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High-Pressure Studies of Azide by Raman and Infrared Spectroscopies † † ‡ † † § # Dongmei Li, Fangfei Li, Yan Li, Xiaoxin Wu, Guangyan Fu, Zhenxian Liu, Xiaoli Wang, † † Qiliang Cui, and Hongyang Zhu*, † State Key Laboratory of Superhard Materials, College of Materials Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun, Jilin 130012, China ‡ College of Physics, Jilin University, Changchun, Jilin 130012, China § Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington, DC 20015, United States # Institute of Condensed Matter Physics, Linyi University, Linyi 276005, China

ABSTRACT: We report the high-pressure studies of RbN3 by Raman and IR spectral measurements at room temperature with the pressure up to 28.5 and 30.2 GPa, respectively. All the fundamental vibrational modes were resolved by combination of experiment and calculation. Detailed spectroscopic analyses reveal two phase transitions at ∼6.5 and ∼16.0 GPa, respectively. Upon compression, the shearing distortion of the unit cell induced the displacive structural transition of phase α → γ. Further analyses of the mid-IR spectra − → indicate the evolution of N3 with the arrangement sequence of orthogonal parallel → orthogonal during the phase transition of phase α → γ → δ. Additionally, the pressure-induced nonlinear/asymmetric existence of N  − N N and the two crystallographically nonequivalent sites of N3 were observed in phase δ.

■ INTRODUCTION the mechanism of pressure-induced phase transitions as well as Inorganic azides have attracted considerable attention during the evolution of azide which might result in the formation the past several decades due to their peculiar structures and of polymeric . − 1 3 Under ambient conditions, RbN3 crystallizes into a body- physicochemical properties. Their important applications as 18 initial , as gas generators, and even as photographic centered tetragonal structure with space group D4h-I4/mcm and cell parameters of a = b = 6.311 62(70) Å, c = 7.540 38(98) materials at low temperature have been used extensively in − − 20 + 3 5 Å, as shown in Figure 1a. The linear symmetrical N3 and Rb industry and military. Additionally, the unique structures, as − − a result of linear-rod-shaped azide ions (N ), make them form alternating layers in the [001] direction with the N3 3 groups inclined at 90° to one another within each plane as logical candidates for the study of the complex nature of ’ chemical bonding and internal molecular structure beyond illustrated in Figure 1b,c. According to Mueller and Joebstl s − 4 7 studies, RbN3 transforms into the cubic structure from the alkali halides and cyanides. It is hence instructive to study − tetragonal structure with N3 anions oriented at random, the inorganic azides intensively and extensively to provide more 21 of a fundamental basis for their industrial applications and for parallel to the edges of the cubic unit cell upon heating. As scientific research. Recently, the studies of alkali azides have the temperature dropped to 82 K, no phase transition was observed in the low-temperature measurement of RbN3 studied opened a new perspective as a distinctive precursor in the 22 formation of polymeric nitrogen, the ultimate example of a by Hathaway and Temple. Our recent high-pressure X-ray ff high-energy-density material (HEDM), due to the lower di raction (XRD) study of RbN3 revealed the pressure-induced  − phase transitions of tetragonal → monoclinic → orthorhom- bonding energy of double bonds N N (418 kJ/mol) in N3  8 bic.20 However, details about azide ions are limited due to the compared to the triple bonds N N (954 kJ/mol) in N2. The N − ions have been found to transform into larger nitrogen minimal contribution of the N atom to the XRD. Therefore, the 3 fi vibrational studies of RbN3 are bene cial to explore the clusters then into polymeric nitrogen nets with application of − pressure, as the reported nonmolecular nitrogen state and evolution of the N3 in the process of phase transitions. The − high-pressure vibrational studies of RbN are restricted to the zigzag chains of N5 rings have been formed in the high- 3 9,10 Raman scattering studies up to pressure of 4 GPa.23,24 It is pressure studies of NaN3 and LiN3. Moreover, the structural, electronic, and optical properties of alkali azides also present abundant changes as explored by experimental and theoretical Received: June 1, 2015 − high-pressure studies.11 19 A comparison of the high-pressure Revised: June 29, 2015 behaviors of these substances would enable an understanding of Published: July 1, 2015

© 2015 American Chemical Society 16870 DOI: 10.1021/acs.jpcc.5b05208 J. Phys. Chem. C 2015, 119, 16870−16878 The Journal of Physical Chemistry C Article

− − − − Figure 1. Crystal structure of RbN3 at ambient conditions along (a) a b c, (b) a b, and (c) b c axes. Blue color represents N atoms, and red color represents Rb atoms.

Figure 2. Experimental (exp.) and calculated (cal.) (a) Raman and (b) IR spectra of RbN3 at ambient pressure. The omitted spectral regions are due to the lack of spectroscopic features. All the assignments of the vibrational modes are labeled above each band. The blue vertical bars label the mode positions (pos.) from the calculations. The black vertical bars label the scale of the absolute IR absorbance intensity. The lines marked with ×1500, × 50, and ×20 indicate that the spectra were at a magnification of 1500, 50, and 20 times. therefore rather significant to investigate the high-pressure vibrational frequencies in Raman and IR spectroscopy without vibrational spectroscopic behaviors of RbN3 by an optical interference. More importantly, the phase transition sequence spectroscopic method. of RbN3 is revealed at the vibrational spectrum level which is In this work, we represent the high-pressure Raman and IR not investigated so far. The detailed spectroscopic analyses measurements of RbN3 at room temperature with diamond based on combined Raman and IR activities of the character- anvil cells (DACs) up to 28.5 and 30.2 GPa, respectively. One istic modes of RbN3 allowed for a more in-depth understanding of the primary objectives is to resolve all of the fundamental of the structure and stability of RbN3.

16871 DOI: 10.1021/acs.jpcc.5b05208 J. Phys. Chem. C 2015, 119, 16870−16878 The Journal of Physical Chemistry C Article

Figure 3. (a−k) Simulated eigenvectors of all the vibrational modes in the primitive cell from the calculations. The blue and red spheres denote N and Rb atoms, respectively. The green arrows marked the vibrational directions of the atoms. (l) The relationship of the coordinate systems between the primitive cell (A−B−C) and the unit cell (a−b−c).

■ EXPERIMENTAL SECTION container, and the data were subsequently acquired. The − −1 The RbN with a purity of 99% was obtained commercially observed range of far-IR spectra was within 60 700 cm . For 3 the mid-IR experiment, KBr powder was used as the pressure from International Laboratory USA Co. The high-pressure transmitting medium. The loaded DAC was placed in the focal Raman experiments were performed in a symmetric DAC with region of the microscope objective lens, through which the culets of 400 μm in diameter. A T301 steel sheet served as the high-flux polychromatic IR beam passed. The range of the mid- gasket with a chamber of 120 μm in diameter and 56 μmin − IR spectra was within 600−4000 cm 1. The spectral resolution thickness for packing the sample. The mixture of methanol and − for all measurements was about 2 cm 1. with a volume ratio of 4:1 was employed as the In order to explore the vibrational spectrum of RbN and pressure transmitting medium. A ruby ball was used to 3 their vibration modes, we have performed the ab initio determine pressure by using the standard ruby fluorescent calculations with plane wave pseudopotential density functional technique. The measurements were performed using a solid- computer code Cambridge Serial Total Energy Package state, diode-pumped Nd:vanadate laser (Coherent Inc.) with 25 (CASTEP). The generalized gradient approximation (GGA) 532 nm wavelength as excitation source. A liquid nitrogen- using Perdew−Burke−Ernzerhof (PBE) parametrization was cooled CCD camera equipped on Acton SpectraPro 500i 26 −1 used to describe the exchange-correlation potential. In our spectrometer with a 1800-groove mm grating was used for ff calculations, convergence tests give the energy cuto Ecutoff as recording the Raman scattering spectra. 770 eV and the electronic Brillouin zone (BZ) integration with The high-pressure IR experiments were performed at the the K-points of 0.031/Å. The internal atomic positions and cell U2A beamline, which is a part of the vacuum ultraviolet (VUV) size of the system were fully relaxed. In the geometry relaxation, ring of the National Synchrotron Light Source (NSLS) at the the self-consistency convergence on the total energy was 5.0 × Brookhaven National Laboratory. The pressure was generated 10−6 eV/atom, and the maximum force on the atom was found by the symmetrical DAC with type II diamonds. The flat culets μ to be 0.01 eV/ Å. The vibrational frequencies of the optimized of the diamonds are 500 m in diameter. The T301 steel sheet structure were then calculated. served as the gasket with a chamber of 120 μm in diameter and 50 μm in thickness. A ruby ball was placed in the sample chamber as the pressure sensor. For the far-IR experiments, ■ RESULTS AND DISCUSSION petroleum jelly was served as the inert pressure transmitting A. Ambient-Pressure Raman and IR Spectra. Under medium. The loaded DAC was placed inside a nitrogen-purged ambient conditions, RbN3 crystallizes into the tetragonal

16872 DOI: 10.1021/acs.jpcc.5b05208 J. Phys. Chem. C 2015, 119, 16870−16878 The Journal of Physical Chemistry C Article

Table 1. Raman Frequencies, IR Frequencies, and Their Assignments of RbN3 Obtained from Calculations, References, and Our Experiments, Respectively

Raman frequencies (cm−1) IR frequencies (cm−1) expt calcd ref expt calcd ref assignment assignment 22 30 + 66 65 66 /65 T(Eg)Rbtranslation 30 − + 100 93 120 T(Eu)N3 and Rb translation 9631 30 − + 117 94 120 T(A2u)N3 and Rb translation 16432 30 ′ − 160 124 146 T (Eu)N3 translation 14431 22 30 − 128 120 128 /128 R(Eg)N3 rotation 22 30 − 140 125 140 /138 R(B1g)N3 rotation 30 ν   620 613 640 2(A2u)NN N bending 30 ν   645 616 644 2(Eu)NN N bending ff 1119 DI1 di erence frequency 30 ν   1251 1250 2 2(B2g) overtone of N N N bending 30 30 ν 1263 1262/1284 /1289 2 2(A1g) ν″ 1285 2 2(A1g) 27 1317 CR1 combination frequency 22 30 ν   1334 1209 1333 /1334 1(B2g)NN N symmetric stretch 22 30 ν   1346 1211 1333 /1334 1(A1g)NN N symmetric stretch 27,28 1353 CR2 combination frequency 30 ν   1867 2032 3(Eu)NN N asymmetric stretch ν ν ν ν 28 3249 CI1 combination frequencies of 2 2 + 3 and 1 + 3

3271 CI2

3293 CI3

3343 CI4

18 structure with space group I4/mcm (D4h) (Figure 1a) with two an ideal single unit cell, while the experimental result is a molecules per primitive cell. The group theoretical analyses collective contribution of the fine powder. For the lattice indicate 24 vibrational modes are associated with RbN3 with the vibrational modes T(Eg), R(Eg), and R(B1g)whichare following irreducible representation associated with the translation of the Rb+ parallel to the (001) plane with atoms in adjacent planes moving in opposite Γ=+acousticAE 2uu (1) − directions (Figure 3a), the hindered rocking of the N3 parallel − to the z axis (Figure 3e), and the hindered rotation of the N3 Γ=+optical42EAuuguggg 2 ++ 22 EB 2 + 2 ABB 2 ++ 2 1 perpendicular to the z axis (Figure 3f), were observed at 66, 128, and 140 cm−1 in the Raman spectra, respectively. The + A1g (2) ′ T(Eu), T(A2u), and T (Eu) modes, which are corresponding to + − where the A and B modes are nondegenerate, the E mode is the translation of the Rb and N3 in the opposite directions doubly degenerate, and the subscripts g and u stand for gerade perpendicular to the z axis (Figure 3b), the translation of the + − Rb and N3 in the opposite directions parallel to the z axis and ungerade, respectively. One A2u and one Eu correspond to − zero frequency acoustic modes, and the rest are optic modes. (Figure 3c), and the translation of the N3 parallel to z axis −1 All gerade modes are Raman-active and ungerade modes are (Figure 3d), were detected at 100, 117, and 160 cm in the far- IR-active, except for B and A , which are neither Raman- nor IR region. The NNN bending modes (Figure 3g,h), 1u 2g ν ν IR-active. The experimental and calculated Raman and IR 2(A2u) and 2(Eu), IR-active due to the linear symmetrical of − −1 spectra were collected as plotted in Figure 2a,b, respectively. N3 , were observed at 620 and 645 cm , while the overtone of   ν ν ν″ The calculated results show quite similar features to those of N N N bending modes, 2 2 (B2g), 2 2(A1g), and 2 2(A1g), Raman-active modes, were observed at 1251, 1263, and 1285 our experimental spectra, and all the vibrational modes within − the primitive cell, including all the degenerate modes, are cm 1, respectively. They have appreciable intensities due to the ν ν displayed intuitively in Figure 3a−k. Figure 3l shows the Fermi resonance interaction of 2 2 and 1 modes similar to 17,27 relationship of the coordinate systems between the primitive those of KN3 and CsN3. For the splitting of the crystal cell and the unit cell. On the basis of the research achievements correlation field, the NNN out-of-phase symmetric stretch ν of our predecessors and our calculations, all the Raman and IR mode 1(B2g) (Figure 3i) and in-phase symmetric stretch mode ν −1 modes along with their frequencies and assignments are 1(A1g) (Figure 3j) were observed at 1334 and 1346 cm , summarized in Table 1. Nevertheless, it should be pointed respectively. All these fundamental frequencies of vibrational out that all the peaks are shifted to low frequency in our modes in experiments and calculations are coincident with each   ν calculated results compared with experimental results. The other well. The N N N asymmetric stretch mode 3(Eu) − discrepancy can be primarily explained by the fact that the (Figure 3k) is located at 1867 cm 1 with a significant intensity ν present calculations concern T = 0 K rather than room in calculated results. However, the 3(Eu) mode was not temperature. Additionally, the certain differences may partly observed in the measured spectra as it locates at the result from the fact that the computations were carried out for multiphonon absorptional region (1850−2500 cm−1) of the

16873 DOI: 10.1021/acs.jpcc.5b05208 J. Phys. Chem. C 2015, 119, 16870−16878 The Journal of Physical Chemistry C Article

− −1 − Figure 4. Selected high-pressure Raman spectra of RbN3 in the region of (a) lattice modes (30 430 cm ) and (b) internal modes (1200 1500 cm−1). The dashed lines serve as visual guides. The mode guided by the green dashed line in part b is from the diamond anvils with D-0 standing for the pressure released to 0 GPa. Parts c and d are the corresponding Raman shifts of all modes as a function of pressure with the vertical dashed line indicating the proposed phase boundaries. diamond anvils. Moreover, according to the vibrational result of reduction of interionic distances. As the pressure spectrum studies of KN3 and CsN3 (isostructural with RbN3 increases up to 6.4 GPa, four new Raman modes (labeled from 27,28 at ambient conditions), the two weak bands at 1317 and LR1 to LR4 from low to high frequencies) in the lattice region 1353 cm−1 in the internal modes region of the Raman spectra (Figure 4a) were observed. Concurrently, the intensity of the ν fi are the combination frequencies of T(Eg)+2 2(B2g) and T(Eg) T(Eg)andR(Eg) modes signi cantly depleted. These ν″ fi α → γ +2 2(A1g), labeled as CR1 and CR2 in this work. Additionally, phenomena indicate that the rst phase transition ( ) the band at 1119 cm−1 in the mid-IR spectra is probably occurred at this pressure. As the pressure increased by a small ff ν − fi attributed to the di erence frequency of 2 2 and T, labeled as value, the intensity of LR1 LR4 modes was signi cantly 27−29 DI1 in this work. The bands at 3249, 3271, 3293, and 3343 enhanced and the intensity of T(Eg) and R(Eg) modes was −1 cm , labeled as CI1, CI2, CI3, and CI4, respectively, are assigned also completely depleted at 6.7 GPa suggesting the completion ν ν ν ν 27−29 fi to the combination frequency of 2 2 + 3 and 1+ 3. The of the rst phase transition at this pressure. The sizable shoulder observed at the high-frequency side of the mode softening of the T(Eg) modes has been predicted to trigger a 23 R(B1g) in the Raman spectrum may be related to the second- phase transition when the pressure is greater than 4 GPa, order scattering.22 which has been proven in this work. In addition, the softening B. Raman Spectra upon Compression. We collected the of the T(Eg) modes upon pressure is also observed in KN3 and 17,33 Raman spectra of RbN3 upon compression with selected TlN3, isostructural with RbN3 at ambient conditions. For spectra shown in Figure 4a,b corresponding to the lattice TlN3, pressure-induced softening of the T(Eg) mode is attribute modes region and internal modes region, respectively. Upon to the shearing distortion of the unit cell that subsequently compression, all the lattice modes except the translational results in the structural phase transition from tetragonal to 34 T(Eg) mode shift monotonically to higher frequencies as the monoclinic. Analogously, the same triggering mechanism was

16874 DOI: 10.1021/acs.jpcc.5b05208 J. Phys. Chem. C 2015, 119, 16870−16878 The Journal of Physical Chemistry C Article

− −1 − −1 − Figure 5. Selected high-pressure IR spectra of RbN3 in the region of far-IR (a) 100 390 cm and mid-IR (b) 600 1500 cm and (c) 3150 3700 cm−1. The dashed lines serve as visual guides. D-0 stands for the pressure released to 0 GPa. Parts d−f are the corresponding IR shifts of all modes as a function of pressure with the vertical dashed line indicating the proposed phase boundaries.

17 38,39 proposed for KN3. In tetragonal-type compounds the T(Eg) compounds, which is however beyond the scope of this 35 mode is related to the shear elastic constant C44. Therefore, work. fi the signi cant softening of the T(Eg) mode in RbN3 reinforces With further compression, an additional lattice mode fi α → γ (labeled as L ) started to burgeon at 11.9 GPa and that the rst phase transition ( ) of RbN3 was induced by R5 the shearing distortion of the unit cell. Moreover, the shearing continuously strengthen up to 16.1 GPa, whereas the LR2 and of layers were also presented in the pressure-induced phase LR3 modes gradually merged in this pressure region. All these γ → δ transition of LiN , NaN , and CsN .12,15,19 The distance features indicate the second phase transition ( ) starts at 3 3 3 11.9 GPa and completes at 16.1 GPa. Compared to the lattice between the neighboring Rb+ of the adjacent layers will be modes, the internal modes are relatively stable due to the increased during the process of the shearing distortions. In this greater strength of quasidouble than that of bonds between case, the softening of the T(Eg) mode was interpreted ions. As shown in Figure 4b, no prominent changes were reasonably. The softening of the T(Eg) mode (or the lowest- observed up to 14.8 GPa, except that the overtone vibrational frequency modes) accompanying phase transitions has been ν ν ν″ modes 2 2 (B2g), 2 2(A1g), and 2 2(A1g) gradually depleted also observed in the high-pressure studies of other layered due to the reduction of interionic distances with increasing 35−37 tetragonal-type compounds, which is probably a con- pressure. However, as the pressure increased up to 16.1 GPa,   ν ν sequence of the instability of the layered tetragonal-type the symmetric stretch modes of N N N 1(B2g) and 1(A1g) ν compounds upon compression. In addition, the softening- split into a doublet set with the new modes labeled as 1C and ν accompanied phase transitions have been observed in other 1D, convincing us of the existence of the two crystallo-

16875 DOI: 10.1021/acs.jpcc.5b05208 J. Phys. Chem. C 2015, 119, 16870−16878 The Journal of Physical Chemistry C Article

− Figure 6. Simulated diagram of the evolution of the N3 with increasing pressure. The green arrows indicate the vibrational direction of the atom. − − α γ δ Parts a, b, and c correspond to the projections of arrangement of N3 in a b plane in phases , , and , respectively. − δ graphically nonequivalent sites of N3 in phase , which is whereas the T(Eu) and T(A2u) modes disappeared simulta- reasonably consistent with our previous study.20 With neously, accompanying the enlarged pressure dependence of ν ν ′ increasing pressure, the 1C and 1D modes gradually T (Eu) and LR1 modes (Figure 5d) as well as the enhanced ν ν strengthened and then separated from 1(B2g) and 1(A1g) intensity of LI1 modes (Figure 5a). These phenomena are modes. As the pressure increased up to the highest pressure ascribed to the phase transition of phase α → γ as reasonably (28.5 GPa) of this work, all modes became extremely weak and consistent with our Raman spectra. In the pressure region 7.4− broad without other changes being observed. To intuitively 10.1 GPa, all the lattice modes were observed independently reflect the transition boundaries and better understand the and clearly with the consistent positive linear pressure transition mechanism, the pressure dependence of all the dependence, suggesting the stable existence of the phase γ in vibrational frequencies is plotted in Figure 4c,d. As shown, in this pressure region. As the pressure was increased to 10.6 GPa, −1 both the lattice and the internal mode region, the disappearing another mode at 200 cm (labeled as LI4) emerged, and then it and appearing of the Raman modes accompanied by the significantly developed into the most intense mode up to 16.5 changes of the pressure dependence of vibrational frequencies GPa. All the lattice modes presented the nonlinear pressure consistently suggested two phase transitions at 6.4 and 16.1 dependence in the pressure region 10.6−16.5 GPa (Figure 5d), GPa, in excellent agreement with our previous XRD study. Of consistent with the evolution of the Raman spectra in the − − note, in the pressure region 6.4 16.1 GPa, the LR2 and LR3 pressure region 11.9 16.1 GPa, which are indicative of the modes presented the significantly different pressure depend- process of the phase transition (γ → δ). Upon subsequent ence of vibrational frequencies as they merged and the compression, phase δ was stable up to the highest pressure of reversion of their relative intensity. Moreover, the pressure this study. ν ν − dependence of frequencies of these 2 2 (B2g), 2 2(A1g), and More crucial details about the evolution of N3 in the ν″ 2 2(A1g) are all nonlinear in this pressure region. All these process of compression can be obtained from the mid-IR significant trends probably indicate that the surrounding spectra. Upon compression, the most significant change in the − fi potential of N3 changed in this pressure region. mid-IR pro le was that the doubly degenerate bending mode ν C. IR Spectra on Compression. Supplementary to the 2(Eu) evolved into the double nondegenerate modes, labeled   ν′ ν″ Raman measurements, the N N N bending and asymmetric as 2(Eu)and 2(Eu),asshowninFigure5b.This stretch modes, as well as these lattice vibrational modes T(Eu), phenomenon is attributed to the pressure-induced elimination ′ ν T(A2u), and T (Eu), were further investigated by the infrared of the degeneration in the 2(Eu) mode, and the essential measurements. The selected spectra of the far-IR and mid-IR reasons may be the rotation or tilt of the equivalent sites of − − are depicted in Figure 5a c with the pressure up to 20.4 and N3 . As the pressure increases up to 6.5 GPa, a new mode at −1 30.2 GPa, respectively. The far-IR spectral region (Figure 5a) 3444 cm (labeled as CI5) appeared whereas the CI1 mode presents the evolution of the lattice modes of RbN3. All the almost depleted simultaneouslyasshowninFigure5c, modes exhibit ordinary blue shifts as the pressure goes below corresponding to the phase transition of phase α → γ. This 3.8 GPa due to the reduction of interionic distances with phase transition was further evidenced by the inflections of the −1 ’ ν′ ν″ increasing pressure. A new mode at 174 cm (labeled as LI1) frequencies pressure dependence of 2(Eu) and 2(Eu) modes with weak intensity occurred at 3.8 GPa. Additionally, Figure at this pressure as shown in Figure 5e. Upon further 5d shows that the pressure dependence of frequencies of T(Eu), compression to 10.8 GPa, two new modes at 1137 and 1374 ′ −1 fl T(A2u), and T (Eu) all decreased above 3.8 GPa. All these cm were observed, accompanied by the in ections of the ν′ phenomena can be attributed to the distortion of the unit cell frequencies with the pressure dependence of 2(Eu) and 20 ν″ as reported in our previous XRD study. From this 2(Eu) modes, suggesting the onset of the second phase γ → δ −1 fi perspective, the appearance of the LI1 mode probably stems transition ( ). The mode at 1137 cm is identi ed as a ′ ff from the elimination of the double degeneration of the T (Eu) di erence frequency as labeled as DI2. The mode with weak mode around this pressure due to the detectable distortion of intensity at 1374 cm−1 is assigned to the NNN symmetric ν the unit cell. Upon further compression, two new modes at 128 stretch mode, labeled as sy temporarily, corresponding to the −1 ν −1 and 156 cm (labeled as LI2 and LI3) occurred at 7.4 GPa 1(A1g) mode at 1375 cm (11.9 GPa) in the Raman spectra.

16876 DOI: 10.1021/acs.jpcc.5b05208 J. Phys. Chem. C 2015, 119, 16870−16878 The Journal of Physical Chemistry C Article ν δ α The sy mode presented as IR-active at this pressure due to the symmetry of phase compared to that of phase , which is NNN becoming nonlinear/asymmetric upon compres- consistent with our previous study. Our analyses imply that 40−42 sion, as is the fact in Ca(N3)2, Ba(N3)2, and Pb(N3)2. In Raman and infrared spectra are useful characterization methods − the process of 10.8 14.9 GPa, the relative intensity of CI4 and for exploring structures and stability properties of RbN3 under CI5 is gradually changed accompanied by the overlap and cross high pressure; even so, further experimental and theoretical ν ν′ of the 2(A2u) and 2(Eu) modes, indicating the process of the investigations are required to explore the behaviors of these γ → δ phase transition ( ). Further compression to 16.1 GPa high-pressure phases of RbN3. resulted in the disappearance of CI2 and CI3 modes ■ CONCLUSIONS accompanied by the merging of CI4 and CI5 (Figure 5f). ν Furthermore, the pressure dependence of 2(A2u), DI2, and DI1 The high-pressure Raman and IR spectroscopic measurements modes changed at this pressure as shown in Figure 5e. These of RbN3 were investigated with the pressure up to 28.5 and phenomena all indicated the completion of the phase transition 30.2 GPa, respectively. All the fundamental vibrational modes (γ → δ). Upon subsequent compression, phase δ is stable up to are resolved comprehensively without interference on the basis 30.2 GPa, the highest pressure of this study, with all modes of our experimental and calculated results. Upon compression, becoming extremely weak and/or broad. two phase transitions were observed at near 6.5 and 16.0 GPa ’ ν′ Curiously, the frequencies pressure dependence of 2(Eu) as evidenced by changes of spectral profiles and the pressure mode has the abnormal evolution from negative to positive dependence of the characteristic vibrational modes over upon compression. Moreover, the pressure dependence of the different pressure ranges. Spectroscopic measurements on ν′ ν″ 2(Eu) and 2(Eu) modes are symmetrical about their center decompression suggest the reversible phase transitions of symmetric line, as shown in Figure 5e, which indicates the RbN3. The abnormal softening of the T(Eg) mode in Raman probable existence of a certain synergistic action force for these spectra reveals the displacive structural transition of phase α → ν′ ν″ two modes. The 2(Eu) and 2(Eu) mode correspond to the γ resulted from the pressure-induced shearing distortion of the − − bending vibration of N3 , which is located in the b and a axes, unit cell. Detailed mid-IR analyses indicate the evolution of N3 respectively, as presented in Figure 6. As we all know, for LiN3, with the arrangement sequence of orthogonal → parallel → − NaN3, and AgN3, the N3 species all undergo a rotation or tile orthogonal during the phase transition sequence of phase α → about the axis through the central nitrogen atom and normal to γ → δ, which is excellent, consistent with our previous study. one plane upon pressure or temperature.12,19,43 In the case of The IR-active NNN symmetric stretch mode in phase δ − RbN3, the N3 located at the b and a axes will rotate about the indicates the pressure-induced nonlinear/asymmetric behavior   − axis through the central nitrogen (the N atoms labeled as 2 and of N N N. Additionally, the N3 evolved into two 5) with increasing pressure as shown in Figure 6. During the crystallographically nonequivalent sites in phase δ evidenced process of the rotation, the distance between the electro- by the splitting of the NNN symmetric stretch modes. negative termini N atom and electropositive central N atom of Generally, our detailed spectroscopic analyses based on − the adjacent N3 anion changed. The distance marked with d26 combined Raman and IR measurements provide a more in- decreased while the distance marked with d35 increased with depth understanding of the structures and stability of RbN3 increasing pressure, resulting in the corresponding attractions under high pressure. increasing and decreasing, respectively. Consequently, the bending vibration of the N − located in the a axis is ■ AUTHOR INFORMATION 3 − strengthened, and that is weakened for the N3 located in the Corresponding Author b axis which corresponds to the significant softening of the *Phone: +8643185168881. Fax: +8643185168881. E-mail: ν′ ν″ 2(Eu) and hardening 2(Eu) mode. Figure 6 demonstrates [email protected]. − the evolution of the projections of the arrangement of N3 in Notes − the a b plane with increasing pressure. This phenomenon is The authors declare no competing financial interest. consistent with our previous conclusion that the rotation of the − → → N3 is rearranged as orthogonal parallel orthogonal ■ ACKNOWLEDGMENTS during the phase transition sequence of phase α → γ → δ.20 In δ − Use of the National Synchrotron Light Source is supported by phase , the N3 remains in the orthogonal arrangement DOE Office of Science, Office of Basic Energy Sciences, under resulting in the monotonous hardening of pressure dependence ν′ ν″ Contract No. DE-AC02-98CH10886. The U2A beamline is of 2(Eu) and 2(Eu) modes (Figure 5e) due to the reduction supported by COMPRES, the Consortium for Materials of interionic distances with increasing pressure. Additionally, − α δ Properties Research in Earth Sciences, under NSF Cooperative the orthogonal arrangement of N3 in phase and phase Agreement Grant No. EAR01-35554 and the U.S. DOE (Figure 6a,c) favorable interactions that are built between the (CDAC, Contract No. DEFC03-03N00144). This work was electronegative termini and electropositive central N atom of − supported financially by the National Natural Science the adjacent N3 are more energetically favorable than that of Foundation of China (11304111, 51172087, 11147007, and the parallel arrangement in phase γ (Figure 6b). Consequently, γ − 11304139), and the National Basic Research Program of China the phase with the parallel arrangement of N3 tends to be (2011CB808204). unstable with increasing pressure, as our previous prediction that the phase γ is possibly the intermediate phase between ■ REFERENCES phase α and phase δ.20 (1) Tornieporth-Oetting, I. C.; Klapötke, T. M. 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16877 DOI: 10.1021/acs.jpcc.5b05208 J. Phys. Chem. C 2015, 119, 16870−16878 The Journal of Physical Chemistry C Article

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16878 DOI: 10.1021/acs.jpcc.5b05208 J. Phys. Chem. C 2015, 119, 16870−16878