Combined Raman Spectroscopy and X-ray Diffraction

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C: Energy Conversion and Storage; Energy and Charge Transport

Exploration of the Energetic Material Ammonium Perchlorate at High Pressures: Combined Raman Spectroscopy and X-ray Diffraction Study Lei Kang, Shourui Li, Bo Wang, Xiaoshuang Li, and Qingguang Zeng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05046 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on July 2, 2018

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The Journal of Physical Chemistry

Exploration of the Energetic Material Ammonium Perchlorate at High Pressures: Combined Raman Spectroscopy and X ‑ ray Diffraction Study Lei Kang,1 Shourui Li,2 Bo Wang,1 Xiaoshuang Li,1 and Qingguang Zeng*,1

1

School of Applied Physics and Materials, Wuyi University, Jiangmen 529020, China 2

National Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang 621900, China

Corresponding author. E-mail: [email protected]

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract High-pressure behaviors of ammonium perchlorate (NH4ClO4, AP), a widely used energetic oxidizer, have been investigated using in situ angle-dispersive X-ray diffraction (ADXRD) and Raman spectroscopy experiments up to 22.0 and 20.4 GPa, respectively. A sluggish structural transformation is identified, which started at ~ 2.4 GPa and completed at ~ 10.6 GPa. The crystal structure of high-pressure (phase II) is refined to possesses a symmetry of P2. The structural transformation is indicated by the obvious changes in XRD patterns, Raman spectra, and discontinuities of peak positions. Variations in lattice constants and volume of unit cell of the ambient structure (phase I) up to ~ 10.0 GPa are also presented. The compressibility of phase I is anisotropic. The compression ratio of b-axis is larger than that of a- and c-axes. The bulk modulus of phase I is B0 = 24.5(4) GPa and its first pressure derivative is B0 ' = 4.5(2). The phase transition is reversible, as the XRD pattern transformed to the initial profile upon release of pressure. Based on the Raman and XRD analysis, the high pressure-induced phase transition of ammonium perchlorate is proposed to be associated with rearrangement of hydrogen-bonded networks.


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Introduction Ammonium perchlorate (NH4ClO4, AP) has drawn considerable attention because of its widely use as a powerful oxidizer in solid propellants and explosives, which are usually subjected to high pressure and high temperature environment during explosion and burning. In the last decades, researchers have devoted a lot of effort to investigate the structural properties of AP.1-4 AP is a typical hydrogen-bonded compound that is usually highly compressible. Structural transition, conformation transformation, and decomposition may occur by applying external temperature or pressure, which in turn is expected to influence its performance and sensitivity. However, little is known about the structural change of AP at extreme conditions because of complexity of its chemical changes. At room temperature and atmospheric pressure, AP belongs to orthorhombic structure, which has a space group of Pnma (phase I) with Z = 4, and a = 9.20 Å, b = 5.82 Å, and c = 7.45 Å.5,6 The ClO4- groups and NH4+ groups are connected, forming a three-dimensional N-H···O network, which is displayed in Figure 1. The structure of AP is supported by hydrogen bonds and electrostatic interactions. AP undergoes a structural transformation to a cube phase at temperatures above 511 - 513 K.7 When cooling, two structural transitions can be detected at about 180 and 40 K by the abnormal changes from Raman spectra.8 Pressure is widely used to study the properties of materials, which is also a powerful tool to design new kind of materials by tuning the distances between molecules and thus structural changes.9-11 Especially for energy materials, pressure is an ideal tool to investigate their physical and chemical properties. For example, motivated by the synthesis of polymeric of nitrogen from azides (N3-), the structural behaviors of inorganic and organic azides at high pressures have been reported.12-16 This kind of materials is of special interest because they have practical and scientific application as propellants and explosives. They can also be used to synthesize polymeric nitrogen, which possesses high density of energy. The energy of N=N bond in azides is lower than that of N≡N in N2 molecules, so the N3- is expected to form polymeric nitrogen more easily than N2. A systematic works were presented on the structural evolutions of inorganic and organic azides. Phase transitions were observed ACS Paragon Plus Environment


through high pressure techniques, such as Raman and XRD. For example, three structural transition pressures of sodium azide NaN3 were determined at 0.3, 17.3, and 28.7 GPa, respectively, the structure at 0.3 GPa was solved to have a C2/m symmetry.12 Two structural transformations were revealed at around 6.5 and 16.0 GPa of rubidium azide and only one structural transition can be detected at 2.9 GPa of ammonium azide.14,16 While strontium azide did not show phase transitions up to 33.5 GPa, only a pressure-induced irreversible amorphization was observed.13 For 4-toluenesulfonyl azide (4-TsN3), it transformed to a solid structure at 0.7 GPa, the first solid-solid structural changes occurred at 2.7 GPa, and a second transition was evidenced at 6.3 GPa，then it turned into amorphous state at a relatively lower pressure compared with that of strontium azide.15 High pressure is also employed to investigate the structural behaviors of AP. The first investigation performed by Bridgman with the structural change at 3.1 GPa.17 Later, shock and static X-ray diffraction measurements confirmed a new phase at 4.7 GPa from some changes in the diffraction patterns. The new phase was reported to be an asymmetrical structure.18 The vanish of ClO4 mode at 1.0 - 2.4 GPa from infrared study also indicated a possible phase transformation, the high-pressure structure was inferred to belong to cube sysytem.19 To further complement the structural properties of AP at high pressures, an optical study was performed up to 26.0 GPa. However, no evidence of phase transitions was observed.7 Interestingly, obvious changes at 0.9 and 3.0 GPa for structural transitions were observed by Peiris et al.20 using power X-ray diffraction, infrared, and Raman spectroscopy up to 5.6 GPa. The new structure cannot be refined to orthorhombic system. Emergence of new XRD pattern above 3.0 GPa indicated another phase transition. The structure was suggested to have a higher symmetry due to fewer peaks compared with that of ambient orthorhombic phase. A recent investigation combining neutron and X-ray diffraction experiments found evidence for a first order structural transformation at 4.6 GPa, and the structure was solved to a new orthorhombic phase.21 The most recent study was performed using Raman spectroscopy, which found three structural transformations at 4.5, 9.9, and 27 GPa, respectively.22 ACS Paragon Plus Environment

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AP is a powerful oxidizer and ammonium is a good fuel, which is widely used as a propellant in rockets and missiles. Therefore, it is very important to clarify the phase diagram of AP, especially, its structural evolution at high pressures. However, the structural behaviors of AP are poorly understood under high pressure with many conflicting results in literatures, a detailed study to clarify the ambiguities is needed. In this paper, we present comprehensive investigations on the high-pressure structural properties of AP. In addition, the equation of state as well as axial compressibility are shown and discussed. This work can provide some insight for exploring the properties of hydrogen bond and the structural behavior of energetic materials at high pressures.



Experimental Section AP crystals were obtained from Alfa Aesar. Diamond anvil cells were used in X-ray diffraction and Raman experiments. The diameter of diamond anvil is 300 µm. A hole in T301 steel stainless gasket served as sample chamber. The diameter of the sample chamber is 100 µm and its thickness is 40 µm. A small amount of sample and a tiny ruby ball were loaded. The standard ruby R1 line was used to calculate pressures,23 and the well separated and sharp ruby peaks confirmed the quasi-hydrostatic pressure environment. All measurements were performed at room temperature. Raman spectra was recorded by Renishaw inVia Raman microscope. The excitation wavelength is 633 nm with power less than 10 mW. The Raman spectra are taken unpolarized. X-ray diffraction experiments were carried out at the 4W2 station at BSRF with the wavelength of 0.6199 Å, the beam size is 30 × 20 µm2. Before data collection, geometric parameter was calibrated with CeO2 sample. Then, FIT2D program was used to convert the obtained 2D images into 1D diffraction patterns.24 The information of the high-pressure structure was obtained by indexing and refining XRD patterns using Material Studio 5.5 program.


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Results and Discussion High-pressure ADXRD measurements was carried out to obtain the structural properties of AP. Figure 2 displays the diffraction patterns of AP up to 22.0 GPa. When pressure is increasing, all peaks shifted towards higher 2-theta angles, which can be explained that the interplanar distances were reduced and thus the decrease of unit cell volume. When pressure is increased to 2.6 GPa, new peaks marked by asterisks appeared. The new peaks did not belong to ambient structure, which indicates the occurrence of a phase transformation. These new peaks gradually increase intensities with increasing pressure (marked by up-facing arrows). Meanwhile, the intensities of peaks of the original phase I such as (011) and (211) (marked by down-facing arrows) decrease with increasing pressure. Upon further compress to 5.9 GPa, another two peaks (marked by asterisks) appear, indicating the continuation of structural transition process. The evolution of d values of diffraction peaks is shown in Figure 3. The original peaks of phase I shifted linearly to lower d-spacing values upon compression until the emergence of new peaks at 2.6 GPa. These results indicate that a pressure-induced phase transition occurs at 2.6 GPa, which are broadly consistent with the previous reports.18,19,21,22 The peaks of phase I did not vanish until 11.5 GPa. The sample transformed to phase II at higher pressure. There is a large pressure coexistence range (2.6-11.5 GPa) between the two phases, which can be also commonly found in other materials.25,26 In addition to non-hydrostatic environment, the large coexistence pressure range may be due to the slow transition kinetics.27,28 When pressure is further increased, no phase transition can be detected up to 22.0 GPa. Moreover, after decompression to 0.7 GPa, the XRD pattern matches well with the pattern of 0.6 GPa, indicating the full reversibility of the proposed phase transition. To explore the compressibility of the ambient phase (phase I) of AP, the lattice constants and volume of unit cell at selected pressures are obtained by Pawley method, as shown in Figure 4(a, b). The parameters decrease by 5.7% of a-axis, 7.6% of b-axis, and 6.4% of c-axis between 0 and 9.3 GPa. Obviously, the unit cell axes show



anisotropic behavior and the a- and c-axes are less compressible than b-axis. The P-V data of phase I are fitted by third-order B-M equation of state:29 3B P(V ) = 0 2

 V  2 3    V  7 3  V  5 3   3 ' 0 0   −    1 + ( B0 − 4 )  0  − 1   V    4  V     V 

Where P represents the external pressure, V represents the unit cell volume at pressure P, and V0 represents the volume at atmospheric pressure. B0 and B0 ' are bulk modulus and its first pressure derivative, respectively. Figure 4(b) presents the fitted result (solid line) with B0 = 24.5(4) GPa and B0 ' = 4.5(2). The volume decreased by 18.8%, which indicate the structure is compressible. The XRD pattern at 11.5 GPa was refined to obtain the information of phase II structure (as shown in Figure 5). At 11.5 GPa, the structure can be refined as a possible P2 symmetry. The lattice constants are a = 7.18 Å, b = 6.259 Å, c = 5.613 Å, and β = 105.981°, and V = 242.55 Å3. The R-factor is RWP = 0.77%. The R-factor is very low, suggesting that the refinement result is acceptable. The phase II possesses a lower symmetry than that of phase I, which is in agreement with previous reports.18 High pressure Raman experiment was carried out to explore the variation of lattice and internal mode vibrations. The space group of AP is Pnma, its point group is D2h. There are 4 molecules per unit cell, each molecule possesses 10 atoms. The mechanical representation of this symmetry is M = 18Ag + 12Au + 12B1g + 18B1u + 18B2g + 12B2u + 12B3g + 18B3u showing 3 acoustic modes Γacoustic = B1u + B2u + B3u and 117 optic modes Γoptic = 18Ag + 12Au + 12B1g + 17B1u + 18B2g + 11B2u + 12B3g + 17B3u Based on the group-theoretical analysis, among the optic mode, the Raman-active modes are 18Ag + 12B1g + 18B2g + 12B3g, and the infrared active modes are 17B1u + 11B2u + 17B3u. Assignment Raman modes is based on literatures.30-32 Figures 6 and 7 shown the Raman spectra up to 20.4 GPa and the peak positions versus pressure, respectively.


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Lattice modes are sensitive to structural changes. As for AP, based on the literatures,30-32 the modes below 240 cm-1 are assigned as lattice modes. However, no lattice mode was shown in the previous high-pressure studies.20,22 As shown in Figure 6(a), two lattice modes can be distinguished, which are sensitive to variations under high pressure due to the weak interionic interaction (hydrogen bonding, electrostatic interaction).33,34 When pressure is applied, the external modes showed blue shifts to higher frequency region. The blue shifts for of external modes are results of increased strength of interactions, because of the decreased interionic distances.34-36 The lattice modes decrease in intensities with increasing pressure, and the mode (marked by a down-facing arrow) finally disappeared at about 2.4 GPa, indicating a structural transition. Upon further compression to 4.3 GPa, a new peak at around 75 cm-1 appeared (marked by an asterisk). The original strong peak (at about 70 cm-1) disappears at 10.6 GPa, implying that the transition is complete, which is consistent with the XRD results. Figure 7(a) displays the pressure dependence of lattice modes at 0 - 20.4 GPa. Discontinuities can be observed at 2.4 GPa, which indicates a phase transition. The changes in the Raman lattice region and the discontinuities of pressure-induced mode positions indicating a structural change at around 2.4 GPa. This result is also consitent with previous Raman measurements.19,20,22 With complete release of pressure, the Raman spectrum shows similar features with that of 1 atm. Internal modes can detective local changes of groups. Figure 6(b) shows the typical Raman spectra in 400 - 1300 cm-1. When applying pressure, the peaks shift continuously towards the higher frequency region owing to the increase of effective force constants and the decrease of bond distances.37 The peak at about 921 cm-1 related to ClO4 symmetric stretching vibration gradually decreases in intensity as pressure is increasing, and disappear completely at 2.4 GPa. Meanwhile, a new peak at about 1141 cm-1 (marked by an asterisk) is observed at 2.4 GPa, indicating the change of crystal structure. When pressure is applied up to 6.0 GPa, another new peak appears (467 cm-1), which is also observed in literature,22 implying the continuation of the structural change. Upon compression to 10.6 GPa, the peak at 465 cm-1 related to ClO4 symmetric bending shows obvious split, which is considered to be an indication ACS Paragon Plus Environment


of lower symmetry.38 Meanwhile, red shift is observed for ClO4 asymmetric bending vibration at 10.6 GPa. The peak positions in 450-700 cm-1 and 900-1250 cm-1 under high pressure are displayed in Figure 7(b, c). Most internal peaks exhibit blue shifts to high frequencies with increasing pressure, and discontinuities are observed, suggesting a structural change. The modes related to NH4 stretching is useful to understand changes of hydrogen bonds. Figure 6(c) presents the evolution of NH4 stretching modes up to 20.4 GPa. There are two NH4 stretching modes that can be resolved in this spectral region. The peak at about 3212 cm-1 can be assigned as NH4 symmetric stretching mode, and peak located at 3350 cm-1 is NH4 asymmetric stretching mode. The NH4 stretching modes at 3350 cm-1 shows red shift up to 6.0 GPa, indicating the strengthening of hydrogen bonding interaction at high pressures.39 For hydrogen bonds with red shifts, the electrostatic attraction between H atoms and O atoms is enhanced due to closer distance with applied pressure, leading to the elongated N-H bond.40 A new NH4 peak marked by an asterisk at 3330 cm-1 appears at 7.9 GPa, and the origial mode at about 3350 cm-1 disappears, implying the reconstruction of hydrogen bonds. Figure 7(d) shows pressure dependence of N-H related peak positions. The discontinuity of NH4 stretching mode suggests the rearrangement of hydrogen bonds. Changes of NH4 stretching modes indicate that the structural transition is related to rearrangement of hydrogen bonded network. The present Raman results indicate that a sluggish structural transition started at around 2.4 GPa and completed at around 10.6 GPa, which is consitent with the changes in our XRD experiment. The present result is broadly consitent with the structural transition proposed by Sandstorm et al.,18 the phase I → II structural transition proposed by Hunter et al.,21 the phase II → III structural transition proposed by Peiris et al.,20 and the phase I → II structural transition proposed by Dunuwille et al..22 There are small differences of phase transition pressure among the experiments，which may be due to the pressure transmitting media, time interval of loadings, and sluggish nature of structural transition. However, the proposed transition cannot be observed from some infrared and Raman spectroscopy investigations.19,22 ACS Paragon Plus Environment

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The phase transition was inferred from the vanish of ClO4 mode (939 cm-1),19 which was also observed in the present Raman spectra. The mode does not disappear until 2.4 GPa, which is shown in Figure 6(b). High-pressure Raman measurements revealed that structural transitions occurred at 4.5 and 9.9 GPa, respectively, based on discontinuous evolution of peak positions under high pressure and appearance of new peaks.22 However, new peaks can still be observed in the pressure region (4.5-9.9 GPa), which means the phase transition is continuing.22 The sluggish transition is also confirmed by the present XRD experiments. New peaks appear at 2.6 GPa in XRD pattern, which cannot be indexed into the ambient phase, and peaks belonging to phase I are still detected. The new peaks continue to gain intensities with increasing pressure while peaks of phase I continually reduce intensities. Meanwhile, more new peaks appear, and the peaks belonging to the ambient phase (phase I) does not disappear completely until 11.5 GPa. The XRD patterns containing both new peaks and original peaks can not be indexed into a single phase, which indicate the coexistence of phases in the pressure region (2.6-11.5 GPa). The coexistence region is also broadly consistent with the pressure region which prevously was assigned to a single phase.22 So the phase transition observed at 9.9 GPa maybe indicate the end of the structural transition starting at 4.5 GPa rather than occurrence of another phase transition. Moreover, lattice modes region in the Raman spectra was not displayed.22 Lattice modes involve the collective movement of all atoms, which is an important indicator of structural changes. Raman spectroscopy is efficient to detecte vibrations of bonds, groups, and chemical changes. However, structural changes deduced from optical spectroscopy are not feasible invariably.41-43 Sometimes, the emergence or vanish of peaks may be due to the different peaks shifted with different rates upon increasing the pressure. Increasing pressure can also result in a removal of the degeneracy of some Raman vibrations, and thus emergence of new Raman peaks. Disappearance of modes may be also due to the mergence of some neighboring modes, because of difference in pressure dependences (dυ/dP). Compared with Raman measurement, XRD technique is a direct probe for crystal structural transitions, that



can detective the ralative movement of ions and molecules, and therefore, the symmetry changes. Based on XRD and Raman results, mechanism for this structural transformation is proposed. There are two principal interactions in AP structure, hydrogen bond and electrostatic interactions. Upon compression, the distances among atoms decrease, so electrostatic interactions of NH4+ and ClO4- groups are strengthened. Hydrogen bonds are also strengthened because of the decrease of lengths. When pressure is applied, AP structure becomes unstable due to the increased Gibbs free energy. At 2.4 GPa, NH4+ and ClO4- groups reorient and hydrogen bonds rearrange in order to reduce energy, which results in structural change. The proposed mechanism of this structural transformation is consistent with the XRD and Raman scattering measurements. The changes of external peaks at 2.4 GPa indicate transformation of the crystal structure. The appearance of new peaks in diffraction patterns confirmed the transition. Phase II was indexed to have the space group of P2, a lower symmetry, which is consistent with split of Raman peak. Moreover, rearrangements of hydrogen bonds is induced, indicated by discontinuities in NH4 modes. New hydrogen bonded networks were constructed from the emergence of new NH4 mode.


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Conclusion The structural and vibrational behaviors of ammonium perchlorate have been studied using X-ray diffraction and Raman scattering experiments. A structural transformation from orthorhombic structure into a possible monoclinic structure is detected, which started at ~ 2.4 GPa and completed at ~ 10.6 GPa. The compressibility of phase I is anisotropic, indicated by the weak compressibility of aand c-axes compared with b-axis. The bulk modulus B0 = 24.5(4) GPa and its first pressure derivative B0 ' = 4.5(2) are obtained. The phase transition is reversible. The structural transformation is proposed to be associated with rearrangements of hydrogen bonds. High-pressure studies of ammonium perchlorate can provide insight for exploring the property of hydrogen bond as well as the structural behaviors of energetic materials under high pressure condition.



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Acknowledgments This work is supported by Innovation Projects of Department of Education of Guangdong

Province

(No.

2017KQNCX207,

2017KQNCX197,

and

2017KQNCX198), NSFC (No. 11504353), the Science and Technology Projects of Jiangmen (No. (2017)307 ， and (2017)149), Cooperative education platform of Guangdong Province (No. (2016)31), Innovative Research Team in University of Guangdong (No. 2015KCXTD027)，Key Laboratory of Optoelectronic materials and Applications in Guangdong Higher Education (No. 2017KSYS011).


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(29) Birch, F. The Effect of Pressure Upon the Elastic Parameters of Isotropic Solids, According to Murnaghan's Theory of Finite Strain. J. Appl. Phys. 1938, 9, 279-288. (30) Zhu, W.; Wei, T.; Zhu, W.; Xiao, H. Comparative DFT Study of Crystalline Ammonium Perchlorate and Ammonium Dinitramide. J. Phys. Chem. A 2008, 112, 4688-4693. (31) Gruzdkov, Y. A.; Winey, J. M.; Gupta, Y. M. Spectroscopic Study of Shock-Induced Decomposition in Ammonium Perchlorate Single Crystals. J. Phys. Chem. A 2008, 112, 3947-3952. (32) Wu, Q.; Li, C.; Tan, L.; Hang, Z.; Zhu, W. Comparative DFT and DFT-D Studies on Structural, Electronic, Vibrational and Absorption Properties of Crystalline Ammonium Perchlorate. RSC Adv. 2016, 6, 48489-48497. (33) Banerji, A.; Deb, S. K. Raman Scattering Study of High-Pressure Phase Transition in Thiourea. J. Phys. Chem. B 2007, 111, 10915-10919. (34) Ciezak, J. A.; Jenkins, T. A.; Liu, Z.; Hemley, R. J. High-Pressure Vibrational Spectroscopy of Energetic Materials: Hexahydro-1,3,5-trinitro-1,3,5-triazine. J. Phys. Chem. A 2006, 111, 59-63. (35) Boldyreva, E. High-Pressure Diffraction Studies of Molecular Organic Solids. A Personal View. Acta Crystallogr., Sect. A 2008, 64, 218-231. (36) Rao, R.; Sakuntala, T.; Godwal, B. K. Evidence for High-pressure Polymorphism in Resorcinol. Phys. Rev. B 2002 , 65, 054108. (37) Ingo, O.; Olga, F.; Burkhard, S. High Pressure Structural Investigations of 2, 5-di(4-pyridyl)-1,3,4-oxadiazole-Importance of Strain Studies for The Description of Intermolecular Interactions. J. Phys.:Condens. Matter 2006, 18, 5269-5278. (38) Dreger, Z. A.; Gupta, Y. M. High Pressure Raman Spectroscopy of Single Crystals of Hexahydro-1, 3, 5-trinitro-1, 3, 5-triazine (RDX). J. Phys. Chem. B 2007, 111, 3893-3903. (39) Hamann, S. D.; Linton, M. The Influence of Pressure on The Infrared Spectra of Hydrogen-Bonded Solids. IV. Miscellaneous Compounds. Aust. J. Chem. 1976, 29, 1825-1827. (40) Li, Q.; Li, S.; Wang, K.; Li, X.; Liu, J.; Liu, B.; Zou, G.; Zou, B. Pressure-Induced Isosymmetric Phase Transition in Sulfamic Acid: A Combined Raman and x-ray Diffraction Study. J. Chem. Phys. 2013, 138, 214505. (41) Zhao, X.; Zhong, G.; Zhang, J.; Huang, Q.; Goncharov, A. F.; Lin, H.; Chen, X.; Combined Experimental and Computational Study of High-pressure Behavior of Triphenylene. Sci Rep. 2016, 6, 25600.



(42) Tumanov, N. A.; Boldyreva, E. V.; Kolesov, B. A.; Kurnosov, A. V.; Quesada, C. R. Pressure-Induced Phase Transitions in L-alanine, Revisited. Acta Crystallogr., Sect. B 2010, 66, 458-471. (43) Tumanov, N. A.; Boldyreva, E. V. X-ray Diffraction and Raman Study of Dl-alanine at High Pressure: Revision of Phase Transitions. Acta Crystallogr., Sect. B 2012, 68, 412-423.


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Figures Figure 1. Crystal structure of AP at room temperature and atmospheric pressure. (a) the unit cell; (b) the hydrogen-bonding network. The hydrogen bonds are marked as dashed lines.



Figure 2. Representative XRD patterns of AP at selected pressures. The numbers above the first pattern are Miller indexes (h k l). The new peaks are marked by asterisks. The up- and down-facing arrows indicate the changes of intensities of the peaks upon compression, respectively.


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Figure 3. Variation of the d-spacing of diffraction peaks at selected pressures, the two dashed lines indicate the pressure range of phase transition.



Figure 4. (a) Variations of lattice constants (a, b, and c) of the ambient phase as a function of pressure. (b) Pressure versus unit cell volume (V) of the ambient phase.


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Figure 5. Pawley refinement of the diffraction pattern at 11.5 GPa. The blue line shows the difference between the observed (purple) and the simulated (orange) profiles.



Figure 6. Raman spectra of AP crystal at selected pressures: (a) 40−240 cm-1; (b) 400−1300 cm-1; (c) 3100−3600 cm-1. The new peaks are marked by asterisks.


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Figure 7. Peak positions of AP under high pressure. (a) 40-220 cm-1, (b) 450-700 cm-1, (c) 900-1250 cm-1, (d) 3200-3400 cm-1. the two dashed lines indicate the pressure range of phase transition.



TOC Graphic


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TOC Graphic 42x21mm (300 x 300 DPI)



Figure 1. Crystal structure of AP with space group Pnma at ambient conditions. (a) the unit cell; (b) the hydrogen-bonding networks. The hydrogen bonds are marked as dashed lines. 58x20mm (300 x 300 DPI)


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Figure 2. Representative synchrotron XRD patterns of AP at high pressures. The numbers above the first pattern are Miller indexes (h k l) of the diffraction peaks of the ambient phase. The new peaks are marked by asterisks. The up- and down-facing arrows indicate the increasing and decreasing intensities of the peaks with increasing pressure, respectively. 101x130mm (300 x 300 DPI)



Figure 3. Variation of the d-spacing of main peaks at high pressures, the two dashed lines indicate the pressure range of phase transition. 99x119mm (300 x 300 DPI)


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Figure 4. (a) Variations of lattice parameters (a, b, and c) of the ambient phase as a function of pressure. The curves serve as guides to the eyes. (b) Pressure dependence of unit cell volume (V) of the ambient phase, the solid line is the fitting data by third-order Birch−Murnaghan equation of state. 101x62mm (300 x 300 DPI)



Figure 5. Pawley refinement of the diffraction pattern collected at 11.5 GPa. The blue line shows the difference between the observed (purple) and the simulated (orange) profiles. 50x34mm (300 x 300 DPI)


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Figure 6. Raman spectra of AP crystal at selected pressures. For clarity the spectra have been divided into three parts: (a) 40−240 cm-1; (b) 400−1300 cm-1; (c) 3100−3600 cm-1. The new peaks are marked by asterisks. 97x53mm (300 x 300 DPI)



Figure 7. Pressure dependence of Raman modes of AP. (a) 40-220 cm-1, (b) 450-700 cm-1, (c) 900-1250 cm-1, (d) 3200-3400 cm-1. the two dashed lines indicate the pressure range of phase transition. 202x245mm (300 x 300 DPI)


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Combined Raman Spectroscopy and X-ray Diffraction

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