Letter pubs.acs.org/JPCL

Color Control of Pr3+ Luminescence by Electron−Hole Recombination Energy Transfer in CaTiO3 and CaZrO3 Zoila Barandiarán,† Marco Bettinelli,‡ and Luis Seijo*,† †

Departamento de Quı ́mica, Instituto Universitario de Ciencia de Materiales Nicolás Cabrera, and Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, 28049 Madrid, Spain ‡ Dipartimento Scientifico e Tecnologico, Università di Verona, and INSTM, UdR Verona, Verona 37129, Italy S Supporting Information *

ABSTRACT: Controlling luminescence in phosphors able to produce several emissions from different stable excited states determines their use in optical devices. We investigate the color control mechanism that quenches the greenish-blue emission in favor of the red one in the archetype phosphor CaTiO3:Pr3+. State-of-the-art ab initio calculations indicate that direct host-to-dopant energy transfer (released by electron−hole recombination following the interband excitation and structural reorganization) selectively populates the 1D2 red luminescent state of Pr3+ and bypasses the 3P0 greenish-blue emitter. Local defects can modulate the electron−hole recombination energy and therefore increase the red emission efficiency, as experimentally observed. The selection of red emission does not happen in CaZrO3:Pr3+ because the electron− hole recombination energy is much higher. The calculations could not support the widely accepted color control mechanism based on metal-to-metal charge transfer states. The conclusion sets new points of view for the color control of lanthanide activated inorganic phosphors.

B

transfer from the 3P0 state of Pr3+ to a cation of the host, which leads to an intermediate metal-to-metal charge transfer (MMCT) state, followed by an electron back-transfer from the host cation to the remaining Pr4+ ion that leads to a Pr3+ in the 1 D2 state. [The intermediate state was initially labeled the intervalence charge transfer (IVCT) state, but this name is conventionally reserved for the MMCT when the two metals differ only in the oxidation state.11] The strongest support for the MMCT model was provided by experimental studies on CaTiO3:Pr3+ and CaZrO3:Pr3+ by Boutinaud and co-workers,12 which showed that the greenishblue 3P0 → 3H4 emission is dominant in CaZrO3:Pr3+ whereas the red 1D2 → 3H4 emission is the only one exhibited by CaTiO3:Pr3+. It was natural to accept as an explanation that the energy of the Pr3+-to-Zr4+ MMCT state was way above 3P0 in CaZrO3:Pr3+, whereas the (lower) Pr3+-to-Ti4+ MMCT state was between 3P0 and 1D2 in CaTiO3:Pr3+ (cf. Figure 4 in ref 12), therefore quenching all of the greenish-blue emission in the latter material. A series of experimental studies, although not ruling out other possibilities, supported this interpretation.5,6,9,13,14 However, direct proof of the MMCT nature of the intermediate state in CaTiO3:Pr3+ was never found. In an attempt to support the MMCT hypothesis, we performed state-of-the-art wave function-based multireference multiconfigurational relativistic embedded-cluster ab initio calculations on CaTiO3:Pr3+ and CaZrO3:Pr3+. We calculated

eing able to control the nonradiative decay between the 3 P0 and 1D2 levels of Pr3+ in oxide materials and the corresponding relative intensities of their greenish-blue (3P0 → 3 H4) and red (1D2 → 3H4) emissions is an important goal in the field of inorganic phosphors since 2 decades ago.1,2 For instance, the search for Pr3+-doped oxide phosphors useful as scintillators for fast decay X-ray computed tomography seeks chemical manipulations that could make the faster 3P0 → 3H4 emission (in the μs scale) dominant over the slower 1D2 → 3H4 emission (in the ms scale).3 The same is true, for example, for long-lasting greenish-blue luminescent materials.4 On the contrary, the search for Pr3+-doped oxide red phosphors useful for solid-state white illumination devices, for lasers, and for other applications seeks chemistry that could make the 1D2 → 3 H4 emission dominant.2,5 It is important, then, to identify the mechanism of the 3 P0−1D2 nonradiative decay in order to be able to design Pr3+doped oxide materials with selective greenish-blue or red emissions. Historically, several interpretations were given for this decay mechanism: A multiphonon relaxation model and a model of cross-relaxation within pairs of Pr3+ ions were considered at some point and later ruled out in CaTiO3:Pr3+ on the basis of experimental observations.1,6 Proposals of intersystem crossing through low-lying levels of the 4f5d configuration1,7 were considered to be unlikely6 and shown to be in large disagreement with ab initio calculations on Lu2O3:Pr3+.8 A fourth model, the Pr-to-metal charge transfer state model6,9 (also known as the virtual recharge model10), found broad acceptance up to now. According to it, the 3 P0−1D2 nonradiative decay takes place through electron © 2017 American Chemical Society

Received: May 10, 2017 Accepted: June 20, 2017 Published: June 21, 2017 3095

DOI: 10.1021/acs.jpclett.7b01158 J. Phys. Chem. Lett. 2017, 8, 3095−3100

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The Journal of Physical Chemistry Letters the 4f2 and 4f5d states of Pr3+ in CaTiO3:Pr3+ and CaZrO3:Pr3+, the Pr3+-to-Ti4+ MMCT states of CaTiO3:Pr3+, and the O2−-toTi4+ and O2−-to-Zr4+ ligand-to-metal charge transfer (LMCT) states of CaTiO3 and CaZrO3. As we will see, these ab initio calculations do not support the MMCT or virtual recharge model. Instead, they indicate that the intermediate state responsible for the nonradiative decay from 3P0 to 1D2 in CaTiO3:Pr3+ is an O2−-to-Ti4+ LMCT state of the host. We performed wave function multireference multiconfigurational relativistic embedded-cluster ab initio calculations on several clusters extracted from the materials CaTiO3:Pr3+ and CaZrO3:Pr3+ with the MOLCAS suite of programs.15 The CaTiO3 unit cell and the clusters used are shown in Figure 1.

Figure 1. CaTiO3 unit cell neighboring Ti atoms (right bottom), and (TiO6Ca8Ti6)32+ shown in gold, Ti in red, O in

Figure 2. Potential energy curves of Pr3+ and Pr4+ in the Ca site of CaTiO3 along the PrO12 breathing mode. The states of the (PrO12)21− cluster with dominant 4f2 configuration are shown in full lines with colors: black for main free-ion character 3H4,5,6, orange for 3F2,3,4, cyan for 1G4, red for 1D2, green for mixtures of 3P0,1,2 and 1I6, and bright green for 1S0. The states of main configurational characters 4f5d(eg) and 4f5d(t2g) are shown in blue and maroon, respectively. Pr4+ states are shown in dashed magenta lines. Green and red emissions from the lowest level of the 3P0,1,2 and 1I6 manifold (usually referred to as 3P0 emission) and the lowest 1D2 level (1D2 emission), respectively, are indicated.

21−

(left top), (PrO12) cluster with top), (PrO12Ti8O24)37− cluster (left cluster (right bottom). Ca atoms are green, and Pr in violet.

We summarize the methods and results here and present further details in the Supporting Information. The reliability of the methods and conditions of choice is supported by the accumulated experience on the ab initio calculation of excited states of a number of lanthanide-containing materials.16 We show the calculated 4f2 and 4f5d energy levels of a Pr3+ in the Ca site of CaTiO3 with long-range compensation as a function of the Pr−O distance, dPr−O, in Figure 2. Detailed spectroscopic constants, assignments, and analysis of the electronic structure of the states are presented in Tables S2 and S3 of the Supporting Information, which also contain the equivalent results for Pr3+ in the Ti site for completeness. These results have been obtained in two-step spin−orbit coupling SA-CASSCF/MS-CASPT2/RASSI-SO DKH calculations on the (PrO12)21− cluster. In a first step, we used the spin−orbit-free many-electron relativistic second-order Douglas−Kroll−Hess (DKH) Hamiltonian,17,18 and we performed state-average complete-active-space self-consistent-field (SACASSCF)19−21 and multistate second-order perturbation theory (MS-CASPT2)22−25 calculations. In the second step, we added to the Hamiltonian the AMFI approximation of the DKH spin−orbit coupling operator,26 and we performed restricted-active-space state-interaction spin−orbit (RASSISO)27,28 calculations. In all of these calculations, the otherwise isolated clusters are embedded in ab initio model potentials (AIMPs)29 that include Coulomb, exchange, and Pauli repulsion interactions from cubic idealizations of the CaTiO3 and CaZrO3 host lattices; they have been obtained here in self-

consistent embedded-ions (SCEI)30 Hartree−Fock calculations and are available from the authors.31 The red emitter is 7Eg, with a very clear 1D2 character. The greenesh-blue emission originates in 5Ag, with 60% 1I and 40% 3 P characters. It is interesting that the emitting label commonly named 3P0 has a larger 1I6 character, according to these calculations. The respective calculated emissions are 19170 and 24360 cm−1, the experimental ones being 16600 and 20209 cm−1.12 Two thirds of the deviations are known to have a freeion origin.32 Regarding the 3P0−1D2 nonradiative decay, this energy diagram confirms that the intermediate state cannot have a 4f5d character because those states lie at much higher energy than 4f2 3P0.6,8 The 4f5deg and 4f5dt2g levels have a very similar equilibrium Pr−O distance between themselves and with the 4f2 states, contrary to what happens in CaF2:Pr3+,32 where the distances follow the pattern dPr−F(4f5deg) < dPr−F(4f2) < dPr−F(4f5dt2g). This is attributed to the very large Pr−O distance and weak ligand field experienced by Pr3+ at the Ca site of CaTiO3. Now, let us discuss the Pr3+-to-Ti4+ MMCT states of CaTiO3:Pr3+, which are the ones invoked in the MMCT or virtual recharge model for the 3P0−1D2 nonradiative decay. The configuration coordinate energy diagram showing them is 3096

DOI: 10.1021/acs.jpclett.7b01158 J. Phys. Chem. Lett. 2017, 8, 3095−3100

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The Journal of Physical Chemistry Letters presented in Figure 3. It was calculated at the diabatic level33,34 using the just-shown SA-CASSCF/MS-CASPT2/RASSI-SO

Figure 4. Isoline representation of the potential energy surfaces of three local states of the material CaTiO3:Pr3+ vs the Pr−O and Ti−O distances. One state of Pr3+ in CaTiO3 is shown in black, one Pr3+-toTi4+ MMCT state is in brown, and one O2−-to-Ti4+ LMCT state is in magenta. Configuration coordinates appropriate for one-dimensional energy diagrams are indicated with lines: for ligand-field Pr3+ states in black, for MMCT states of the pair Pr3+/Ti4+ in brown, and for O-toTi LMCT states in magenta.

Having ruled out the participation of Pr3+ 4f5d states and Pr3+-to-Ti4+ MMCT states in the 3P0−1D2 nonradiative decay of CaTiO3:Pr3+, let us consider now the role that O2−-to-Ti4+ LMCT states of CaTiO3 can play in such a decay because it is known that energy is transferred from these states to Pr3+ (CaTiO3 interband transitions excite Pr3+ luminescence12). An O2−-to-Ti4+ electron transfer in CaTiO3:Pr3+ should not have an important impact on the Pr−O distance, so that, in a first approximation, one of the corresponding LMCT states is represented in Figure 4 with magenta isolines and the LMCT configuration coordinate is the magenta line, which passes through the minima of the Pr3+ 4f2 states. A representation of the Pr3+ and O2−-to-Ti4+ LMCT states of CaTiO3:Pr3+ in a common energy diagram along the LMCT configuration coordinate is made in Figure 5, together with the equivalent diagram for Pr3+ and O2−-to-Zr4+ LMCT states of CaZrO3:Pr3+. The LMCT states have been obtained in SA-CASSCF/MSCASPT2 calculations on the (TiO 6 Ca 8 Ti 6 ) 32 + and (ZrO6Ca8Zr6)32+ clusters embedded in CaTiO3 and CaZrO3, respectively. The configurational complete active spaces correspond to the distribution of 6 electrons in the 6 MOs of main character t1g(O2pπ) and t2g(Ti3d or Zr4d) for g states and of 12 electrons in the 9 MOs of main characters t1u(O2pπ), t2u(O2pπ), and t2g(Ti3d or Zr4d) for u states. The results are summarized in Table 1. Further details are given in the Supporting Information. The first important observation from Figure 5 and Table 1 is that the LMCT states of CaTiO3 lie in the same energy range as the 3P0 and 1D2 states of Pr3+, whereas the LMCT states of CaZrO3 lie at much higher energy. This implies that they can be intermediates in a 3P0−1D2 nonradiative decay mechanism in the first host but irrelevant for that in the second host. Let us look at the LMCT states in more detail. The electric dipole-allowed LMCT absorptions (which have 1A1g → 1T1u many-electron character and O2pπt1u,t2u → Ti3dt2g(Zr4dt2g) main

Figure 3. MMCT diagram of Pr3+/Ti4+ pairs in CaTiO3.

results on the (PrO12)21− and (PrO12)20− clusters and SACASSCF/MS-CASPT2 results on the (TiO6)8− and (TiO6)9− clusters, all of them embedded in CaTiO3, with adiabatic corrections calculated in SA-CASSCF/MS-CASPT2 calculations on the (PrO12Ti8O24)37− cluster at the Pr3+-to-Ti4+ equilibrium structure (cf. Supporting Information for further details). Before discussing further the MMCT energy diagram in Figure 3, let us comment that the configuration coordinate used for it is indicated in Figure 4 (and represented in Figure S8 of the Supporting Information). In this figure, potential energy surfaces of several states are represented with isolines vs the Pr−O and Ti−O distances: One of the Pr3+ 4f2 states is shown in black isolines, and the configuration coordinate used in Figure 2 (breathing mode of PrO12) is represented with a black line. One of the Pr3+-to-Ti4+ MMCT states is shown with brown isolines, and the brown line is the configuration coordinate used in Figure 3. It corresponds to simultaneous contractions of the Pr−O bonds and expansions of the Ti−O bonds that accompany the electron transfer from Pr3+ to Ti4+. What we observe in Figure 3 is that the Pr3+-to-Ti4+ MMCT energy levels, that is, the energy levels of Pr4+−Ti3+ pairs, are too high in energy so as to play a relevant role in the 3P0−1D2 nonradiative decay. This means that these calculations do not support the MMCT or virtual recharge model to explain such decay. 3097

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Figure 5. Configuration coordinate energy diagrams combining the Pr3+ states with the O2−-to-Ti4+ LMCT states of CaTiO3:Pr3+(left) and the O2−to-Zr4+ LMCT states of CaZrO3:Pr3+(right).

LMCT states are much higher than the 3P0 and 1D2 states of Pr3+, and they cannot play any role in the selective population of 1D2. Then, the usual nonradiative decay mechanisms in Pr3+ materials lead to partial populations of the states 3P0 and 1D2, and both the greenish-blue and red emissions occur. At this point, it is interesting to recall that the efficiency of the red emission of CaTiO3:Pr3+ was found to be tunable with the preparation method, which can lead to different charge compensations.38 It was also found to improve by Ba and Sr substitutions for Ca39 and by addition of Lu2O3, La2O3, and Gd2O3.40 In the same line, the photoluminescence and afterglow efficiency of CaTiO3:Pr3+ were improved by excess Ca41 and ammonia treatment.2 These behaviors are in agreement with the present interpretation of the reasons behind the quenching of the greenish-blue emission and enhancement of the red emission. The reason is that all of these treatments are expected to produce local defects, which can modulate the electron−hole recombination energy in their proximities and, as a consequence, the selective energy transfer from the host to the 1D2 level of close Pr3+ ions. In order to illustrate this, we calculated the LMCT states of CaTiO3 with a Ca vacancy next to Ti. Such vacancies have been reported to produce a shoulder in the diffuse reflectance spectra of CaTiO3:Pr3+ at sligtly lower energy than the valence-toconduction absorption band edge and to play an important role in the enhancement of its red emission by addition of rare earth oxides.40 We observed a 2400 cm−1 lowering of the LMCT states. Although this result was obtained in MS-CASPT2 calculations on a symmetrized Ca vacancy with the (TiO6Ti6)16+ cluster embedded in CaTiO3 without additional geometry relaxations and is not expected to be taken quantitatively, it clearly indicates that local defects can influence

one-electron character) are shown in bold font in Table 1 for CaTiO3 and CaZrO3 (minimum-to-minimum transition) and with a magenta arrow in Figure 5 (left) for CaTiO3 (vertical transition). The calculated vertical allowed transition in CaTiO3 is 31400 cm−1. It should be compared with the experimental interband transition maximum, which is found to be 30000 cm−1 by Pinel et al. (band A in Figure 4 of ref 12). A DFT calculation with periodic boudary conditions reported 31500 cm−1 for the direct band gap.35 The calculated minimum-tominimum allowed transition is 29500 cm−1. It agrees with the origin of the interband transition found in the excitation spectrum at around 28000 cm−1.12 The calculated vertical and minimum-to-minimum allowed transition energies (interband absorption maximum and origin, respectively) of CaZrO3 are 50600 and 49300 cm−1. Kaneyoshi36 found a peak at 47600 cm−1 in the excitation spectrum of CaZrO3, and Rosa et al.37 reported a theoretical DFT band gap of 46000 cm−1. However, the LMCT states involved in the electric dipoleallowed absorptions are not the lowest ones. Several LMCT states that have O2pπt1g → Ti3dt2g main one-electron character and are involved in electric dipole-forbidden absorptions lie between the 3P0 and 1D2 states of Pr3+ in CaTiO3:Pr3+. They can provide an efficient nonradiative decay mechanism to selectively populate 1D2. In this mechanism (Figure 5), the allowed host interband absorption O2−-to-Ti4+ at around 30000 cm −1 is followed by structural reorganization (Ti−O elongation) and electronic relaxation toward the lowest LMCT state. In this material, this state lies below 3P0 and sligtly above 1D2, so that a host electron−hole recombination Ti3+-to-O− takes place and its energy is nonradiatively transferred directly to the 1D2 level of Pr3+, which finally emits in red. In CaZrO3:Pr3+, however, all of the O2−-to-Zr4+ 3098

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In the photoluminescence excitation (PLE) spectrum of the D2 emission of CaTiO3:Pr3+, Zhang et al.40 report a broad band at 330 nm (30300 cm−1) and another one at 370 nm (27000 cm−1). The same bands are shown in the continuouswave excitation spectrum of the 1D2 emission by Pinel et al.12 (The first band is attributed by the former authors to Pr3+ 4f5d states and by the latter authors to the lowest interband transition, or O2−-to-Ti4+ LMCT. All these authors attribute the second band to Pr-to-Ti MMCT states.) Besides, the two bands appear also in the diffuse reflectance spectra of CaTiO3:Pr3+.40 According to the present results, both bands can be attributed to O2−-to-Ti4+ LMCT, the upper one corresponding to the pure host and the lower one to Ca2+ vacancies or other local defects. The coincidence between reflectance and PLE spectra is the result of the host-to-Pr(1D2) energy transfer mechanism proposed here. In summary, all of the results discussed in this Letter indicate that the current model that attributes the 3P0−1D2 nonradiative decay of Pr3+ in some oxides to intersystem crossing through Pr-to-Ti charge transfer states is not sustained by ab initio calculations. Instead, the present ab initio calculations indicate that the mechanism responsible for a selective population of the 1 D2 level in CaTiO3:Pr3+, which leads to quenching the greenish-blue emission from the 3P0 level and the sole presence of the 1D2 red emission, is direct electron−hole recombination energy transfer from the host to the 1D2 level of the Pr3+ dopant.

Table 1. Spectroscopic Constants of the O-to-Ti and O-to-Zr LMCT States after MS-CASPT2 Calculations on the (TiO6Ca8Ti6)32+ and (ZrO6Ca8Zr6)32+ Clusters Embedded in CaTiO3 and CaZrO3, Respectively, along with Ti−O and Zr−O Equilibrium Distances dTi−O,e and dZr−O,e (Å), Breathing Mode Harmonic Vibrational Frequencies ωa1g(cm−1), and Minimum-to-Minimum Transition Energies Te(cm−1)a CaTiO3 level

ωa1g

1 1A1g

1.957

794

1

2.001 2.000 1.999 1.999 1.995 1.995 2.006 2.006 1.996 1.996 2.006 2.007 2.005 2.006 2.008 2.010 2.004 2.003 2.001 2.006 2.008 2.009 2.004 2.008

775 786 777 775 793 793 817 800 791 792 801 803 807 801 801 777 783 746 724 812 801 803 789 799

1 1 1 1 1 1 1 1 1 1 1 2 1 1 1 2 1 2 2 3 1 3 4 4 a

dTi−O,e

A2g Eg 3 Eg 3 A2g 1 T1g 1 T2g 1 Eu 3 Eu 3 T2g 3 T1g 1 A1u 3 Eu 3 T1u 3 A1u 1 A2u 1 Eu 1 T1u 1 T2u 3 T2u 1 T1u 3 A2u 3 T1u 1 T2u 3 T2u 1

1

CaZrO3 Te Ground State 0 LMCT States 20720 21110 23490 23790 26650 26720 27240 27790 27830 28030 28620 29200 29320 29630 29800 28900 29470 30410 30500 30710 30890 31110 31140 31690

dZr−O,e

ωa1g

2.105

775

0

2.143 2.139 2.145 2.145 2.140 2.139 2.147 2.150 2.141 2.141 2.152 2.150 2.147 2.152 2.150 2.155 2.145 2.147 2.148 2.147 2.151 2.145 2.149 2.162

744 732 763 761 765 764 505 830 767 769 767 965 736 771 771 711 782 898 763 732 771 668 688 755

43360 44010 44960 45680 48910 48720 48430 48810 50550 50020 50530 50050 49410 51290 49650 49450 49290 51130 50620 52800 50910 53130 55140 54540

Te



ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b01158. Further details of the calculations, detailed spectroscopic constants and analyses of the wave functions, and additional configuration coordinate diagrams (PDF)



The allowed absorptions (1A1g → 1T1u) are highlighted in bold.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zoila Barandiarán: 0000-0001-7166-6844 Luis Seijo: 0000-0002-0621-3694

the energies of the lowest LMCT states sufficiently so as to be able to modulate the relative 3P0 and 1D2 populations. Let us further discuss some other experimental observations. Pinel et al.12 reported that a sample of CaTiO3:Pr3+ prepared by solid-state reaction at 1200 °C under air for 430 h instead of the regular 6 h became yellowish-brown, with an intense very broad absorption band ranging from 650 nm (15400 cm−1) to less than 400 nm (more than 25000 cm−1) not followed by luminescence. These featuers were attributed to the presence of Pr4+, which was confirmed by an EPR study,6 so that the sample is in fact a mixed-valence material CaTiO3:Pr3+,Pr4+. We have computed an ab initio IVCT configuration coordinate energy diagram at the diabatic level42,43 for a mixed-valence Pr3+/Pr4+ pair in CaTiO3. It is presented in Figure S5 of the Supporting Information. The diagram predicts the lowest Pr3+-to-Pr4+ IVCT absorptions to have peeks extending from 12000 to 24000 cm−1, the full width at half-maximum (fwhm) of each of them being typically around 5000 cm−142 and being followed by no emission. All of this suggests that the yellowish-brown color of this sample and its corresponding very broad absorption band are due to Pr3+-to-Pr4+ IVCT absorption.

Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was partially supported by Ministerio de Economiá y Competitividad, Spain (Dirección General de Investigación y Gestión del Plan Nacional de I+D+i, MAT2014-54395-P).



REFERENCES

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DOI: 10.1021/acs.jpclett.7b01158 J. Phys. Chem. Lett. 2017, 8, 3095−3100

Color Control of Pr3+ Luminescence by Electron-Hole Recombination Energy Transfer in CaTiO3 and CaZrO3.

Controlling luminescence in phosphors able to produce several emissions from different stable excited states determines their use in optical devices. ...
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