High-Pressure Studies of Rubidium Azide by Raman and Infrared

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High-Pressure Study of Rubidium 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 J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b05208 • Publication Date (Web): 01 Jul 2015 Downloaded from http://pubs.acs.org on July 7, 2015

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High-pressure Studies of Rubidium Azide by Raman and Infrared Spectroscopies Dongmei Li†, Fangfei Li†, Yan Li‡, Xiaoxin Wu†, Guangyan Fu†, Zhenxian Liu§, Xiaoli Wang#, Qiliang Cui†, Hongyang Zhu†* †

State Key Laboratory of Superhard Materials, College of Materials Science and

Engineering, Jilin University, Changchun, Jilin 130012, China ‡

College of Physics, Jilin University, Changchun, 130012, China

§

Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington, DC 20015, USA

#

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

∗

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

E-mail address: [email protected]

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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 GPa 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

non-linear/asymmetric existence of N=N=N and the two crystallographically nonequivalent sites of N3− were observed in phase δ. Keywords: High-pressure; Vibrational spectra; Phase transition; Soft mode

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INTRODUCTION Inorganic azides have been attracted considerable attention during the past several decades due to their peculiar structures and physico-chemical properties.1-3 Their application importance as initial explosives, as gas generators, and even as photographic materials at low temperature have been used extensively in industry and military.3-5 Additionally, the unique structures, as result of linear-rod-shaped azide ions (N3−), make them logical candidates to study the complex nature of chemical bonding and internal molecular structure beyond alkali halides and cyanides.4-7 It is hence instructive to study the inorganic azides intensively and extensively to provide more fundamental basis for their industrial applications and scientific researches. Recently, the studies of alkali azides have open a new perspective as a distinctive precursor in the formation of polymeric nitrogen, the ultimate example of a high-energy-density material (HEDM), due to the lower bonding energy of double bonds N=N (418 KJ/mol) in N3− compared to the triple bonds N≡N (954 KJ/mol) in N2.8 The N3− ions have been found to transform into larger nitrogen clusters then polymeric nitrogen nets with application of pressure, as reported the non-molecular nitrogen state and zigzag chains of N5− rings have been formed in the high-pressure studies of NaN3 and LiN3.9,10 Moreover, the structural, electronic, and optical properties of alkali azides also present abundant changes as explored by experimental and theoretical high-pressure studies.11-19 Since a comparison of the high-pressure behaviors of these substances would enable an understanding of the mechanism of pressure-induced phase transitions as well as the evolution of azide ions which might

3


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result in the formation of polymeric nitrogen.

Figure 1. Crystal structure of RbN3 at ambient conditions along (a) a-b-c and (b) a-b (c) b-c axes. Blue color represents N atoms, red color represents Rb atoms. Under ambient conditions, RbN3 crystallizes into a body-centered tetragonal structure with space group D 18 4h -I4/mcm and cell parameters of a = b = 6.31162(70) Å, c = 7.54038(98) Å,20 as shown in Figure 1a. The linear symmetrical N3− and Rb+ form alternating layers in the [001] direction with the N3− groups inclined at 90° to one another within each plane as illustrated in Figure 1b and Figure 1c. According to Mueller and Joebstl’s studies, RbN3 transforms into the cubic structure from the tetragonal structure with N3− anions oriented at random parallel to the edges of the cubic unit cell upon heating.21 As temperature dropped to 82 K, no phase transition was observed in the low temperature measurement of RbN3 studied by Hathaway and Temple.22 Our recent high-pressure X-ray diffraction (XRD) study of RbN3 revealed the pressure-induced phase transitions of the tetragonal → monoclinic → orthorhombic.20 However, details about azide ions are limited due to the minimal contribution of N atom to the XRD. Therefore the vibrational studies of RbN3 are beneficial to explore the evolution of the N3− in the process of phase transitions. The high-pressure vibrational studies of RbN3 are restricted to the Raman scattering 4



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studies up to pressure of 4GPa.23,24 It is therefore rather significant to investigate the high-pressure vibrational spectroscopic behaviors of RbN3 by optical spectroscopic method. In this work, we represent the high-pressure Raman and IR measurements of RbN3 at room temperature with diamond anvil cells (DAC) up to 28.5 GPa and 30.2 GPa, respectively. One of the primary objective is to resolve all of the fundamental vibrational frequencies in Raman and IR spectroscopy without interference. More importantly, the phase transition sequence of RbN3 is revealed at the vibrational spectrum level which is not investigated so far. The detailed spectroscopic analyses based on combined Raman and IR activities of the characteristic modes of RbN3 allowed for a more in-depth understanding of the structure and stability of RbN3. EXPERIMENT The RbN3 with a purity of 99% was obtained commercially from International Laboratory USA Co.. The high-pressure Raman experiments were performed in a symmetric DAC with culets of 400 µm in diameter. A T301 steel sheet served as the gasket with a chamber of 120 µm in diameter and 56 µm in thickness for packed the sample. The mixture of methanol and ethanol with a volume ratio of 4:1 was employed as the pressure transmitting medium. A ruby ball was used to determine pressure by using the standard ruby fluorescent technique. The measurements were performed using a solid-state, diode-pumped Nd: Vanadate laser (Coherent Inc.) with 532 nm wavelength as excitation source. A liquid nitrogen-cooled CCD camera equipped on Acton SpectraPro 500i spectrometer with a 1800-groove mm-1 grating

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was used for recording the Raman scattering spectra. The high-pressure IR experiments were performed at the U2A beamline, which is a part of the Vacuum Ultraviolet (VUV) ring of the National Synchrotron Light Source (NSLS) at the Brookhaven National Laboratory. The pressure was generated by the symmetrical DAC with type II diamonds. The flat culets of the diamonds is 500 µm in diameter. The T301 steel sheet 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, petroleum jelly was served as the inert pressure transmitting medium. The loaded DAC was placed inside a nitrogen-purged container and the data were subsequently acquired. The observed range of far-IR spectra was within 60-700 cm-1. For the mid-IR experiment, KBr powder was used as the pressure transmitting medium. The loaded DAC was placed in the focal region of the microscope objective lens, through which the high flux polychromatic IR beam passed. The range of the mid-IR spectra was within 600-4000 cm-1. The spectral resolution for all measurements was about 2 cm-1. In order to explore the Vibrational spectrum of RbN3 and their vibration modes, we have performed the ab initio calculations with plane wave pseudo potential density functional computer code Cambridge Serial Total Energy Package (CASTEP).25 The generalized gradient approximation (GGA) using Perdew-Burke-Ernzerhof (PBE) parameterization was used to describe the exchange-correlation potential.26 In our calculations, convergence tests give the energy cutoff Ecutoff as 770 eV and the electronic Brillouin zone (BZ) integration with the K-points of 0.03 1/ Å. The internal

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atomic positions and cell size of the system was fully relaxed. In the geometry relaxation, the self-consistency convergence on the total energy was 5.0×10-6 eV/atom and the maximum force on the atom was found to be 0.01 eV/ Å. The vibrational frequencies of the optimized structure were then calculated. RESULTS AND DISCUSSION A. Ambient-pressure Raman and IR spectra Under ambient conditions, RbN3 crystallizes into the tetragonal structure with space group I4/mcm (D 18 4h ) (Figure 1a) with two molecules per primitive cell. The group theoretical analyses indicate 24 vibrations modes are associated with RbN3 with following irreducible representation Γ acoustic = A2u + Eu

(1)

Γ optical = 4Eu + 2A2u + 2Eg + 2 B2u + 2A2g + B2g + B1g + A1g

(2)

where A and B mode are nondegenerate, E mode is doubly degenerate, the subscripts g and u stand for gerade and ungerade, respectively. One A2u and one Eu correspond to zero frequency acoustic modes, and the rest are optic modes. All gerade modes are Raman-active and ungerade modes are IR-active, except for B1u and A2g, which are neither Raman- nor IR- active. The experimental and calculated Raman and IR spectra were collected as plotted in Figure 2a and Figure 2b, respectively. The calculated results show quite similar features to our experimental spectra, and all the vibrational modes within the primitive cell, including all the degenerate modes, are displayed intuitively in Figure 3a-k. Figure 3l shown the relationship of the coordinate systems between the primitive cell and the unit cell. Based on the research achievements of

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predecessors and our calculations, all the Raman and IR modes along with their frequencies and assignments are summarized in Table 1. Nevertheless, it should be pointed out that all the peaks are shift to low frequency in our calculated results compared with experimental results. The discrepancy can be primarily explained by the fact that the present calculations concern T = 0 K rather than the room temperature. Additionally, the certain differences may be partly resulted from that the computations were carried out for an ideal single unit cell, while the experimental result is a collective contribution of the fine powder. For the lattice vibrational modes T(Eg), R(Eg), and R(B1g) which are associated with the translation of the Rb+ parallel to the (001) plane with atoms in adjacent planes moving in opposite directions (Figure 3a), the hindered rocking of the N3− parallel to the z axis (Figure 3e), and the hindered rotation of the N3− perpendicular to the z axis (Figure 3f), were observed at 66 cm-1, 128 cm-1, and 140 cm-1 in the Raman spectra, respectively. The T(Eu), T(A2u), and T’(Eu) modes, which are corresponding to the translation of the Rb+ and N3− in the opposite directions perpendicular to the z axis (Figure 3b), the translation of the Rb+ and N3− in the opposite directions parallel to the z axis (Figure 3c), and the translation of the N3− parallel to z axis (Figure 3d), were detected at 100 cm-1, 117 cm-1, and 160 cm-1 in the Far-IR region. The N=N=N bending modes (Figure 3g and Figure 3h),

ν 2(A2u) and ν 2(Eu), IR-active due to the linear symmetrical of N3−, were observed at 620 cm-1 and 645 cm-1, while the overtone of N=N=N bending modes, 2ν 2 (B2g), 2ν 2(A1g), and 2ν ’’2(A1g), Raman-active modes, were observed at 1251 cm-1, 1263 cm-1, and 1285 cm-1, respectively. They have appreciable intensities due to the Fermi

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resonance interaction of 2ν 2 and

ν1

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mode similar to KN3 and CsN3.17,27 For the

splitting of the crystal correlation filed, the N=N=N out-of-phase symmetric stretch mode

ν 1(B2g) (Figure 3i) and in-phase symmetric stretch mode ν 1(A1g) (Figure 3j)

were observed at 1334 cm-1 and 1346 cm-1, respectively. All these fundamental frequencies of vibrational modes in experiments and calculations are coincident with each other well. The N=N=N asymmetric stretch mode

ν 3(Eu) (Figure 3k) is located

at 1867 cm-1 with a significant intensity in calculated results. However, the

ν 3(Eu)

mode was not observed in the measured spectra as it locates at the multiphonon absorptional region (1850-2500 cm-1) of the diamond anvils. Moreover, according to the vibrational spectrum studies of KN3 and CsN3 (iso-structural with RbN3 at ambient conditions),27,28 the two weak bands at 1317 cm-1 and 1353 cm-1 in the internal modes region of the Raman spectra are the combination frequencies of T(Eg) + 2ν 2 (B2g) and T(Eg) + 2ν ’’2(A1g), labeled as CR1 and CR2 in this work. Additionally, the band at 1119 cm-1 in the mid-IR spectra is probable attributed to the difference frequency of 2ν 2 and T, labeled as DI1 in this work.27-29 The bands at 3249 cm-1, 3271 cm-1, 3293 cm-1, and 3343 cm-1, labeled as CI1, CI2, CI3, and CI4, respectively, are assigned to the combination frequency of 2ν 2 +ν 3 and

ν 1+ν 3.27-29

The shoulder

observed at the high-frequency side of the mode R(B1g) in the Raman spectrum may be related to the second-order scattering.22

9


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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 assignment 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 meaning the spectra were at a magnification of 1500, 50, and 20 times.

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Figure 3. (a)-(k) the simulated eigenvectors of all the vibrational modes in the primitive cell from the calculations. The blue and red colored 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). Table 1. Raman frequencies, IR frequencies, and their assignments of RbN3 obtained from calculations, references, and our experiments, respectively. Raman frequencies (cm-1) Exp.

Cal.

Ref.

66

65

6622/6530

IR frequencies (cm-1) Exp.

Cal.

Assignment

Ref. T(Eg)

11


Rb+ translation

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100

12030

93

T(Eu)

9631 117

12030

94

124

14630

Rb+

translation T(A2u)

16432 160

N3− and

N3 −

and

Rb+ translation T’(Eu)

N3− translation

14431 128

120

12822/12830

R(Eg)

N3− rotation

140

125

14022/13830

R( B1g )

N3− rotation

ν 2 (A2u)

N=N=N

620

613

64030

bending 645

616

64430

ν 2( Eu )

N=N=N bending

1119

DI1

difference frequency

1251

125030

2ν 2 (B2g)

1263

1262/128430/1

2ν 2(A1g)

28930

overtone 2ν ’’2(A1g)

1285

N=N=N bending

1317

CR1

12


combination

of


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frequency27 1334

1209

133322/133430

ν 1(B2g)

N=N=N symmetric stretch

1346

1211

133322/133430

ν 1 (A1g)

N=N=N symmetric stretch

1353

CR2

combination frequency27, 28

1867

203230

ν 3 (Eu)

N=N=N asymmetric stretch

3249

CI1

combination

3271

CI2

frequencies of

3293

CI3

2ν 2 +

3343

CI4

ν 1 + ν 328

ν 3 and

B. Raman spectra upon compression We collected the Raman spectra of RbN3 upon compression with selected spectra shown in Figure 4a and Figure 4b corresponding to the lattice modes region and internal modes region, respectively. Upon compression, all the lattice modes except the translational T(Eg) mode shift monotonically to higher frequencies as the result of reduction of interionic distances. As the pressure up to 6.4GPa, four new Raman

13


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modes (labeled from LR1 to LR4 from low to high frequencies) in the lattice region (Figure 4a) were observed. Concurrently, the intensity of T(Eg) and R(Eg) modes exhibited significantly depleted. These phenomena indicate that the first phase transition (α → γ) occurred at this pressure. As the pressure increased by a small value, the intensity of LR1-LR4 modes exhibited significantly enhanced and the intensity of T(Eg) and R(Eg) modes also completely depleted at 6.7 GPa suggesting the completion of the first phase transition at this pressure. The sizable softening of the T(Eg) modes has been predicted to trigger a phase transition when the pressure greater than 4GPa,23 which has been proved in this work. In addition, the softening of the T(Eg) modes upon pressure is also observed in KN3 and TlN3, iso-structural with RbN3 at ambient conditions.17, 33 For TlN3, pressure-induced softening of the T(Eg) mode is attribute to the a shearing distortion of the unit cell that subsequently results in the structural phase transition from tetragonal to monoclinic.34 Analogously, the same triggering mechanism was proposed for KN3.17 In tetragonal-type compounds the T(Eg) mode is related to the shear elastic constant C44.35 Therefore, the significant softening of the T(Eg) mode in RbN3 reinforces that the first phase transition (α → γ) of RbN3 was induced by the shearing distortion of the unit cell. Moreover, the shearing of layers were also presented in the pressure-induced phase transition of LiN3, NaN3, and CsN3.12, 15, 19, The distance between the neighbouring Rb+ of the adjacent layers will be increased during the process of the shearing distortions, In this case, the softening of T(Eg) mode were interpreted reasonably. The softening of T(Eg) mode (or the lowest-frequency modes) accompanied phase transitions have been also observed in

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the high-pressure studies of other layered tetragonal-type compounds,35-37 which is probably in consequence of the instability of the layered tetragonal-type compounds upon compression. In addition, the softening-accompanied phase transitions have been observed in other compounds,38,39 which is however beyond the scope of this work.

Figure 4. Selected high-pressure Raman spectra of RbN3 in the region of (a) lattice modes (30-430 cm-1) (b) internal modes (1200-1500 cm-1). The dashed lines serve as

15


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visual guides. The mode guided by the green dashed line in (b) is from the diamond anvils with D-0 stands for the pressure released to 0 GPa. (c) and (d) are the corresponding Raman shift of all modes as a function of pressure with the vertical dashed line indicate the proposed phase boundaries. With further compression, additional lattice mode (labeled as LR5) started to burgeon at 11.9 GPa and continuously strengthen up to 16.1 GPa, whereas the LR2 and LR3 modes gradually merged in this pressure region. All these features indicate the second phase transition (γ → δ) starts at 11.9 GPa and completes at 16.1 GPa. Compared to the lattice modes, the internal modes are relatively stable due to the greater strength of quasi-double than that of bonds between ions. As shown in Figure 4b, no prominent changes were observed up to 14.8 GPa, except that the overtone vibrational modes 2ν 2 (B2g), 2ν 2(A1g), and 2ν ’’2(A1g) gradually depleted due to the reduction of interionic distances with increasing pressure. However, as the pressure up to 16.1 GPa, the symmetric stretch modes of N=N=N a doublet set with the new modes labeled as

ν 1(B2g) and ν 1 (A1g) split into

ν 1C and ν 1D, convincing the existence

of the two crystallographically nonequivalent sites of N3− in phase δ, which is reasonably consistent with our previous study.20 With increasing pressure, the and

ν 1D

modes gradually strengthen then separated from

ν 1C

ν 1(B2g) and ν 1 (A1g)

modes. As the pressure up to the highest pressure (28.5 GPa) of this work, all modes become extremely weak and broad without other changes were observed. To intuitively reflect the transitions boundaries and better understand the transition mechanism, the pressure dependence of all the vibrational frequencies are plotted in Figure 4c and Figure 4d. As shown, in both lattice and internal modes region the

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disappearing and appearing of the Raman modes accompanied by the changes of the pressure dependence of vibrational frequencies consistently suggested two phase transitions at 6.4 and 16.1 GPa, excellent agreement with our previous XRD study. Of note, in the pressure region of 6.4-16.1 GPa, the LR2 and LR3 mode presented the significant different pressure dependence of vibrational frequencies as they merging and the reversion of their relative intensity. Moreover, the pressure dependence of frequencies of these 2ν 2 (B2g), 2ν 2(A1g), and 2ν ’’2(A1g) are all non-linear in this pressure region. All these significant trends probably indicate the surrounding potential of N3− changed in this pressure region. C. IR spectra on compression Supplementary to the Raman measurements, the N=N=N bending and asymmetric stretch modes, as well as these lattice vibrational modes T(Eu), T(A2u), and T’(Eu) were further investigated by the infrared measurements. The selected spectra of the far-IR and mid-IR are depicted in Figure 5a-c with the pressure up to 20.4 GPa and 30.2 GPa, respectively. The far-IR spectra region (Figure 5a) presents the evolution of the lattice modes of RbN3. All the modes exhibit ordinary blue shifts as the pressure below 3.8 Gpa due to the reduction of interionic distances with increasing pressure. A new mode at 174 cm-1 (labeled as LI1) with weak intensity occurred at 3.8 GPa. Additionally, Figure 5d shows that the pressure dependence of frequencies of T(Eu), T(A2u), and T’(Eu) all decreased above 3.8 GPa. All these phenomena can be attributed to the distortion of the unit cell as reported in our previous XRD study.20 From this perspective, the appearance of the LI1 mode probably stems from the elimination of

17


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the doubly degeneration of the T’(Eu) mode around this pressure due to the detectable distortion of the unit cell. Upon further compression, two new modes at 128 cm-1 and 156 cm-1 (labeled as LI2 and LI3) occurred at 7.4 GPa whereas the T(Eu) and T(A2u) modes disappeared simultaneously, accompanying the enlarged pressure dependence of T’(Eu) and LR1 modes (Figure 5d) as well as the enhanced intensity of LI1 modes (Figure 5a). These phenomena are ascribed to the phase transition of phase α → γ as reasonably consistent with our Raman spectra. In the pressure region of 7.4-10.1 GPa, all the lattice modes exhibited independently and clearly with the consistent positive linear pressure dependence, suggesting the stable existence of the phase γ in this pressure region. As the pressure was increased to 10.6 GPa, another mode at 200 cm-1 (labeled as LI4) emerged, then it significantly developed into the most intense mode up to 16.5 GPa. All the lattice modes presented the non-linear pressure dependence in the pressure region of 10.6-16.5 GPa (Figure 5d), consistent with the evolution of the Raman spectra in the pressure region of 11.9-16.1 GPa, which are indicative of the process of the phase transition (γ → δ). Upon subsequent compression, phase δ was stable up to the highest pressure of this study.

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Figure 5. Selected high-pressure IR spectra of RbN3 in the region of Far-IR (a) 100-390 cm-1 and mid-IR (b) 600-1500 cm-1 and (c) 3150-3700 cm-1. The dashed lines serve as visual guides. D-0 stands for the pressure rel eased to 0 GPa. (d)-(f) are the corresponding IR shift of all modes as a function of pressure with the vertical dashed line indicate the proposed phase boundaries. More crucial details about the evolution of N3− in the process of compression can be obtained from the mid-IR spectra. Upon compression, the most significant change in the mid-IR profile was that the the doubly degenerate bending mode evolved into the double nondegenerate modes, labeled as

ν 2(Eu)

ν ’2(Eu) and ν ’’2(Eu),

as

shown in Figure 5b. This phenomenon is attributed to the pressure-induced elimination of the degeneration in the

ν 2(Eu) mode, and the essential reasons may be 19


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the rotation or tilt of the equivalent sites of N3−. As the pressure up to 6.5 GPa, a new mode at 3444 cm-1 (labeled as CI5) was appeared whereas the CI1 mode almost depleted simultaneously as shown in Figure 5c, corresponding to the phase transition of phase α → γ. This phase transition was further evidenced by the inflexions of the frequencies pressure dependence of

ν ’2(Eu)

and

ν ’’2(Eu)

mode at this pressure as

shown in Figure 5e. Upon further compression to 10.8 GPa, two new modes at 1137 cm-1 and 1374 cm-1 were observed, accompanied by the inflexions of the frequencies the pressure dependence of

ν ’2(Eu) and ν ’’2(Eu) modes, suggesting the onset of the

second phase transition (γ → δ). The mode at 1137 cm-1 is identified to a difference frequency as labeled as DI2. The mode with weak intensity at 1374 cm-1 is assigned to the N=N=N symmetric stretch mode, labeled as the

ν 1(A1g)

presented

ν sy

temporarily, corresponding to

mode at 1375 cm-1 (11.9 GPa) in Raman spectra. The

the

IR-active

at

this

pressure

due

to

the

ν sy

N=N=N

mode

becomes

non-linear/asymmetric upon compression, as the fact in Ca(N3)2, Ba(N3)2, and Pb(N3)2.38-40 In the process of 10.8-14.9 GPa, the relative intensity of CI4 and CI5 is gradually changed accompanied by the overlap and cross of the

ν 2(A2u) and ν ’2(Eu)

mode, indicating of the process of the phase transition (γ → δ). Further compression to 16.1 GPa resulted in the disappearance of CI2 and CI3 mode accompanied by the merging of CI4 and CI5 (Figure 5f). Furthermore, the pressure dependence of

ν 2(A2u),

DI2, and DI1 mode changed at this pressure as shown in Figure 5e. These phenomena are all indicated the completion of the phase transition (γ → δ). Upon subsequent compression, phase δ is stable up to 30.2 GPa, the highest pressure of this study, with

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all modes become extremely weak and/or broad.

Figure 6. The simulated diagram of the evolution of the N3− with increasing pressure. The green arrows indicates the vibrational direction of the atom. (a), (b), and (c) correspond to the projections of arrangement of N3− in a-b plane in phase α, γ, and δ, respectively. Curiously, the frequencies pressure dependence of

ν ’2(Eu) mode has the abnormal

evolution of from negative to positive upon compression. Moreover, the pressure dependence of

ν ’2(Eu)

and

ν ’’2(Eu)

mode are symmetrical about their center

symmetric line, as shown in Figure 5e, which indicates the probable existence of a certain synergistic action force for these two modes. The

ν ’2(Eu) and ν ’’2(Eu) mode

correspond to the bending vibration of N3−, which located in b and a axis, respectively, as presented in Figure 6. As we all knows, for the LiN3, NaN3, and AgN3, the N3− all undergo an rotation or tile about the axis through the central nitrogen atom and normal to one plane upon pressure or temperature.12,19,41 In the case of RbN3, the N3− located in b and a axis will rotate about the axis through the central nitrogen (the N atoms labeled as 2 and 5) with increasing pressure as shown in Figure 6. During the process of the rotation, the distance between the electronegative termini N-atom and electropositive central N-atom of the adjacent N3− anion changed. The distance

21


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marked with d26 decreased while the distance marked with d35 increased with increasing pressure, resulting in the corresponding attractions increased and decreased, respectively. Consequently, the bending vibration of the N3− located in a axis is strengthened and that is weakened for the N3− located in b axis which corresponding to the significant softening of the

ν ’2(Eu)

and hardening

ν ’’2(Eu)

mode. Figure 6

demonstrates the evolution of the projections of the arrangement of N3− in a-b plane with increasing pressure. This phenomenon is consistent with our previous conclusion that the rotation of the N3− is rearranged as orthogonal → parallel → orthogonal during the phase transition sequence of phase α → γ → δ.20 In phase δ, the N3− remains the orthogonal arrangement result in the monotonously hardening of pressure dependence of

ν ’2(Eu)

and

ν ’’2(Eu)

mode (Figure 5e) due to the reduction of

interionic distances with increasing pressure. Additionally, the orthogonal arrangement of N3− in phase α and phase δ (Figure 6a and Figure 6c), favorable interaction is built between the electronegative termini and electropositive central N-atom of the adjacent N3−, are more energetically favorable than that of parallel arrangement in phase γ (Figure 6b). Consequently, the phase γ with the parallel arrangement of N3− tends to be unstable with increasing pressure, as our previous prediction that the phase γ is possible the intermediate phase between phase α and phase δ.20 As pressure is released to ambient conditions, all the vibrational spectra recovered to the tetragonal structure, indicates the reversibility of the phase transitions of RbN3. Additionally, the increased number of the total vibrational modes in phase δ indicates

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a lower structural symmetry of phase δ compared to phase α, which is consistent with our previous study. Our analyses imply that Raman and Infrared spectra are useful characterization methods for exploring structures and stability properties of RbN3 under high pressure, even so, further experimental and theoretical investigations are required to explore the behaviors of these high-pressure phases of RbN3. CONCLUSIONS The high-pressure Raman and IR spectroscopic measurements of RbN3 were investigated with the pressure up to 28.5 GPa and 30.2 GPa, respectively. All the fundamental vibrational modes are resolved comprehensively without interference based on our experimental and calculated results. Upon compression, two phase transitions were observed at near 6.5 and 16.0 GPa evidenced by changes of spectral profiles and the pressure dependence of the characteristic vibrational modes over different pressure ranges. Spectroscopic measurements on decompression suggesting the reversible phase transitions of RbN3. The abnormal softening of the T(Eg) mode in Raman spectra reveal the displacive structural transition of phase α → γ resulted from the pressure-induced shearing distortion of the unit cell. Detailed mid-IR analyses indicate the evolution of N3− with the arrangement sequence of orthogonal → parallel → orthogonal during the phase transition sequence of phase α → γ → δ, which is excellent consisted with our previous study. The IR-active N=N=N symmetric stretch mode in phase δ indicates the pressure-induced non-linear/asymmetric behavior of N=N=N. Additionally, the N3− evolved into two crystallographically nonequivalent sites in phase δ evidenced by the splitting of the N=N=N symmetric stretch modes.

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Generally, our detailed spectroscopic analyses based on combined Raman and IR measurements provide a more in-depth understanding of the structures and stability of RbN3 under high pressure. ACKMOWLEDGMENTS Use of the National Synchrotron Light Source is supported by DOE Office of Science,

Office

of

Basic

Energy

Sciences,

under

Contract

No.

DE-AC02-98CH10886. The U2A beamline is supported by COMPRES, the Consortium for Materials Properties Research in Earth Sciences, under NSF Cooperative Agreement Grant No. EAR01-35554 and the U. S. DOE (CDAC, Contract No. DEFC03-03N00144). This work was supported financially by the National Natural Science Foundation of China (11304111, 51172087, 11147007, and 11304139), and the National Basic Research Program of China (2011CB808204). REFERENCES 1. Tornieporth-Oetting, I. C.; M.Klapötke, T. Covalent Inorganic Azides. Angew. Chem. Int. Ed. Engl. 1995, 34, 511-520. 2. Gray, P. Chemistry of the Inorganic Azides. Q. Rev. Chem. Soc, 1963, 17, 441-473. 3. Evavs, B. L.; Yoffe, A. D.; Gray, P. Physics and Chemistry of the Inorganic Azides. Chem. Rev, 1959, 59, 515-568. 4. Dewaele, A.; Belonoshko, A. B.; Garbarino, G.; Occelli, F.; Bouvier, P.; Hanfland M.; Mezouar, M. High-pressure–high-temperature Equation of State of KCl and KBr. Phys. Rev. B, 2012, 85, No. 214105. 5. Hofneister, A.M. IR Spectroscopy of Alkali Halides at Very High Pressures:

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Calculation of Equations of State and of the Response of Bulk Moduli to the B1-B2 Phase Transition. Phys. Rev. B, 1997, 56, 5835-5855. 6. Fuith, A. The KSCN family: Structural Properties and Phase Transitions of Crystals with Three-atomic Linear Anions. Phase Transitions, 1997, 62, 1-93. 7. Freund, J.; Ingalls, R.; Crozier, E. D. Extended X-ray-absorption Fine-structure Study of Alkali-metal Halides Under High Pressure. Phys. Rev. B, 1991, 43, 9894-9905. 8. Talawar, M. B.; Sivabalan, R.; Asthana, S. N.; Singh, H. Novel Ultrahigh-Energy Materials. Combust. Explo. Shock Waves, 2005, 41, 264-277. 9. 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, 10618-10623. 10. 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, No. 164710. 11. 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. 12. 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, No. 195404. 13. Zhu, W.; Xiao, H. First-principles Band Gap Criterion for Impact Sensitivity of Energetic Crystals: a Review. Struct. Chem, 2010, 21, 657-665.

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14. Babu, K. R.; Lingam, C.; Tewari, S. P.; Vaitheeswaran, G. High-Pressure Study of Lithium Azide from Density-Functional Calculations. J Phys Chem A, 2011, 115, 4521-4529. 15. 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, 2011, 84, No. 064127. 16. 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, 736-739. 17. Ji, C.; Zheng, R.; Hou, D.; Zhu, H.; Wu, J., Chyu, M.-C.; Ma, Y. Pressure-induced Phase Transition in Potassium Azide up to 55Gpa. J. Appl. Phys., 2012, 111, No. 112613. 18. Ramesh Babu, K.; Vaitheeswaran, G. Lattice Dynamics and Electronic Structure of Energetic Solids LiN3 and NaN3: A First Principles Study. Chem. Phys. Lett, 2013, 586, 44-50. 19. 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, No. 033511. 20. 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, No. 071903. 21. Mueller, H. J.; Joebstl, J. A. High-temperature Modifications of Alkali Azides. Z. Kristallogr, 1965, 121, 385-391.

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22. Hathaway, C. E.; Temple, P. A. Raman Spectra of the Alkali Azides: KN3, RbN3, CsN3. Phys. Rev. B, 1971, 3, 3497-3503. 23. Christoe, C.W.; Iqbal, Z. Raman Scattering in Alkali Azides at High Pressure. Chem. Phys. Lett, 1976, 39, 511-514. 24. Pistorius, C. W. F. T. Phase Diagrams to High Pressures of the Univalent Azides Belonging to the Space Group D4h18I4/mcm. J. Chem. Phys, 1969, 51, No. 2604. 25. Segall, M. D.; Lindan, P. J. D.; Probert, M. J.; Pickard, C. J.; Hasnip, P. J.; Clark, S. J. Payne, M. C. First-principles Simulation: Ideas, Illustrations and the CASTEP Code. J. Phys: Condens. Matter, 2002, 14, 2717-2744. 26. Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett., 1996, 77, 3865-3868. 27. Bryant, J. I. Vibrational Spectrum of Cesium Azide Crystals. J. Chem. Phys, 1966, 45, 689-699. 28. Bryant, J. I. Infrared Vibrations of Crystalline Potassium Azide. J. Chem. Phys, 1963, 38, 2845-2854. 29. Iqbal, Z. Infrared Absorption Spectrum of KN3 Crystal: Selection Rules and Analysis of Internal ± Internal and Internal ±External Mode Absorptions. J. Chem. Phys, 1972, 57, 2422-2430. 30. Ti, S. S.; Kettle, S. F. A.; Ra, Ø. Weaker Spectral Features in the Raman Spectra of MN3, (M = K, Rb, Cs). J. Raman Spectrosc, 1977, 6, 5-12. 31. Malhotra, M. L.; Müller, K. D. Lattice Vibrations of KN3, RbN3, and CsN3. Phys. Lett, 1970, 31, 73-74.

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32. Govindarajan, J.; Haridasan, T. M. Lattice Vibrations in Alkali Azides. Phys. Lett, 1969, 28, 701-702. 33. Christoe, C. W.; Iqbal, Z. Mode Coupling and the Pressure Induced Phase Transition in Thallium Azide (TIN3). Solid State Commun, 1974, 15, 859-862. 34. Christoe, C. W.; Iqbal, Z. Raman Scattering Study of the Dyanmics of the Pressure-induced Phase Transition in Thallium Azide (TIN3). J. Phys. Chem. Solids, 1977, 38, 1391-1394. 35. Panchal, V.; López-Moreno, S.; Santamaría-Pérez, D.; Errandonea, D.; Manjón, F. J.; Rodríguez-Hernandez, P.; Muñoz, A.; Achary, S. N.; K.Tyagi, A. Zircon to Monazite Phase Transition in CeVO4: X-ray Diffraction and Raman-scattering Measurements. Phys. Rev. B, 2011, 84, No. 024111. 36. Manjón, F. J.; Rodríguez-Hernández, P.; Muñoz, A.; Romero, A. H.; Errandonea, D.; Syassen, K. Lattice Dynamics of YVO4 at High Pressures. Phys. Rev. B, 2010, 81, No. 075202. 37. Panchal, V.; Manjón, F. J.; Errandonea, D.; Rodriguez-Hernandez, P.; López-Solano, J.; Muñoz, A.; Achary, S. N.; Tyagi, A. K. High-pressure Study of ScVO4 by Raman Scattering and ab Initio Calculations. Phys. Rev. B, 2011, 83, No. 064111. 38. Wang, Z.; Lazor, P.; Saxena, S. K.; O’Neill, H. S. C. High Pressure Raman Spectroscopy of Ferrite MgFe2O4. Mater. Res. Bull, 2002, 37, 1589-1602. 39. Wang, Z.; Schiferl, D.; Zhao, Y.; O'Neill, H. S. C. High Pressure Raman Spectroscopy of Spinel-type Ferrite ZnFe2O4. J. Phys. Chem. Solids, 2003, 64,

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Figure captions Figure 1. Crystal structure of RbN3 at ambinent conditions along (a) a-b-c and (b) a-b (c) b-c axes. Blue color represents N atoms, 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 assignment 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 meaning the spectra were at a magnification of 1500, 50, and 20 times. Figure 3. (a)-(k) the simulated eigenvectors of all the vibrational modes in the primitive cell. The blue and red colored 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). Figure 4. Selected high-pressure Raman spectra of RbN3 in the region of (a) lattice modes (30-430 cm-1) (b) internal modes (1200-1500 cm-1). The dashed lines serve as visual guides. The mode guided by the green dashed line in (b) is from the diamond anvils with D-0 stands for the pressure released to 0 GPa. (c) and (d) are the corresponding Raman shift of all modes as a function of pressure with the vertical dashed line indicate the proposed phase boundaries. Figure 5. Selected high-pressure IR spectra of RbN3 in the region of Far-IR (a) 100-390 cm-1 and mid-IR (b) 600-1500 cm-1 and (c) 3150-3700 cm-1. The dashed lines

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serve as visual guides. D-0 stands for the pressure released to 0 GPa. (d)-(f) are the corresponding IR shift of all modes as a function of pressure with the vertical dashed line indicate the proposed phase boundaries. Figure 6. The simulated diagram of the evolution of the N3− with increasing pressure. The green arrows indicates the vibrational direction of the atom. (a), (b), and (c) correspond to the projections of arrangement of N3− in a-b plane in phase α, γ, and δ, respectively.

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High-Pressure Studies of Rubidium Azide by Raman and Infrared

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