High-Pressure Studies of Rubidium Azide by Raman and Infrared

Article pubs.acs.org/JPCC 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, 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 ions which might result in the formation the past several decades due to their peculiar structures and of polymeric nitrogen. − 1 3 Under ambient conditions, RbN3 crystallizes into a body- physicochemical properties. Their important applications as 18 initial explosives, 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. ethanol 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.

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