; Liu, J.; Wang, Z.; Liu, S.; Li, X.; Yang, Z.; Li, P. Design of a Novel Near-Infrared Phosphor by Controlling Cationic Coordination Environment. Cryst. Growth Des. 2018, 18, 4691−4700. (18) Kamimura, H.; Sugano, S.; Tanabe, Y. Ligand Field Theory and Its Applications; Syokabo, Tokyo. 1969. (19) De Guzman, G. N. A.; Fang, M. H.; Liang, C. H.; Bao, Z.; Hu, S. F.; Liu, R. S. Near-Infrared Phosphors and Their Full Potential: A Review on Practical Applications and Future Perspectives. J. Lumin. 2020, 219, 116944−116953. (20) Shannon, R. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32, 751−767. (21) Malysa, B.; Meijerink, A.; Jüstel, T. Temperature Dependent Cr3+ Photoluminescence in Garnets of the Type X3Sc2Ga3O12 (X = Lu, Y, Gd, La). J. Lumin. 2018, 202, 523−531. (22) Grinberg, M. Spectroscopic Characterization of Disordered Materials Doped with Chromium. Opt. Mater. 2002, 19, 37−45. (23) Wiśniewski, K.; Zorenko, Y. U.; Gorbenko, V.; Zorenko, T.; Kukliński, B.; Grinberg, M. High Pressure Spectroscopy Study of SCF Tb3Al5O12:Mn. J. Phys. Conf. Ser. 2010, 249, 24−29. (24) Mill, B. V.; Maksimov, B. A.; Pisarevsky, Y. V.; Danilova, N. P.; Markina, M. P.; Pavlovska, A.; Werner, S.; Schneider, J. In Phase Transitions in Langasite Family Crystals, Proceedings of the 2004 IEEE International Frequency Control Symposium and Exposition, Aug. 23− 27, 2004; pp 52−60.
Article
(25) Pavlovska, A.; Werner, S.; Maximov, B.; Mill, B. PressureInduced Phase Transitions of Piezoelectric Single Crystals from the Langasite Family: La3Nb0.5Ga5.5O14 and La3Ta0.5Ga5.5O14. Acta Crystallogr., Sect. B: Struct. Sci. 2002, 58, 939−947. (26) Shao, Q.; Ding, H.; Yao, L.; Xu, J.; Liang, C.; Jiang, J. Photoluminescence Properties of a ScBO3:Cr3+ Phosphor and Its Applications for Broadband Near-Infrared LEDs. RSC Adv. 2018, 8, 12035−12042.
J
https://dx.doi.org/10.1021/acs.inorgchem.0c02055 Inorg. Chem. XXXX, XXX, XXX−XXX
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