High-Pressure Raman and Infrared Spectroscopic Studies of Cesium

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High-Pressure Raman and Infrared Spectroscopic Studies of Cesium Azide Dongmei Li, Peifen Zhu, Junru Jiang, Miaoran Li, Yanmei Chen, Bingbing Liu, Xiao-li Wang, Qiliang Cui, and Hongyang Zhu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09811 • Publication Date (Web): 10 Nov 2016 Downloaded from http://pubs.acs.org on November 14, 2016

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The Journal of Physical Chemistry

High-Pressure Raman and Infrared Spectroscopic Studies of Cesium Azide Dongmei Lia, Peifen Zhub, Junru Jianga, Miaoran Lia, Yanmei Chena, Bingbing Liua, Xiaoli Wangc, Qiliang Cuia, Hongyang Zhua* a

State Key Laboratory of Superhard Materials, Jilin University, Changchun, Jilin 130012,

China b

Department of Physics and Engineering Physics, the University of Tulsa, OK74104, Tulsa, OK74104, USA

c

Institute of Condensed Matter Physics, Linyi University, Linyi, shandong 276005, China

∗

Corresponding author. Tel: +8643185168881; fax: +8643185168881.

E-mail address: [email protected]

1

ACS Paragon Plus Environment


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Abstract In this work, we present the effects of high pressure on the structure and stability of cesium azide (CsN3) with the pressure up to ~ 30.0 GPa, as studied by Raman and IR spectroscopy. Three phase transitions of Phase II → III→ IV → V were revealed at ~0.5, ~3.7, and ~16.0 GPa. The abnormal softening behavior of T(Eg) mode reveals the shearing distortion during Phase II → III transition. Moreover, changes of lattice modes and splitting of the degenerate T(Eg) and R(Eg) modes in Phase III indicate the breaking of crystallographically equivalent condition of azide ions. Phase IV was found to possess the C2/m structure, and Phase V has a lower symmetry structure than other phases. The IR measurements shown the evolution of the N=N=N bending modes and the IR-active behavior of the symmetric stretch

ν 1 mode

under pressure, which collectively reveal the rotation and bending of the azide ions upon compression. The azide ions groups were found to further bend under pressure, and the bent azide ions might enhance propensity of nitrogen polymerization.

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Introduction The development of inorganic azides have gained wide attention in the past several decades due to their practical applications, including nitrogen sources, photographic materials, and initial explosives.1-3 Recently, a new perspective was opened towards high-pressure studies of inorganic azides due to their promising applications in forming the polymeric nitrogen, a new generation of high energy density material.

4-6

The bond energy

of N-N, N=N, and N≡N is 160, 418, and 954 kJ/mole, respectively.7 Therefore, it is expected that the azides with N=N bond might form a polymeric nitrogen network more readily than N2 molecules with N≡N bond due to the lower bond energy of N=N. The pioneering work by Eremets et al. presented the successful transformation from azide ions of NaN3 into polymeric nitrogen form at pressure above 120 GPa.4 Alkali azides are ideal candidates for forming polymeric nitrogen due to their comparably stablilities

in

solid

state

under

ambient

conditions.

Moreover,

the

‘chemical

pre-compression’ caused by metal elements can lower the synthesis pressure of polymeric nitrogen.8-11 Therefore, the high-pressure studies about these compounds are significant for the synthesis of the energetic polymeric nitrogen. Additionally, these materials should be environmentally clean because their final product of the transformation is nitrogen. Consequently, we conducted a series of high-pressure experimental studies about alkali azides as NaN3,12 KN3,13,14 RbN3,15,16 and CsN3,17 and pressure-induced phase transitions have been revealed in these substances. Furthermore, lots of theoretical investigations about alkali azides were also carried out,18-23 and the azide ions were found to transform into larger nitrogen cluster then into polymeric nitrogen nets at higher pressure.

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At ambient conditions, CsN3 has a body-centered tetragonal lattice with space group of I4/mcm and lattice parameters of a = 6.5412 Å and c = 8.0908 Å, as shown in Figure 1a (Phase II), which is isostructural with RbN3.24 As shown in Figure 1b, Phase II has a layered structure in the [001] direction with alternating planes of azide (N3−) and Cs+ ions. The linear symmetrical azide ions groups perpendicular to their adjacent ones within each plane, as illustrated in Figure 1c. Our previous high-pressure X-ray diffraction study revealed the phase transition sequence of Phase II → III→ IV → V in CsN3 within pressure of 55.4 GPa.17 However, limited information about N atom were revealed due to the minimal contribution to X-ray diffraction. Moreover, radiation damage from X-ray illumination may affect the X-ray diffraction results. Therefore, the spectroscopic studies are beneficial for investigating the evolution of azide ions and the phase transitional mechanism of this compound. Recently, Medvedev et al. presented the high-pressure Raman spectroscopic study of CsN3 with pressures up to 30 GPa,25 and the orientational disordering and ordering behaviors of azide ions were revealed. Additionally, CsN3 was found to has a relatively low polymerization pressure compared with LiN3, NaN3, and KN3.26 Although amount of experimental and theoretical studies about CsN3 have been carried out, several problems about CsN3 under pressure are still unsolved: (1) A controversy was currently existed about the structure of phase IV between recent Raman and X-ray diffraction study.17, 25 (2) The pressure-induced evolution of T(Eg) mode has been found to trigger structural phase transitions in the tetragonal KN3, RbN3, and TlN3,14,16,27 however, it was absent in recent high-pressure study of CsN3 due to the limitation of measuring range.25 (3) The high-pressure behaviors of N=N=N bending vibrations of CsN3 have not yet been conducted so far, which have been

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found to play a significant role for exploring the evolution of azide ions under pressure.16,28

Figure 1. Crystal structure of CsN3 along (a) a – b – c, (b) b – c, and (c) a – b axes. Therefore, the significance and absence of high-pressure spectroscopic study of CsN3 prompted our endeavor to present more in-depth investigates to explore its structural behavior, which may bring in new structures and properties for the highly energetic polymeric nitrogen. In this work, we present the high-pressure studies of CsN3 using Raman and IR method with all the three aspects are discussed. Experimental Section Raman measurements. The CsN3 powder with a purity of 99.99% was purchased from Sigma-Aldrich (USA). The symmetric diamond anvil cells (DACs) with type I diamonds (500 µm in diameter) were used in the Raman measurements. A sample compartment (diameter: 150 µm, thickness: 60 µm) preindented by the T301 steel gasket was drilled to load the sample. The mixture of methanol and ethanol was used as a pressure transmitting medium to provided the hydrostatic conditions. A ruby sphere (diameter: ~10µm) was loaded into the compartment to determine the pressures by using the nonlinear shift of the wavelength of the R1 line. An argon ion laser (532 nm) with the output power of 0.3 W was selected as the exciting source of the Raman spectra. All Raman spectra were collected using a backscattering configuration with the liquid nitrogen cooled CCD camera, equipped with a 5



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diffraction grating of an 1800-groove mm-1. The acquisition time of each Raman spectrum was 60s with the spectra ranged from 25 to1600 cm-1. IR measurements. The DACs with low fluorescence type II diamonds (400 µm in diameter) were used in the IR measurements. A sample compartment (diameter: 100 µm, thickness: 55 µm) preindented by the T301 steel gasket was drilled to load the sample. The powdery KBr was chosen to ensure quasi hydrostatic pressure environment. A ruby sphere (diameter: ~10µm) was placed in the sample compartment as the pressure sensor. The high-pressure IR spectra were recorded using a liquid nitrogen-cooled CCD camera with a spectral resolution set at 2 cm-1 of the Bruker Vertex80 V FTIR spectrometer. The excitation wavelength was 514 nm and the acquisition time for each spectrum was 520 s. The spectra ranged from 500 to 4000 cm-1. Results and Discussion A. The Raman Measurements at Ambient and High Pressure The group theoretical analyses indicate five fundamental vibrational modes are Raman 25, 29 active for the space group I4/mcm (D 18 Three lattice modes: a translational Eg 4h ) of CsN3.

mode corresponding to Cs+ translations parallel to the (001) plane and two librational Eg and B1g modes corresponding to the hindered rotational motions of the azide ions. Two internal modes: A1g and B2g modes associated with the symmetric stretch modes (ν 1) of the azide ions. The ambient Raman spectra of CsN3 were collected as shown in Figure 2. Three lattice modes T(Eg) (R1), R(Eg) (R2), and R(B1g) (R3) were observed at 42, 109, and 151 cm-1 and two internal modes

ν 1(A1g) and ν 1(B2g) were detected at 1328 and 1338 cm-1, in agreement

with previous study.29 Additionally, two overtone modes of the

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ν 2, 2ν 2(A1g) and 2ν 2(B2g),

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were observed at 1243 and 1256 cm-1, and the CR1 mode at 1346 cm-1 was assigned to the combinational mode of

ν 1(B2g)+T'(Eu).29 Notably, the translation T(Eg) mode was resolved

in our work, and we will discuss its abnormal high-pressure behavior in next section.

Figure 2. Representative Raman spectra of CsN3 at selected pressures in the region of (a) 20-497 cm-1 (b)1200-1325 cm-1 (c)1280-1470 cm-1. All the spectra in (b) were at a magnification of 20 times. This solid diamonds(◆) denote the pressure-induced new modes, and the dashed red lines serve as visual guides for evolution of all modes. The high-pressure Raman measurements were represent at room temperature with the pressure up to ~ 30.0 GPa. The selected spectra were plotted in Figure 2a, 2b, and 2c, corresponding to the lattice modes region, overtone modes of the

ν 2 region, and N=N=N

symmetric stretching modes region, respectively. Upon compression, the Phase II was stable up to 0.48 GPa with all observed modes increasing monotonically. At 0.48 GPa, three new lattices modes (labeled as Ri, i=4, 5, 6,) (Figure 2a), two new overtone modes (labeled as 2ν 2A and 2ν 2B) (Figure 2b), and one combinational mode (labeled as CR2) (Figure 2c) were observed simultaneously. These obvious changes collectively suggest the Phase II to III 7



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transition, in agreement with the recent high-pressure X-ray diffraction and Raman studies.17,25 With increasing pressure, a series of new lattice modes (labeled from R9 to R17) emerged at 3.78 GPa, whereas, all the characteristic modes of Phase III disappeared. Additionally, three new overtone modes (labeled as 2ν 2C, 2ν 2D, and 2ν 2E) started to appear and the 2ν 2(A1g) and 2ν 2B modes disappeared at this pressure. The R9 and R12 modes were maintained in small pressure range then disappeared at 4.55 GPa. All the mentioned phenomena consistently suggest the Phase III to IV transition starts at 3.78 GPa and completes at 4.55 GPa. Upon subsequent compression, the spectral profiles of Phase IV maintain stable up to 15.8 GPa. In the 15.8-21.7 GPa range, the lattice modes labeled from R18 to R22 appeared successively, meanwhile the symmetric stretch mode (ν 1) of the azide ions split into a double set, indicating the occurrence of the Phase IV to V transition. With further compression, Phase V was stable up to the highest pressure (30.0 GPa) of this work where all modes were extremely weak and broad. All the pressure dependences of the observed Raman modes are presented in Figure 3a and 3b to provide important insight into the phase transitional mechanisms. Intriguingly, the T(Eg) mode presented softening behavior throughout the entire Phase II and III range, and its completion was accompanied by the onset of the phase III to IV transition as shown in Figure 3a. For KN3, RbN3, and TlN3, isostructural with CsN3 at ambient conditions, the softening of T(Eg) mode are all related to the shearing distortion of the tetragonal structure upon compression.14,16,27 Therefore, we infer that the Phase III still possesses the layered structure, similar to that of Phase II, and a shift of layers parallel to the (001) plane is triggered by compression. The distances of the neighbor Cs+ ions between the adjacent layers should be

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increased due to the shearing distortion of the tetragonal structure, which in turn, weaken the corresponding translational vibrations. Consequently, the T(Eg) mode was softening in whole process of Phase II and III. Of note, the displacive structural transitions from tetragonal to monoclinic structure in KN3, RbN3, and TlN3 are all accompanied by the pressure-dependent softening of T(Eg) mode.14,16,27 Furthermore, onsets of these phase transitions and the completions of the T(Eg)’s softening are simultaneous in these azides. For RbN3, the abnormal softening behavior of T(Eg) triggered a displacive I4/mcm to C2/m transition under pressure.16 Moreover, recent experimental study of CsN3 by Medvedev et al. also suggested that the Phase IV might be related to the C2/m structure.25 Therefore, we speculate that the high-presure Phase IV of CsN3 probably possess the C2/m symmetry of monoclinic structure. Additionally, the C2/m structure has been found to be stable at pressures above 6.0 GPa in previous theoretical work.26

Figure 3. Pressure-induced Raman shifts of the (a) lattices modes and (b) internal modes. The dotted line and the shadow represent boundaries of different phases. In Phase III, the T(Eg) and R(Eg) (R1and R2) mode gradually split into a double set (new 9



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modes are R7 and R8) as shown in Figure 2a and Figure 3a, indicating that they evolved into nondegenerate modes upon compression. From this perspective, the crystallographically equivalent condition of azide ions were broken, which in turn, results in a change of the populations of the lattice modes and a splitting of the degenerate Eg modes. Furthermore, these phenomena may attributed to the partial orientational disordering of azide ions in this phase.25 In Phase V, all the observed modes shown multiple splitting, corresponding to a lower symmetry structure that predicted in previous study.17,25 Another remarkable change is the obvious splitting of the internal

ν 1(A1g) and ν 1(B2g) modes, convincing that two sets of

crystallographically nonequivalent azide ions sites were existed in Phase V.17,25 Additionally, the complicated evolution of the 2ν 2 modes at 0.48 and 3.78 GPa may stem from the changes of

ν 2 modes at these corresponding pressure points, which is further discussed in

next section. B. The IR Measurements at Ambient and High Pressure For the space group I4/mcm (D 18 4h ) of CsN3, the N=N=N bending modes and the N=N=N asymmetric stretch

ν 2(A2u) and ν 2(Eu)

ν 3(Eu) mode are active in mid-IR range.29 The

ambient mid-IR spectra of CsN3 were shown in Figure 4a and 4b. The

ν 2(Eu) modes was

observed at 639 cm-1, whereas the two modes at 618 and 648 cm-1 were assigned to the combinational modes of 2 ν 2(B2g)- ν 2(Eu) and 2 ν 2(A1g)''- ν 2(Eu).29 However, the ν 3(Eu) mode was not observed due to it was screened by the multiphonon absorption of diamonds. The series of modes at 3200, 3222, 3246, 3260, 3271, 3285, 3318, 3331, and 3356 cm-1 were assigned to combinational modes of 2ν 2+ν 3 and

ν 1+ν 2, and they were labeled with CI1-CI9

in sequence in this work with their detail assignments are summarized in Ref 29.29

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Figure 4. Representative IR spectra of CsN3 at selected pressures in the region of (a) 600-1500 cm-1 (b) 3170-3650 cm-1. The solid diamonds(◆) denote the pressure-induced new modes, and the dashed lines serve as visual guides for evolution of all modes. Figure 4a and 4b present the selected high-pressure IR spectra of CsN3 with the pressure up to ~ 30.0GPa at room temperature. Additionally, all the pressure dependences of the observed modes are plotted in Figure 5a and 5b for further investigation of the phase transition mechanisms. Phase II is stable up to 0.55GPa, and the original spectral profiles were maintained with all frequencies of the observed modes increased monotonically. At 0.55 GPa, a new mode labeled as

ν 2A (Figure 4a) and a series of combinational modes labeled

from CI10 to CI16 (Figure 4b) emerged simultaneously, indicating the Phase II to III transition. As Bryant reported that the

ν 2(A2u) mode is extremely weak at ambient conditions, however,

it is observable in thin crystal spectra measured at 77 K.29 Therefore, it is reasonable to

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assigned the

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ν 2A mode to ν 2(A2u) considering the effect of pressure and the sample was

thin upon compression. Moreover, the existence of

ν 2(A2u) mode might further reinforces

the layered structure of Phase III due to it related to the N=N=N bending vibrations parallel the c axis of the crystal. Further compression results in significant changes of spectral profiles in pressure range of 3.58-4.41 GPa. As shown in Figure 4a, the appearance of

ν 2D,

and

ν 2E

modes accompanied by the disappearance of

ν 2B, ν 2C,

ν 2(Eu), ν 2A,

and

2ν 2(B2g)-ν 2(Eu) modes were observed in N=N=N bending region. Additionally, the merging of the CI17, CI18, and CI19 modes and the disappearance of the CI12, CI13, and CI16 modes were also presented (Figure 4b and Figure 5b). These phenomena are attributed to the Phase III to IV transition, and this two phases were coexistent in 3.58-4.41 GPa range. The Phase IV was stable up to 16.7 GPa, as shown in Figure 5a and 5b. With increasing pressure, the Phase IV to V transition was observed to start at 16.7 GPa and complete at 21.3 GPa as characterized by the changes of of all observed modes’ pressure dependence (Figure 5a and 5b). Upon subsequent compression, the Phase V was stable up to the highest pressure of this work.

Figure 5. Pressure-induced IR shifts in the region of (a) 600-1500 cm-1 and (b) 3170-3650 cm-1. The dotted line and the shadow represent boundaries of different phases. 12


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Interestingly, at 11.7 GPa, a new mode ( labeled as

ν sy ) in the fundamental ν 1 mode

region was observed at 1371 cm-1, which corresponds to the Raman active fundamental

ν 1(A1g) mode locates at 1374 cm-1 at 11.4 GPa. Therefore, we assigned this mode to N=N=N symmetric stretch mode. The fundamental

ν 1 symmetric stretch vibrations are IR inactive in

azides with linear symmetric azide ions,14,16,30,31 while they become IR active for these azides possess nonlinear or/and asymmetric azide ions.32-35 Therefore, we infer that the azide ions become nonlinear and/or asymmetric starting from 11.7 GPa. This phenomenon was also found in IR study of RbN3.16 Curiously, the evolution of pressure dependence of

ν 2C andν 2D

modes are symmetric

about their center symmetrical line in Phase IV and V, as shown in Figure 5a. The

ν 2C mode

present the evolution from softening to hardening upon compression, while the hardening character of

ν 2D mode is maintained throughout the Phase IV and V process. This similar

symmetrical phenomenon was also observed in high-pressure studies of RbN3. Additionally, in tetragonal phase of AgN3, the

ν 2(B2u)

and

ν 2(B3u)

bending modes also present the

similar symmetrical evolution upon compression. The symmetrical phenomena of the these bending modes have been reasonably interpreted by the rotation of azide ions in RbN3 and AgN3.16,28 In this perspective, the symmetrical behavior of

ν 2C and ν 2D modes probably

attributed to the rotation of azide ions groups upon compression. For the adjacent azide ions, the interatomic distances between the central N atom of one azide ion and the termini N atom of the other azide ion changed due to the rotation of azide ions. Therefore, their corresponding vibrational force constants were increased/decreased and, consequently, lead to the hardening/softening of the

ν 2C and ν 2D modes.16,28 Another interesting phenomenon 13



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is that the pressure dependence of the N=N=N symmetric stretch

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ν 2C and ν 2D were changed starting from 11.7 GPa, and

ν sy mode emerged simultaneously. The interrelation of the

two phenomena can be interpreted as following: the azide ions groups became bent at 11.7 GPa due to the effect of compression, which results in the significantly increase or decrease of the interatomic distances between the central N atom and the termini N atom of the adjacent azide ions. Consequently, their corresponding vibrational force constants were changed, resulting in the weakening or strengthening of the of

ν 2C and ν 2D modes’ pressure

dependence as shown in Figure 5a. Moreover, the bent azide ions chain might readily approach each other tend to form N chains or rings when the nearest neighbor N atoms close enough at higher pressure. In this respect, the bent azide ions of CsN3 could greatly enhance the propensity of nitrogen polymerization and induce a low polymerization pressure as that found in recent studies.26,32 Additionally, more information regarding the effect of pressure on N=N=N angle might be obtained from the change in intensity of Figure 4a, the relative intensity of this IR-active

ν sy mode. As shown in

ν sy mode was continuously strengthened

with increasing pressure, implying that the azide ions further bend upon compression. This phenomenon was also predicted in recent study of Sr(N3)2.32 Conclusions In conclusion, we present the high-pressure Raman and IR measurements of CsN3 with the pressure up to ~ 30.0 GPa. All the fundamental vibrational modes at ambient conditions were assigned. Upon compression, three phase transitions of Phase II → III→ IV → V were revealed at ~0.5, ~3.7, and ~16.0 GPa, respectively. The softening character of T(Eg) mode was observed throughout the entire Phase II and III range, which reveals the shearing

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distortion of the crystal during this phase transition. In Phase III, the T(Eg) and R(Eg) mode split into a double set due to the breaking of the crystallographically equivalent of azide ions upon compression. The softening of the T(Eg) triggered the Phase III to IV transition, and Phase IV was found to possess the C2/m structure. Phase V has a lower symmetry structure with two sets of crystallographically nonequivalent azide ions sites, indicated by the splitting of the internal

ν1

symmetric stretch mode. The high-pressure IR measurements of CsN3

were present and all the phase transitions sequence are agreement with Raman results. With increasing pressure, the symmetrical evolution of character of symmetric stretch

ν1

ν 2C

andν 2D modes and the IR-active

mode were observed. These phenomena collectively

revealed the rotation and bending of the azide ions upon compression. Moreover, the bent azide ions might enhance the propensity of nitrogen polymerization, and the azide ions groups were found to further bend under pressure. Acknowledgments We thank Ran Liu for his technical support with the IR measurements. This work is supported by the National Natural Science Foundation of China (11304111, 11674144). Project 2016069 supported by Graduate Innovation Found of Jilin University. References 1. Tornieporth-Oetting, I. C.; Klapötke, T. M. Covalent Inorganic Azides. Angew. Chem. Int.Ed. Engl. 1995, 34, 511-520. 2. Evans, B. L.; Yoffe, A. D.; Gray, P. Physics and Chemistry of the Inorganic Azides. Chem. Rev. 1959, 59, 515-568. 3. Gray, P. Chemistry of the Inorganic Azides. Q.Rev., Chem. Soc. 1963, 17, 441-473.

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4. Eremets, M. I.; Popov, M. Y.; Trojan, I. A.; Denisov, V. N.; Boehler, R; Hemley, R. J. Polymerization of Nitrogen in Sodium Azide. J. Chem. Phys. 2004, 120, 10618-23. 5. Eremets, M. I.; Gavriliuk, A. G.; Trojan, I. A.; Dzivenko, D. A.; Boehler, R. Single-Bonded Cubic Form of Nitrogen. Nat. Mater. 2004, 3, 558-563. 6. Eremets, M. I.; Hemley, R. J.; Mao, H.-k.; Gregoryanz, E. Semiconducting Non-Molecular Nitrogen up to 240 Gpa and Its Low-Pressure Stability. Nature. 2001, 411, 170-174. 7. Talawar. M. B.; Sivabalan. R.; Asthana. S. N; Singh. H. Novel Ultrahigh-Energy Materials. Combust, Explos, Shock Waves. 2005, 41, 264-277. 8. Zhang, M.; Yin, K.; Zhang, X.; Wang, H.; Li, Q.; Wu, Z. Structural and Electronic Properties of Sodium Azide at High Pressure: A First Principles Study. Solid State Commun. 2013, 161, 13-18. 9. Wang, X.; Li, J.; Botana, J.; Zhang, M.; Zhu, H.; Chen, L.; Liu, H.; Cui, T.; Miao, M. Polymerization of Nitrogen in Lithium Azide. J. Chem. Phys. 2013, 139, 164710. 10. Medvedev, S. A.; Trojan, I. A.; Eremets, M. I.; Palasyuk, T.; Klapötke, T. M.; Evers, J. Phase Stability of Lithium Azide at Pressures up to 60 Gpa. J. Phys.: Condens. Matter. 2009, 21, 195404. 11. Li, J.; Wang, X.; Xu, N.; Li, D.; Wang, D.; Chen, L. Pressure-Induced Polymerization of Nitrogen in Potassium Azides. Europhys.Lett. 2013, 104, 16005. 12. Zhu, H.; Zhang, F.; Ji, C.; Hou, D.; Wu, J.; Hannon, T.; Ma, Y. Pressure-Induced Series of Phase Transitions in Sodium Azide. J. Appl. Phys. 2013, 113, 033511. 13. Ji, C.; Zhang, F.; Hou, D.; Zhu, H.; Wu, J.; Chyu, M.-C.; Levitas, V. I.; Ma, Y. High Pressure X-Ray Diffraction Study of Potassium Azide. J. Phys.Chem.Solids. 2011, 72,

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736-739. 14. Ji, C.; Zheng, R.; Hou, D.; Zhu, H.; Wu, J.; Chyu, M.-C.; Ma, Y. Pressure-Induced Phase Transition in Potassium Azide up to 55 Gpa. J. Appl. Phys. 2012, 111, 112613. 15. Li, D.; Wu, X.; Jiang, J.; Wang, X.; Zhang, J.; Cui, Q.; Zhu, H. Pressure-Induced Phase Transitions in Rubidium Azide: Studied by in-Situ X-Ray Diffraction. Appl. Phys. Lett. 2014, 105, 071903. 16. Li, D.; Li, F.; Li, Y.; Wu, X.; Fu, G.; Liu, Z.; Wang, X.; Cui, Q.; Zhu, H. High-Pressure Studies of Rubidium Azide by Raman and Infrared Spectroscopies. J. Phys. Chem. C. 2015, 119, 16870-16878. 17. Hou, D.; Zhang, F.; Ji, C.; Hannon, T.; Zhu, H.; Wu, J.; Ma, Y. Series of Phase Transitions in Cesium Azide under High Pressure Studied by in Situ x-Ray Diffraction. Phys. Rev. B: Condens. Matter Matwe. Phys. 2011, 84, 064127. 18. Zhang, M.; Yan, H.; Wei, Q.; Liu, H. A New High-Pressure Polymeric Nitrogen Phase in Potassium azide. RSC Adv. 2015, 5, 11825-11830. 19. Zhang, M.; Yan, H.; Wei, Q.; Wang, H.; Wu, Z. Novel High-Pressure Phase with Pseudo-Benzene “N6” Molecule of LiN3. Europhys.Lett. 2013, 101, 26004. 20. Zhu, W.; Xiao, H. Ab Initio Molecular Dynamics Study of Temperature Effects on the Structure and Stability of Energetic Solid Silver Azide. J. Phys. Chem. C. 2011, 115, 20782-20787. 21. Zhu, W.; Xiao, J.; Xiao, H. Comparative First-Principles Study of Structural and Optical Properties of Alkali Metal Azides. J. Phys. Chem. B. 2006, 110, 9856-9862. 22. Babu, K. R.; Lingam C. B.; Tewari, S. P.; Vaitheeswaran, G. High-Pressure Study of

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Lithium Azide from Density-Functional Calculations. J. Phys. Chem. A. 2011, 115, 4521-4529. 23. Vaitheeswaran, G.; Babu, R. K. Metal Azides Under Pressure: An Emerging Class of High Energy. J. Chem. Sci. 2012, 124, 1391–1398. 24. Müller. V. U. Verfeinerung Der Kristallstrukturen Von KN3, RbN3, CsN3, Und TiN3 Z. Anorg. Allg. Chem. 1972, 392, 159-166. 25. Medvedev, S. A.; Barkalov, O. I.; Naumov, P.; Palasyuk, T.; Evers, J.; Klapötke, T. M.; Felser, C. Phase Transitions of Cesium Azide at Pressures up to 30 Gpa Studied Using in Situ Raman Spectroscopy. J. Appl. Phys. 2015, 117, 165901. 26. Wang, X.; Li, J.; Zhu, H.; Chen, L.; Lin, H. Polymerization of Nitrogen in Cesium Azide under Modest Pressure. J. Chem. Phys. 2014, 141, 044717. 27. Christoe, C. W.; Iqbal, Z. Mode Coupling and the Pressure Induced Phase Transition in Thallium Azide (TiN3). Solid State Commun. 1974, 15, 859-862. 28. Li, D.; Zhu, P.; Wang, Y.; Liu, B.; Jiang, J.; Huang, X.; Wang, X.; Zhu, H.; Cui, Q. High-Pressure Spectroscopic Study of Silver Azide. RSC Adv. 2016, 6, 82270-82276. 29. Bryant, J. I. Vibrational Spectrum of Cesium Azide Crystals. J. Chem. Phys. 1966, 45, 689-699. 30. Bryant, J. I. Vibrational Spectrum of Sodium Azide Single Crystals. J.Chem. Phys. 1964, 40, 3195-3203. 31. Hathaway, C. E.; Temple, P. A. Raman Spectra of the Alkali Azides: KN3, RbN3, CsN3. Phys. Rev. B: Condens. Matter Matwe.Phys. 1971, 3, 3497-3503. 32. Zhu, H.; Han, X.; Zhu, P.; Wu, X.; Chen, Y.; Li, M.; Li, X.; Cui, Q. Pressure-Induced

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Amorphization of Strontium Azide. J. Phys. Chem. C. 2016, 120, 12423-12428. 33. Reckeweg, O.; Simon, A. Azide Und Cyanamide – äHnlich Und Doch Anders. Z. Naturforsch. B. 2003, 58, 1097-1104. 34. Iqbal, Z.; Brown, C. W.; Mitra, S. S. Vibrational Spectrum of Barium Azide Single Crystals. J.Chem.Phys. 1970, 52, 4867-4874. 35. Iqbal, Z.; Garrett.W; Brown, C. W.; Mitra, S. S. Infrared and Raman Spectra of Single-Crystal α-Lead Azide. J. Chem. Phys. 1971, 55, 4528-4535.

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Figure 1. Crystal structure of CsN3 along (a) a – b – c, (b) b – c, and (c) a – b axes. 55x18mm (300 x 300 DPI)



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Figure 2. Representative Raman spectra of CsN3 at selected pressures in the region of (a) 20-497 cm-1 (b)1200-1325 cm-1 (c)1280-1470 cm-1. All the spectra in (b) were at a magnification of 20 times. This solid diamonds(◆) denote the pressure-induced new modes, and the dashed red lines serve as visual guides for evolution of all modes. 85x42mm (300 x 300 DPI)


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Figure 3. Pressure-induced Raman shifts of the (a) lattices modes and (b) internal modes. The dotted line and the shadow represent boundaries of different phases. 91x46mm (300 x 300 DPI)



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Figure 4. Representative IR spectra of CsN3 at selected pressures in the region of (a) 600-1500 cm-1 (b) 3170-3650 cm-1. The solid diamonds(◆) denote the pressure-induced new modes, and the dashed lines serve as visual guides for evolution of all modes. 116x79mm (300 x 300 DPI)


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Figure 5. Pressure-induced IR shifts in the region of (a) 600-1500 cm-1 and (b) 3170-3650 cm-1. The dotted line and the shadow represent boundaries of different phases. 69x28mm (300 x 300 DPI)



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High-Pressure Raman and Infrared Spectroscopic Studies of Cesium

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