Pressure-induced reversible phase transition in thiourea dioxide crystal Qinglei Wang, Tingting Yan, Kai Wang, Hongyang Zhu, Qiliang Cui, and Bo Zou Citation: The Journal of Chemical Physics 142, 244701 (2015); doi: 10.1063/1.4922842 View online: http://dx.doi.org/10.1063/1.4922842 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/142/24?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Deviatoric stress-induced phase transitions in diamantane J. Chem. Phys. 141, 154305 (2014); 10.1063/1.4897252 Response to “Comment on ‘The origins of pressure-induced phase transitions during the surface texturing of silicon using femtosecond laser irradiation’” [J. Appl. Phys. 113, 126102 (2013)] J. Appl. Phys. 113, 126103 (2013); 10.1063/1.4796126 Vibrational and structural properties of tetramethyltin under pressure J. Chem. Phys. 138, 024307 (2013); 10.1063/1.4774022 High pressure investigation of α -form and CH 4 -loaded β -form of hydroquinone compounds J. Chem. Phys. 130, 124511 (2009); 10.1063/1.3097763 High-pressure synthesis and physical properties of an orthorhombic phase of chromium dioxide J. Appl. Phys. 99, 053909 (2006); 10.1063/1.2179967
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THE JOURNAL OF CHEMICAL PHYSICS 142, 244701 (2015)
Pressure-induced reversible phase transition in thiourea dioxide crystal Qinglei Wang,1,a) Tingting Yan,1,a) Kai Wang,1,2,b) Hongyang Zhu,1 Qiliang Cui,1 and Bo Zou1,b) 1 2
State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, People’s Republic of China College of Chemistry, Jilin University, Changchun 130012, People’s Republic of China
(Received 9 March 2015; accepted 9 June 2015; published online 22 June 2015) The effect of high pressure on the crystal structure of thiourea dioxide has been investigated by Raman spectroscopy and angle-dispersive X-ray diffraction (ADXRD) in a diamond anvil cell up to 10.3 GPa. The marked changes in the Raman spectra at 3.7 GPa strongly indicated a structural phase transition associated with the distortions of hydrogen bonding. There were no further changes up to the maximum pressure of 10.3 GPa and the observed transition was completely reversible when the system was brought back to ambient pressure. This transition was further confirmed by the changes of ADXRD spectra. The high-pressure phase was indexed and refined to an orthorhombic structure with a possible space group Pbam. The results from the first-principles calculations suggested that this phase transition was mainly related to the changes of hydrogen-bonded networks in thiourea dioxide. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4922842]
I. INTRODUCTION
Hydrogen-bonded molecular crystals have been the attention of chemists, physicists, and materials scientists for their importance in determining the structural stability of chemical systems in recent years.1–4 Pressure, a well-known basic thermodynamic parameter, can be used as an ideal tool to study hydrogen-bonded systems.5–11 It is well established that pressure can reduce the distances between molecules and atoms. Thus, they reorient themselves and tend to achieve close packing, eventually resulting in new crystal structures. Moreover, pressure can break and form hydrogen bonding. This means that the distortions and rearrangements in hydrogen-bonded networks can be easily induced by compression. Besides, the balance between hydrogen bonding and van der Waals interactions can be altered with applied pressure, which may lead to structural changes. All of the aforementioned results demonstrate that pressure can be used as a powerful tool to explore the nature of hydrogen bonding and the corresponding cooperativity. Therefore, it is very necessary to perform high-pressure investigations on hydrogen-bonded molecular crystals to acquire further knowledge on the structure stability. Meanwhile, such investigations are also of fundamental and significant importance for chemistry and chemical industry. Over the past few decades, a number of structural transitions induced by high pressure on hydrogen-bonded structures have been reported. Meanwhile, considerable efforts have been devoted to studying the pressure-induced phase transitions of hydrogen-bonded systems by our group.12–16 Urea, a typical system for studying the structure and phase stabilities of hydrogen-bonded molecular crystals, has been investigated systematically under high pressure.17,18 Single crystals of urea phases I, III, and IV are grown in situ in a diamond anvil cell a)Q. L. Wang and T. T. Yan contributed equally to this work. b)Electronic addresses: [email protected] and [email protected]
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(DAC), and their structures are determined by X-ray diffraction. Urea undergoes a phase transition at 0.48 GPa, yielding phase III (phase I → phase III). There is another transition to phase IV at 2.8 GPa. The molecular reorientations in urea phases I, III, and IV, and the changes of hydrogen bonding can illustrate the importance of the angular dimensions and directionality of hydrogen bonding for the solid-state phase transitions. Biurea has been studied by in situ Raman spectroscopy and angle-dispersive x-ray diffraction (ADXRD) in a DAC up to 5 GPa.16 The significant changes in Raman spectra provide evidence for a pressure-induced structural phase transition in the pressure range of 0.6–1.5 GPa. ADXRD spectra further confirm this phase transition with symmetry transformation from C2/c to a possible space group P2/n. The structural transformation was proposed to be a result of the rearrangement of the hydrogen-bonded networks. In addition, a highpressure Raman spectroscopic study of phase transitions in thiourea has been reported.19–22 The changes in Raman spectra with increasing and decreasing pressure have been followed to approximately 11 GPa. The existence of three more transitions in this system to phases VII, VIII, and IX at 1, 3, and 6.1 GPa, respectively, has been reserved. The high-pressure behavior of thiourea, the molecular formula and structure of which are similar to those of urea except that the O atom in urea is replaced by S, also exhibits various structural changes. Moreover, all the transitions have been found to be completely reversible. Overall, the above results suggest that the variations in hydrogen bonding play an important part in structural transition in these hydrogen-bonded structures. Thiourea dioxide ((NH2)2CSO2, abbreviated as TDO), a typical example of hydrogen-bonded molecular crystals, has attracted considerable interests for its structural and characteristic properties.23–25 The compound has a wide range of applications in chemical industry. Besides, thiourea dioxide has a number of unique properties including strong reducing property,26 excellent bleaching property,27 organocatalytic
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property,28 and its stability. Jensen carried out a series of single crystal diffraction studies and quantitative first-principle calculations to investigate the features and reactivity of TDO.25 The crystal molecular structure of TDO at ambient pressure was determined. Pei conducted the studies of TDO in aqueous phase by means of density functional theory, UV absorption, and Raman spectroscopy to investigate its structure and property.29 As shown in Fig. 1, the molecule as a whole is nonplanar and TDO exhibits three-dimensional hydrogen-bonded networks. The thiourea portion of the molecule has a planar conformation. When the two oxygen atoms are included, the sulfur atom is at the apex of a trigonal pyramid formed with the two oxygen atoms and the carbon atom as the base. TDO crystallizes into an orthorhombic structure belonging to Pnma space group at room temperature with the unit cell parameters a = 10.66(9) Å, b = 10.11(9) Å, c = 3.91(5) Å, V = 422.67(3) Å3, and Z = 4. The TDO molecules in the crystals form numbers of layers parallel to (010). As depicted in the Fig. 1, within the layers, each molecule is linked to four neighbor molecules primarily by weak N–H · · · O hydrogen bonding (intralayer hydrogen bonding) under ambient pressure conditions.30 The adjacent layers are linked by much weaker N–H · · · O hydrogen bonding (interlayer hydrogen bonding). Importantly, the crystal structure is stabilized mainly by hydrogen bonding and van der Waals interactions. Thus, it is very critical for us to realize the importance of the balance between hydrogen bonding and van der Waals interactions to further study the structure of hydrogen-bonded molecular crystals under high pressure. To the best of our knowledge, there is still no research reported on TDO applied by high pressure until now. Thus, such systematic high pressure experiments are expected to conduct and further get much more valuable information on TDO crystal under high pressure. In this paper, we have performed in situ high-pressure Raman scattering measurements and ADXRD studies up to 10.3 GPa on TDO. Raman spectroscopy and ADXRD pattern have proven to be powerful tools for interrogating the structural properties at high pressure.31,32 Analyses of structural changes and cooperative effects between hydrogen bonding and van der Waals interactions are performed. The firstprinciple calculations are also employed to explore the mechanisms of the observed experimental results. The main goal of this work is to provide much valuable information on the nature of the hydrogen bonding as well as the cooperative effects under high pressure conditions.
II. EXPERIMENTAL
TDO for this study was purchased from Alfa Aesar [purity 99%] and was used without further purification. The sample
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was ground to a fine powder using a mortar and pestle. A symmetric DAC furnished with 0.4 mm culet-size diamonds was used to perform in situ Raman scattering and ADXRD measurements at room temperature. A T301 stainless steel gasket was preindented to a thickness of 0.04 mm, and a center hole with a diameter of 0.13 mm was drilled as the sample chamber. Subsequently, TDO was loaded into the chamber. Liquid argon was employed as the pressure-transmitting medium (PTM) to ensure hydrostatic pressure conditions. A ruby ball was used to measure the pressure.33 High-pressure Raman spectra were measured in the standard backscattering geometry with the Acton Spectra Pro 2500 spectrograph (500 mm focal length) equipped with a CCD that was cooled with liquid nitrogen (PyLoN:100B). The excitation source was a single-mode diode-pumped solid-state (DPSS) laser at 532 nm with the output power at 10 mW. Each acquisition was carried out several minutes after elevating pressure, aiming to restrain any kinetic factor during the measurements. High-pressure ADXRD experiments were performed on the as-prepared samples at Cornell High Energy Synchrotron Source (CHESS) of Cornell University. The 0.4859 Å beam was adopted as the incidence light source. CeO2 was used as the standard sample to do the calibration of geometric parameters before data collection. The Bragg diffraction rings were recorded using a Mar345 imaging-plate detector. The FIT2D software was used to integrate the ADXRD patterns.34 The geometrical optimization and electronic properties were calculated using the pseudopotential plane-wave method within the density functional theory implemented in the CASTEP code.35,36 The generalized gradient approximation37 of Perdew-Burke-Ernzerhof and Vanderbilt-type ultrasoft pseudopotentials38 is used. These calculations are performed with a 2 × 2 × 7 Monkhorst-Pack k-point mesh and a 300 eV plane-wave cutoff, both of which result in good convergence.
III. RESULTS AND DISCUSSION
The Raman spectra of TDO at ambient pressure observed by us matched very well with those reported in the literature, so we make the assignments of Raman peaks on the basis of previous results.29,39,40 We tentatively separate the Raman active modes into two distinct vibrational regions (external vibrations and internal ones). The external mode involves collective motions of all atoms in the unit cell. The internal mode is related to the vibration of specific groups. In this work, the Raman spectra of TDO are measured up to 10.3 GPa, the highest pressure applied in the experiment. The evolution of Raman spectra of TDO in the spectral regions of 50–250, 250–1800, and FIG. 1. The crystal structure and hydrogen-bonded networks of TDO under ambient pressure. Pink and blue dot lines represent interlayer N–H · · ·O hydrogen bonding and intralayer N–H · · ·O hydrogen bonding, respectively.
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FIG. 3. Dependence of peak positions in external region versus pressure. The vertical line means the boundaries of ambient phase I and high-pressure phase II. FIG. 2. Selected Raman spectra of external modes of TDO at various pressures in the range of 50–250 cm−1.
2850–3490 cm−1 at different pressures are presented in Fig. 2, Figs. 5(a) and 5(b), and Fig. 7(a), respectively. The observed simultaneous variations in several Raman modes indicate that TDO has undergone a structural phase transition (phase I → phase II) at 3.7 GPa. These observations are accompanied by appearance of several new modes and disappearance of some original modes. The Raman spectra of the new phase are found to be stable up to the maximum pressure (10.3 GPa). Upon decreasing the pressure, a small hysteresis of the transition can be observed from the Raman spectra. The highpressure phase (phase II) returned to the original phase at about 2.9 GPa, as indicated by the Raman spectra on decompression. In addition, the effect of high pressure on the deuterated thiourea dioxide has been investigated by Raman spectroscopy in a DAC up to 10.4 GPa. Similarly, the marked changes in the Raman spectra at 4.3 GPa strongly indicated a structural phase transition in the deuterated thiourea dioxide crystal and the high-pressure phase returned to the original phase at about 3.7 GPa (see the supplementary material49). The hysteresis manifests itself as a difference in the transition pressure in the forward and reverse directions in pressure-induced phase transition. The barrier dynamics is often considered to be the main factor controlling the hysteresis. The reversible phase transition does not occur unless extra energy must be supplied to get the system over the transition barrier.41 The small hysteresis of the transition in thiourea dioxide crystal is mainly due to small barrier energy. Thus, the sample is already in the phase I at decompression at 2.9 GPa. The phenomenon showed that the pressure-induced phase transition of TDO was completely reversible. The pressure-induced frequency shifts of the Raman modes are illustrated in Fig. 3, Figs. 6(a) and 6(b), and
Fig. 7(b), separately. At 3.7 GPa, significant discontinuities occur, consistent with the proposed phase transition. Although it is difficult to determine the crystal structure of the highpressure phase (phase II) from the Raman data, the observed Raman spectra can shed light on this phase transition. It is well-known that the external modes of hydrogenbonded crystals are very sensitive to pressure-induced phase transition. Figures 2 and 3 summarize the pressure dependence of external modes and the corresponding peak positions versus pressure. The Raman spectra of TDO observed at ambient pressure condition consist of seven external modes (89, 99, 107, 116, 125, 136, and 161 cm−1). At higher pressure, most external modes display normal blue-shifts but at different rates. These blue-shifts indicate external modes hardening because of the enhancements in intermolecular interactions.42,43 Below 3.7 GPa, there are not obvious changes for the line shape of the Raman active modes. All the Raman active modes have a narrow linewidth. Meanwhile, the Raman linewidth of most external modes remained essentially the same with increasing pressure. At 3.7 GPa, the spectra shape changed abruptly, which can be visualized as evidence for the phase transition induced by increased pressure. The modes (89, 99, 107, 116, 125, 136, and 161 cm−1) disappeared when pressure is increased to 3.7 GPa. Some new Raman peaks appear with high intensity at 100, 116, 154, and 160 cm−1. As shown in Fig. 4, there is a drastic change of the full width at half maximum (FWHM) of several Raman peaks in external modes region at 3.7 GPa. The linewidth of the Raman modes exhibits an increase suddenly. The observed behavior of Raman linewidth is probably because the available phase space for the reorienting molecules in thiourea dioxide crystal becomes more restricted at higher pressures.44 As shown in Fig. 3, significant discontinuities occur at 3.7 GPa, consistent
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FIG. 4. FWHM of the Raman modes at 89 cm−1, 99 cm−1, and 1100 cm−1 versus pressure.
with the proposed phase transition. On further compression to 10.3 GPa, these new Raman peaks gain in intensity progressively. However, the Raman spectra still remained essentially similar to those at 3.7 GPa. This implies that phase II is stable and does not undergo any further changes in this experiment. The results from external modes can provide strong evidence for the occurrence of a pressure-induced phase transition at 3.7 GPa. Upon total release of pressure, the Raman spectra returned to its initial state, implying this transition was reversible. Moreover, the linewidths also returned to nearly the original values after the release of the pressure. The changes of internal vibration modes of TDO molecules can also reveal the existence of this phase transition. We assign these observed Raman peaks in the frequency range of 250–3490 cm−1 to internal modes (Figs. 5(a) and 5(b) and Fig. 7(a)). The internal modes in the range of 250–1800 cm−1
FIG. 6. Dependence of peak positions in the range of (a) 320–840 and (b) 950–1750 cm−1 in internal region versus pressure. The vertical line means the boundaries of ambient phase I and high-pressure phase II.
can be used to explore local variations in the chemical environment around specific groups. For these internal modes, considerable variations in the Raman spectra can be detected. With increasing pressure, most of the internal modes start to gradually shift to higher frequencies. These increases in frequencies can be explained by the decreased length of the covalent bonding in TDO molecules and the increase in the effective force constants as the crystal is compressed.45,46 In Fig. 5(a), the frequencies of the modes assigned to NH2 rocking at 501 cm−1 and NH2CNH2 symmetric stretching at 687 cm−1 of TDO initially decreased below 3.7 GPa, whereas the frequencies started to increase with elevated pressure above 3.7 GPa. The results are the outcome of the strengthened hydrogen
FIG. 5. Selected Raman spectra of TDO at various pressures in the range of (a) 250–1250 and (b) 1400– 1800 cm−1. The exclamations denote the emergence of new peaks.
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FIG. 7. The N–H stretching region of 2850–3490 cm−1 of TDO at various pressures: (a) Raman spectra and (b) frequency shifts of these modes. The vertical line represents the boundary of the two phases.
bonding under high pressure. The most dramatic change in the Raman spectra is at 3.7 GPa and several new features are observed. These changes at 3.7 GPa are accompanied by the appearance of two new modes at 791 and 1611 cm−1 (marked with the exclamations) with gradually increasing high intensity. Besides, it is apparent that the Raman active modes at 1100 cm−1 for the symmetric and anti-symmetric NH2 modes get broadened abruptly at 3.7 GPa. We suggest that the broadening is mainly due to the distortion of hydrogen bonding under high pressure. As pressure further increasing, the mode at 1073 cm−1 progressively loses its intensities, whereas the two new modes show continuous increases in intensity. The pressure dependence of peak positions of the corresponding internal modes is depicted in Figs. 6(a) and 6(b). Most of the internal modes shift gradually toward higher frequencies. On the whole, there is an obvious feature in the evolution of internal modes as a function of pressure. There are larger pressure coefficients of external modes compared with those of internal modes. This is because that noncovalent interactions are weak in essence and show much higher compressibility than covalent bonds. These significant changes indicate that the hydrogen-bonded networks in the TDO crystal tend to change remarkably. In Figs. 7(a) and 7(b), we present Raman spectra in the N–H stretching region and frequency shifts at selected pressures. Apart from the aforementioned internal modes, the dependence of N–H stretching vibrations also plays an important role in monitoring the structural change at high pressure. At 0.6 GPa, two modes (labeled ν1 and ν2) can be detected. Mode ν1 at 3063 cm−1 is identified as weak intralayer N–H · · · O hydrogen bonding, and mode ν2 at 3278 cm−1 corresponds to much weaker interlayer N–H · · · O hydrogen bonding. The mode ν1 shows a shift to low frequencies with increasing pressure below 3.7 GPa, while mode ν2 shift to
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higher frequencies. The red shifts of ν1 are in accordance with general rules that an increase of pressure decreases the D–H stretching frequencies of weak and moderate strength D–H · · · A bonds. The N–H bond length is extended by the increasing van der Waals interactions between H · · · O with compression, thus reducing the frequency of N–H stretching vibration.47,48 Based on the calculated results, abnormal blue shifts regarding stretching vibrations are expected due to the changes of angles of interlayer N–H · · · O hydrogen bonding at high pressure. Above 3.7 GPa, mode ν1 and mode ν2 are both red-shifted. Meanwhile, mode ν2 progressively loses intensity with increasing pressure and becomes broadened. Detailed information on the peak positions versus pressure is shown in Fig. 7(b). The changes at 3.7 GPa further confirm the transition from phase I to phase II which is stable up to 10.3 GPa. The weak intralayer N–H · · · O hydrogen bonding and weaker interlayer N–H · · · O hydrogen bonding in the TDO crystal are relatively strengthened under high pressure. According to the response of N–H stretching vibrations to high pressure, we propose that this phase transition is mainly caused by the distortions in the N–H · · · O hydrogen-bonded networks. In order to confirm the pressure-induced phase transition, we performed the ADXRD experiment as great as 10.3 GPa, which is believed to offer straightforward evidence for a phase transition. Figure 8 shows the angle-dispersive X-ray diffraction patterns of TDO with increasing pressures. Below 3.8 GPa, all of the diffraction peaks shift to greater angles with increasing pressure, which is indicative of a decrease of the interplanar distance of crystal planes. Several peaks decrease progressively as increased pressure and disappear completely when pressures reach 3.8 GPa. The obvious changes in the ADXRD patterns indicate that a phase transition is occurring
FIG. 8. Representative ADXRD patterns of TDO at high pressure. The wavelength for data collection is 0.4859 Å. The upward-facing arrows denote the emergence of new peaks.
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at 3.8 GPa. Four new diffraction peaks marked with upwardfacing arrows are observed at 3.8 GPa, in which its intensity gradually increased with pressure. Besides, after phase transition, the diffraction peaks have a lower shift rate. This means that the molecule arrangements become more closed. With further compression up to 10.3 GPa, no significant variations of the diffraction patterns of phase II are observed, which indicates that the high-pressure phase is stable up to the highest pressure employed in our experiment. We performed Pawley refinement of the ADXRD pattern at 4.1 GPa. The crystal structure of phase II could be indexed and refined to an orthorhombic structure with a possible space group Pbam. The indexed lattice constants obtained were a = 9.52(1) Å, b = 6.76(9) Å, c = 5.19(6) Å, and the unit cell volume V = 334.89(9) Å3 with Z = 4. The ADXRD experiments conducted with increasing pressure confirmed the results from Raman experiments, demonstrating that TDO undergoes a phase transition at 3.7 GPa. Upon release of pressure, the diffraction patterns of phase II returned to its initial state and this finding revealed that the phase transition observed was reversible. The results for Raman and ADXRD experiments provide strong guidance of the phase transition of TDO. However, the existing experimental data cannot provide detailed information on the geometry of hydrogen bonding at high pressure. Therefore, the first-principle calculations have been performed to understand what sort of changes of the structure of TDO occur on earth at high pressure and further investigate the mechanism of phase transition. The calculated results reveal that high-pressure variations mainly occur at the much weaker layer N–H · · · O hydrogen bonding. As shown in Fig. 9, the high-pressure model is formed mainly by the distortion of the interlayer N–H · · · O hydrogen bonding (parallel to the x-axis) and intralayer N–H · · · O hydrogen bonding (parallel to the yaxis). At ambient conditions, hydrogen bonding and van der Waals interactions are the dominant interactions within TDO crystal. The behaviors of TDO under high pressure are expected to stem from the cooperativity of these two interactions. Given the knowledge of the calculated model, we infer the mechanism of the phase transition as follows. The results from the first-principle calculations reveal that, below 3.7 GPa, the neighboring molecules in the TDO crystal become closer to each other inevitably owing to the applied pressure. In addition, hydrogen bonding and van der Waals interactions will be strengthened, which results in the increasing total Gibbs free energy. Consequently, the structure starts to become unstable. When the applied pressure is sufficiently high, the structure cannot afford the increasing free energy anymore. Thus, phase transition and large distortions of the interlayer N–H · · · O hydrogen bonding occur to reduce the total energy to get new equilibrium positions. The features observed in Raman spectra under high pressure are consistent with the proposed mechanism of this phase transition, and the Raman spectra can also show some details of phase transition. For example, the external modes play an important function in monitoring structural changes under high pressure. The dramatic changes in these external modes such as disappearance of the original peaks and appearance of numerous new peaks indicate that the intermolecular interac-
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FIG. 9. Calculated intralayer and interlayer hydrogen-bond networks: (a) at ambient pressure and (b) at higher pressure. Dashed blue lines represent hydrogen bonding.
tions are bound to have the great changes. Before phase transition, the red-shift of the N–H stretching is attributed to the strengthening of intralayer hydrogen bonding, while the blueshift of the N–H stretching is mainly caused by the distorted interlayer hydrogen bonding. During phase transition, the obvious changes in the N–H stretching Raman peaks significantly modify the hydrogen-bond networks. The interlayer N–H · · · O hydrogen bonding occurs to have a large distortion due to high pressure as shown in Fig. 9. With further increasing pressure above 3.7 GPa, the frequencies of Raman peaks of the high-pressure phase at 3063 and 3278 cm−1 decreased. The process is attributed to the strengthening of interlayer and intralayer N–H · · · O hydrogen bonding under high pressure. After releasing pressure, the distorted hydrogen-bonded networks can be restored, which can explain the reversible pressure-induced phase transition. The above discussed results reveal that the hydrogen bonding plays a key role in the reversible structural transition of TDO crystal. However, it is necessary and essential to have further high pressure single crystal XRD experiments and neutron diffraction studies to provide reliable information on the exact hydrogen atomic positions at high pressure.
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IV. CONCLUSION
In summary, we have studied the pressure-induced phase transition of TDO using in situ Raman scattering and ADXRD measurements. The dramatic changes at 3.7 GPa in the Raman spectroscopy experiments indicated a significant phase transition. Phase II was stable up to 10.3 GPa, the highest pressure we investigated. The ADXRD experimental data at high pressure further confirmed this reversible transition. The results from the first-principle calculations and Raman spectra clarified the transition mechanism in terms of the distortions of hydrogen-bond networks. Meanwhile, the present study can help improve the current understanding on the nature of hydrogen bonding as well as cooperative effects under highpressure conditions. ACKNOWLEDGMENTS
We thank Zhongwu Wang for his technical support with the synchrotron X-ray diffraction measurements at Cornell High Energy Synchrotron Source (CHESS). This work is supported by NSFC (Grant Nos. 91227202 and 11204101), RFDP (Grant No. 20120061130006), Changbai Mountain Scholars Program (Grant No. 2013007), National Basic Research Program of China (Grant No. 2011CB808200), China Postdoctoral Science Foundation (Grant No. 2012M511327), and Project No. 2014087 supported by the Graduate Innovation Fund of Jilin University. 1T.
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THE JOURNAL OF CHEMICAL PHYSICS 142, 244701 (2015)
Pressure-induced reversible phase transition in thiourea dioxide crystal Qinglei Wang,1,a) Tingting Yan,1,a) Kai Wang,1,2,b) Hongyang Zhu,1 Qiliang Cui,1 and Bo Zou1,b) 1 2
State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, People’s Republic of China College of Chemistry, Jilin University, Changchun 130012, People’s Republic of China
(Received 9 March 2015; accepted 9 June 2015; published online 22 June 2015) The effect of high pressure on the crystal structure of thiourea dioxide has been investigated by Raman spectroscopy and angle-dispersive X-ray diffraction (ADXRD) in a diamond anvil cell up to 10.3 GPa. The marked changes in the Raman spectra at 3.7 GPa strongly indicated a structural phase transition associated with the distortions of hydrogen bonding. There were no further changes up to the maximum pressure of 10.3 GPa and the observed transition was completely reversible when the system was brought back to ambient pressure. This transition was further confirmed by the changes of ADXRD spectra. The high-pressure phase was indexed and refined to an orthorhombic structure with a possible space group Pbam. The results from the first-principles calculations suggested that this phase transition was mainly related to the changes of hydrogen-bonded networks in thiourea dioxide. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4922842]
I. INTRODUCTION
Hydrogen-bonded molecular crystals have been the attention of chemists, physicists, and materials scientists for their importance in determining the structural stability of chemical systems in recent years.1–4 Pressure, a well-known basic thermodynamic parameter, can be used as an ideal tool to study hydrogen-bonded systems.5–11 It is well established that pressure can reduce the distances between molecules and atoms. Thus, they reorient themselves and tend to achieve close packing, eventually resulting in new crystal structures. Moreover, pressure can break and form hydrogen bonding. This means that the distortions and rearrangements in hydrogen-bonded networks can be easily induced by compression. Besides, the balance between hydrogen bonding and van der Waals interactions can be altered with applied pressure, which may lead to structural changes. All of the aforementioned results demonstrate that pressure can be used as a powerful tool to explore the nature of hydrogen bonding and the corresponding cooperativity. Therefore, it is very necessary to perform high-pressure investigations on hydrogen-bonded molecular crystals to acquire further knowledge on the structure stability. Meanwhile, such investigations are also of fundamental and significant importance for chemistry and chemical industry. Over the past few decades, a number of structural transitions induced by high pressure on hydrogen-bonded structures have been reported. Meanwhile, considerable efforts have been devoted to studying the pressure-induced phase transitions of hydrogen-bonded systems by our group.12–16 Urea, a typical system for studying the structure and phase stabilities of hydrogen-bonded molecular crystals, has been investigated systematically under high pressure.17,18 Single crystals of urea phases I, III, and IV are grown in situ in a diamond anvil cell a)Q. L. Wang and T. T. Yan contributed equally to this work. b)Electronic addresses: [email protected] and [email protected]
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(DAC), and their structures are determined by X-ray diffraction. Urea undergoes a phase transition at 0.48 GPa, yielding phase III (phase I → phase III). There is another transition to phase IV at 2.8 GPa. The molecular reorientations in urea phases I, III, and IV, and the changes of hydrogen bonding can illustrate the importance of the angular dimensions and directionality of hydrogen bonding for the solid-state phase transitions. Biurea has been studied by in situ Raman spectroscopy and angle-dispersive x-ray diffraction (ADXRD) in a DAC up to 5 GPa.16 The significant changes in Raman spectra provide evidence for a pressure-induced structural phase transition in the pressure range of 0.6–1.5 GPa. ADXRD spectra further confirm this phase transition with symmetry transformation from C2/c to a possible space group P2/n. The structural transformation was proposed to be a result of the rearrangement of the hydrogen-bonded networks. In addition, a highpressure Raman spectroscopic study of phase transitions in thiourea has been reported.19–22 The changes in Raman spectra with increasing and decreasing pressure have been followed to approximately 11 GPa. The existence of three more transitions in this system to phases VII, VIII, and IX at 1, 3, and 6.1 GPa, respectively, has been reserved. The high-pressure behavior of thiourea, the molecular formula and structure of which are similar to those of urea except that the O atom in urea is replaced by S, also exhibits various structural changes. Moreover, all the transitions have been found to be completely reversible. Overall, the above results suggest that the variations in hydrogen bonding play an important part in structural transition in these hydrogen-bonded structures. Thiourea dioxide ((NH2)2CSO2, abbreviated as TDO), a typical example of hydrogen-bonded molecular crystals, has attracted considerable interests for its structural and characteristic properties.23–25 The compound has a wide range of applications in chemical industry. Besides, thiourea dioxide has a number of unique properties including strong reducing property,26 excellent bleaching property,27 organocatalytic
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property,28 and its stability. Jensen carried out a series of single crystal diffraction studies and quantitative first-principle calculations to investigate the features and reactivity of TDO.25 The crystal molecular structure of TDO at ambient pressure was determined. Pei conducted the studies of TDO in aqueous phase by means of density functional theory, UV absorption, and Raman spectroscopy to investigate its structure and property.29 As shown in Fig. 1, the molecule as a whole is nonplanar and TDO exhibits three-dimensional hydrogen-bonded networks. The thiourea portion of the molecule has a planar conformation. When the two oxygen atoms are included, the sulfur atom is at the apex of a trigonal pyramid formed with the two oxygen atoms and the carbon atom as the base. TDO crystallizes into an orthorhombic structure belonging to Pnma space group at room temperature with the unit cell parameters a = 10.66(9) Å, b = 10.11(9) Å, c = 3.91(5) Å, V = 422.67(3) Å3, and Z = 4. The TDO molecules in the crystals form numbers of layers parallel to (010). As depicted in the Fig. 1, within the layers, each molecule is linked to four neighbor molecules primarily by weak N–H · · · O hydrogen bonding (intralayer hydrogen bonding) under ambient pressure conditions.30 The adjacent layers are linked by much weaker N–H · · · O hydrogen bonding (interlayer hydrogen bonding). Importantly, the crystal structure is stabilized mainly by hydrogen bonding and van der Waals interactions. Thus, it is very critical for us to realize the importance of the balance between hydrogen bonding and van der Waals interactions to further study the structure of hydrogen-bonded molecular crystals under high pressure. To the best of our knowledge, there is still no research reported on TDO applied by high pressure until now. Thus, such systematic high pressure experiments are expected to conduct and further get much more valuable information on TDO crystal under high pressure. In this paper, we have performed in situ high-pressure Raman scattering measurements and ADXRD studies up to 10.3 GPa on TDO. Raman spectroscopy and ADXRD pattern have proven to be powerful tools for interrogating the structural properties at high pressure.31,32 Analyses of structural changes and cooperative effects between hydrogen bonding and van der Waals interactions are performed. The firstprinciple calculations are also employed to explore the mechanisms of the observed experimental results. The main goal of this work is to provide much valuable information on the nature of the hydrogen bonding as well as the cooperative effects under high pressure conditions.
II. EXPERIMENTAL
TDO for this study was purchased from Alfa Aesar [purity 99%] and was used without further purification. The sample
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was ground to a fine powder using a mortar and pestle. A symmetric DAC furnished with 0.4 mm culet-size diamonds was used to perform in situ Raman scattering and ADXRD measurements at room temperature. A T301 stainless steel gasket was preindented to a thickness of 0.04 mm, and a center hole with a diameter of 0.13 mm was drilled as the sample chamber. Subsequently, TDO was loaded into the chamber. Liquid argon was employed as the pressure-transmitting medium (PTM) to ensure hydrostatic pressure conditions. A ruby ball was used to measure the pressure.33 High-pressure Raman spectra were measured in the standard backscattering geometry with the Acton Spectra Pro 2500 spectrograph (500 mm focal length) equipped with a CCD that was cooled with liquid nitrogen (PyLoN:100B). The excitation source was a single-mode diode-pumped solid-state (DPSS) laser at 532 nm with the output power at 10 mW. Each acquisition was carried out several minutes after elevating pressure, aiming to restrain any kinetic factor during the measurements. High-pressure ADXRD experiments were performed on the as-prepared samples at Cornell High Energy Synchrotron Source (CHESS) of Cornell University. The 0.4859 Å beam was adopted as the incidence light source. CeO2 was used as the standard sample to do the calibration of geometric parameters before data collection. The Bragg diffraction rings were recorded using a Mar345 imaging-plate detector. The FIT2D software was used to integrate the ADXRD patterns.34 The geometrical optimization and electronic properties were calculated using the pseudopotential plane-wave method within the density functional theory implemented in the CASTEP code.35,36 The generalized gradient approximation37 of Perdew-Burke-Ernzerhof and Vanderbilt-type ultrasoft pseudopotentials38 is used. These calculations are performed with a 2 × 2 × 7 Monkhorst-Pack k-point mesh and a 300 eV plane-wave cutoff, both of which result in good convergence.
III. RESULTS AND DISCUSSION
The Raman spectra of TDO at ambient pressure observed by us matched very well with those reported in the literature, so we make the assignments of Raman peaks on the basis of previous results.29,39,40 We tentatively separate the Raman active modes into two distinct vibrational regions (external vibrations and internal ones). The external mode involves collective motions of all atoms in the unit cell. The internal mode is related to the vibration of specific groups. In this work, the Raman spectra of TDO are measured up to 10.3 GPa, the highest pressure applied in the experiment. The evolution of Raman spectra of TDO in the spectral regions of 50–250, 250–1800, and FIG. 1. The crystal structure and hydrogen-bonded networks of TDO under ambient pressure. Pink and blue dot lines represent interlayer N–H · · ·O hydrogen bonding and intralayer N–H · · ·O hydrogen bonding, respectively.
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FIG. 3. Dependence of peak positions in external region versus pressure. The vertical line means the boundaries of ambient phase I and high-pressure phase II. FIG. 2. Selected Raman spectra of external modes of TDO at various pressures in the range of 50–250 cm−1.
2850–3490 cm−1 at different pressures are presented in Fig. 2, Figs. 5(a) and 5(b), and Fig. 7(a), respectively. The observed simultaneous variations in several Raman modes indicate that TDO has undergone a structural phase transition (phase I → phase II) at 3.7 GPa. These observations are accompanied by appearance of several new modes and disappearance of some original modes. The Raman spectra of the new phase are found to be stable up to the maximum pressure (10.3 GPa). Upon decreasing the pressure, a small hysteresis of the transition can be observed from the Raman spectra. The highpressure phase (phase II) returned to the original phase at about 2.9 GPa, as indicated by the Raman spectra on decompression. In addition, the effect of high pressure on the deuterated thiourea dioxide has been investigated by Raman spectroscopy in a DAC up to 10.4 GPa. Similarly, the marked changes in the Raman spectra at 4.3 GPa strongly indicated a structural phase transition in the deuterated thiourea dioxide crystal and the high-pressure phase returned to the original phase at about 3.7 GPa (see the supplementary material49). The hysteresis manifests itself as a difference in the transition pressure in the forward and reverse directions in pressure-induced phase transition. The barrier dynamics is often considered to be the main factor controlling the hysteresis. The reversible phase transition does not occur unless extra energy must be supplied to get the system over the transition barrier.41 The small hysteresis of the transition in thiourea dioxide crystal is mainly due to small barrier energy. Thus, the sample is already in the phase I at decompression at 2.9 GPa. The phenomenon showed that the pressure-induced phase transition of TDO was completely reversible. The pressure-induced frequency shifts of the Raman modes are illustrated in Fig. 3, Figs. 6(a) and 6(b), and
Fig. 7(b), separately. At 3.7 GPa, significant discontinuities occur, consistent with the proposed phase transition. Although it is difficult to determine the crystal structure of the highpressure phase (phase II) from the Raman data, the observed Raman spectra can shed light on this phase transition. It is well-known that the external modes of hydrogenbonded crystals are very sensitive to pressure-induced phase transition. Figures 2 and 3 summarize the pressure dependence of external modes and the corresponding peak positions versus pressure. The Raman spectra of TDO observed at ambient pressure condition consist of seven external modes (89, 99, 107, 116, 125, 136, and 161 cm−1). At higher pressure, most external modes display normal blue-shifts but at different rates. These blue-shifts indicate external modes hardening because of the enhancements in intermolecular interactions.42,43 Below 3.7 GPa, there are not obvious changes for the line shape of the Raman active modes. All the Raman active modes have a narrow linewidth. Meanwhile, the Raman linewidth of most external modes remained essentially the same with increasing pressure. At 3.7 GPa, the spectra shape changed abruptly, which can be visualized as evidence for the phase transition induced by increased pressure. The modes (89, 99, 107, 116, 125, 136, and 161 cm−1) disappeared when pressure is increased to 3.7 GPa. Some new Raman peaks appear with high intensity at 100, 116, 154, and 160 cm−1. As shown in Fig. 4, there is a drastic change of the full width at half maximum (FWHM) of several Raman peaks in external modes region at 3.7 GPa. The linewidth of the Raman modes exhibits an increase suddenly. The observed behavior of Raman linewidth is probably because the available phase space for the reorienting molecules in thiourea dioxide crystal becomes more restricted at higher pressures.44 As shown in Fig. 3, significant discontinuities occur at 3.7 GPa, consistent
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FIG. 4. FWHM of the Raman modes at 89 cm−1, 99 cm−1, and 1100 cm−1 versus pressure.
with the proposed phase transition. On further compression to 10.3 GPa, these new Raman peaks gain in intensity progressively. However, the Raman spectra still remained essentially similar to those at 3.7 GPa. This implies that phase II is stable and does not undergo any further changes in this experiment. The results from external modes can provide strong evidence for the occurrence of a pressure-induced phase transition at 3.7 GPa. Upon total release of pressure, the Raman spectra returned to its initial state, implying this transition was reversible. Moreover, the linewidths also returned to nearly the original values after the release of the pressure. The changes of internal vibration modes of TDO molecules can also reveal the existence of this phase transition. We assign these observed Raman peaks in the frequency range of 250–3490 cm−1 to internal modes (Figs. 5(a) and 5(b) and Fig. 7(a)). The internal modes in the range of 250–1800 cm−1
FIG. 6. Dependence of peak positions in the range of (a) 320–840 and (b) 950–1750 cm−1 in internal region versus pressure. The vertical line means the boundaries of ambient phase I and high-pressure phase II.
can be used to explore local variations in the chemical environment around specific groups. For these internal modes, considerable variations in the Raman spectra can be detected. With increasing pressure, most of the internal modes start to gradually shift to higher frequencies. These increases in frequencies can be explained by the decreased length of the covalent bonding in TDO molecules and the increase in the effective force constants as the crystal is compressed.45,46 In Fig. 5(a), the frequencies of the modes assigned to NH2 rocking at 501 cm−1 and NH2CNH2 symmetric stretching at 687 cm−1 of TDO initially decreased below 3.7 GPa, whereas the frequencies started to increase with elevated pressure above 3.7 GPa. The results are the outcome of the strengthened hydrogen
FIG. 5. Selected Raman spectra of TDO at various pressures in the range of (a) 250–1250 and (b) 1400– 1800 cm−1. The exclamations denote the emergence of new peaks.
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FIG. 7. The N–H stretching region of 2850–3490 cm−1 of TDO at various pressures: (a) Raman spectra and (b) frequency shifts of these modes. The vertical line represents the boundary of the two phases.
bonding under high pressure. The most dramatic change in the Raman spectra is at 3.7 GPa and several new features are observed. These changes at 3.7 GPa are accompanied by the appearance of two new modes at 791 and 1611 cm−1 (marked with the exclamations) with gradually increasing high intensity. Besides, it is apparent that the Raman active modes at 1100 cm−1 for the symmetric and anti-symmetric NH2 modes get broadened abruptly at 3.7 GPa. We suggest that the broadening is mainly due to the distortion of hydrogen bonding under high pressure. As pressure further increasing, the mode at 1073 cm−1 progressively loses its intensities, whereas the two new modes show continuous increases in intensity. The pressure dependence of peak positions of the corresponding internal modes is depicted in Figs. 6(a) and 6(b). Most of the internal modes shift gradually toward higher frequencies. On the whole, there is an obvious feature in the evolution of internal modes as a function of pressure. There are larger pressure coefficients of external modes compared with those of internal modes. This is because that noncovalent interactions are weak in essence and show much higher compressibility than covalent bonds. These significant changes indicate that the hydrogen-bonded networks in the TDO crystal tend to change remarkably. In Figs. 7(a) and 7(b), we present Raman spectra in the N–H stretching region and frequency shifts at selected pressures. Apart from the aforementioned internal modes, the dependence of N–H stretching vibrations also plays an important role in monitoring the structural change at high pressure. At 0.6 GPa, two modes (labeled ν1 and ν2) can be detected. Mode ν1 at 3063 cm−1 is identified as weak intralayer N–H · · · O hydrogen bonding, and mode ν2 at 3278 cm−1 corresponds to much weaker interlayer N–H · · · O hydrogen bonding. The mode ν1 shows a shift to low frequencies with increasing pressure below 3.7 GPa, while mode ν2 shift to
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higher frequencies. The red shifts of ν1 are in accordance with general rules that an increase of pressure decreases the D–H stretching frequencies of weak and moderate strength D–H · · · A bonds. The N–H bond length is extended by the increasing van der Waals interactions between H · · · O with compression, thus reducing the frequency of N–H stretching vibration.47,48 Based on the calculated results, abnormal blue shifts regarding stretching vibrations are expected due to the changes of angles of interlayer N–H · · · O hydrogen bonding at high pressure. Above 3.7 GPa, mode ν1 and mode ν2 are both red-shifted. Meanwhile, mode ν2 progressively loses intensity with increasing pressure and becomes broadened. Detailed information on the peak positions versus pressure is shown in Fig. 7(b). The changes at 3.7 GPa further confirm the transition from phase I to phase II which is stable up to 10.3 GPa. The weak intralayer N–H · · · O hydrogen bonding and weaker interlayer N–H · · · O hydrogen bonding in the TDO crystal are relatively strengthened under high pressure. According to the response of N–H stretching vibrations to high pressure, we propose that this phase transition is mainly caused by the distortions in the N–H · · · O hydrogen-bonded networks. In order to confirm the pressure-induced phase transition, we performed the ADXRD experiment as great as 10.3 GPa, which is believed to offer straightforward evidence for a phase transition. Figure 8 shows the angle-dispersive X-ray diffraction patterns of TDO with increasing pressures. Below 3.8 GPa, all of the diffraction peaks shift to greater angles with increasing pressure, which is indicative of a decrease of the interplanar distance of crystal planes. Several peaks decrease progressively as increased pressure and disappear completely when pressures reach 3.8 GPa. The obvious changes in the ADXRD patterns indicate that a phase transition is occurring
FIG. 8. Representative ADXRD patterns of TDO at high pressure. The wavelength for data collection is 0.4859 Å. The upward-facing arrows denote the emergence of new peaks.
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at 3.8 GPa. Four new diffraction peaks marked with upwardfacing arrows are observed at 3.8 GPa, in which its intensity gradually increased with pressure. Besides, after phase transition, the diffraction peaks have a lower shift rate. This means that the molecule arrangements become more closed. With further compression up to 10.3 GPa, no significant variations of the diffraction patterns of phase II are observed, which indicates that the high-pressure phase is stable up to the highest pressure employed in our experiment. We performed Pawley refinement of the ADXRD pattern at 4.1 GPa. The crystal structure of phase II could be indexed and refined to an orthorhombic structure with a possible space group Pbam. The indexed lattice constants obtained were a = 9.52(1) Å, b = 6.76(9) Å, c = 5.19(6) Å, and the unit cell volume V = 334.89(9) Å3 with Z = 4. The ADXRD experiments conducted with increasing pressure confirmed the results from Raman experiments, demonstrating that TDO undergoes a phase transition at 3.7 GPa. Upon release of pressure, the diffraction patterns of phase II returned to its initial state and this finding revealed that the phase transition observed was reversible. The results for Raman and ADXRD experiments provide strong guidance of the phase transition of TDO. However, the existing experimental data cannot provide detailed information on the geometry of hydrogen bonding at high pressure. Therefore, the first-principle calculations have been performed to understand what sort of changes of the structure of TDO occur on earth at high pressure and further investigate the mechanism of phase transition. The calculated results reveal that high-pressure variations mainly occur at the much weaker layer N–H · · · O hydrogen bonding. As shown in Fig. 9, the high-pressure model is formed mainly by the distortion of the interlayer N–H · · · O hydrogen bonding (parallel to the x-axis) and intralayer N–H · · · O hydrogen bonding (parallel to the yaxis). At ambient conditions, hydrogen bonding and van der Waals interactions are the dominant interactions within TDO crystal. The behaviors of TDO under high pressure are expected to stem from the cooperativity of these two interactions. Given the knowledge of the calculated model, we infer the mechanism of the phase transition as follows. The results from the first-principle calculations reveal that, below 3.7 GPa, the neighboring molecules in the TDO crystal become closer to each other inevitably owing to the applied pressure. In addition, hydrogen bonding and van der Waals interactions will be strengthened, which results in the increasing total Gibbs free energy. Consequently, the structure starts to become unstable. When the applied pressure is sufficiently high, the structure cannot afford the increasing free energy anymore. Thus, phase transition and large distortions of the interlayer N–H · · · O hydrogen bonding occur to reduce the total energy to get new equilibrium positions. The features observed in Raman spectra under high pressure are consistent with the proposed mechanism of this phase transition, and the Raman spectra can also show some details of phase transition. For example, the external modes play an important function in monitoring structural changes under high pressure. The dramatic changes in these external modes such as disappearance of the original peaks and appearance of numerous new peaks indicate that the intermolecular interac-
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FIG. 9. Calculated intralayer and interlayer hydrogen-bond networks: (a) at ambient pressure and (b) at higher pressure. Dashed blue lines represent hydrogen bonding.
tions are bound to have the great changes. Before phase transition, the red-shift of the N–H stretching is attributed to the strengthening of intralayer hydrogen bonding, while the blueshift of the N–H stretching is mainly caused by the distorted interlayer hydrogen bonding. During phase transition, the obvious changes in the N–H stretching Raman peaks significantly modify the hydrogen-bond networks. The interlayer N–H · · · O hydrogen bonding occurs to have a large distortion due to high pressure as shown in Fig. 9. With further increasing pressure above 3.7 GPa, the frequencies of Raman peaks of the high-pressure phase at 3063 and 3278 cm−1 decreased. The process is attributed to the strengthening of interlayer and intralayer N–H · · · O hydrogen bonding under high pressure. After releasing pressure, the distorted hydrogen-bonded networks can be restored, which can explain the reversible pressure-induced phase transition. The above discussed results reveal that the hydrogen bonding plays a key role in the reversible structural transition of TDO crystal. However, it is necessary and essential to have further high pressure single crystal XRD experiments and neutron diffraction studies to provide reliable information on the exact hydrogen atomic positions at high pressure.
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IV. CONCLUSION
In summary, we have studied the pressure-induced phase transition of TDO using in situ Raman scattering and ADXRD measurements. The dramatic changes at 3.7 GPa in the Raman spectroscopy experiments indicated a significant phase transition. Phase II was stable up to 10.3 GPa, the highest pressure we investigated. The ADXRD experimental data at high pressure further confirmed this reversible transition. The results from the first-principle calculations and Raman spectra clarified the transition mechanism in terms of the distortions of hydrogen-bond networks. Meanwhile, the present study can help improve the current understanding on the nature of hydrogen bonding as well as cooperative effects under highpressure conditions. ACKNOWLEDGMENTS
We thank Zhongwu Wang for his technical support with the synchrotron X-ray diffraction measurements at Cornell High Energy Synchrotron Source (CHESS). This work is supported by NSFC (Grant Nos. 91227202 and 11204101), RFDP (Grant No. 20120061130006), Changbai Mountain Scholars Program (Grant No. 2013007), National Basic Research Program of China (Grant No. 2011CB808200), China Postdoctoral Science Foundation (Grant No. 2012M511327), and Project No. 2014087 supported by the Graduate Innovation Fund of Jilin University. 1T.
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