Phase transitions of cesium azide at pressures up to 30 GPa studied using in situ Raman spectroscopy

Abstract

Cesium azide has been studied by Raman spectroscopy at pressures up to %30 GPa at room temperature. The sequence of phase transitions to Phase III (at 0.5 GPa), Phase IV (at 4.3 GPa), and Phase V (at %19 GPa) has been observed in agreement with recent X-ray diffraction studies. Phase III has been found to adopt a monoclinic C2/m structure with two azide anions in nonequivalent positions, where one set of azide anions appears to be orientationally disordered according to the observed Raman spectra. The transition to Phase IV has been associated with orientational ordering of azide anions, while the transition to Phase V has been shown to proceed with a lowering of crystal symmetry. Moreover, spectroscopic features indicate a possible change of bonding in CsN 3 toward formation of covalent bonds at high pressures. V

Lithium Pentazolate Synthesized by Laser Heating-Compressed Lithium Azide and Nitrogen

“…The broad peak around 635 cm –1 marked by a black solid point can be attributed to the IR-active bending mode of N 3 – . Due to the pressure-induced distortion of azide ions, the IR-active bending vibration mode of N 3 – around 640 cm –1 becomes Raman-active and can be observed in Raman measurements (e.g., NaN 3 and CsN 3 ) under high pressures. , The Raman spectrum indicates that the LiN 3 remains stable in the C 2/ m crystal structure, and the N 2 has occurred phase transitions to ε-N 2 , as reported at 41.1 GPa. Meanwhile, after laser heating, all the Raman modes that belong to LiN 3 disappear.…”

High-Pressure-Induced Structural and Chemical Transformations in NaN3

“…The vibrational mode at 642 cm –1 is attributed to the IR-active bending mode of N 3 – . Due to the pressure-induced distortion of the azide group, this IR-active bending vibration mode of N 3 – becomes Raman active and can be observed in azides under high pressure. , The Raman modes at 865, 1187, and 1839 cm –1 do not belong to any internal modes of N 3 – , so it can be suggested that they originate from a pressure-induced chemical transformation to new kinds of Na–N compounds instead of the structural transition (γ-NaN 3 to δ-NaN 3 ) reported by Zhu et al The hexagonal P 6/ m -NaN 3 has been predicted in compressed NaN 3 , which is a metallic phase without Raman-active modes according to the theoretical calculations. So our spectral observation of cold-compressed NaN 3 shows unique pressure-induced chemical transformation behaviors.…”

High-Pressure Raman and Infrared Spectroscopic Studies of Cesium Azide

“…Additionally, CsN 3 was found to has a relatively low polymerization pressure compared with LiN 3 , NaN 3 , and KN 3 . Although amount of experimental and theoretical studies about CsN 3 have been carried out, several problems about CsN 3 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. , (2) The pressure-induced evolution of T­(E g ) mode has been found to trigger structural phase transitions in the tetragonal KN 3 , RbN 3 , and TlN 3 ; ,, however, it was absent in recent high-pressure study of CsN 3 due to the limitation of measuring range . (3) The high-pressure behaviors of NNN bending vibrations of CsN 3 have not yet been conducted so far, which have been found to play a significant role for exploring the evolution of azide ions under pressure. , …”

“…(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(E g ) mode has been found to trigger structural phase transitions in the tetragonal KN 3 , RbN 3 , and TlN 3 ; 14,16,27 however, it was absent in recent high-pressure study of CsN 3 due to the limitation of measuring range. 25 (3) The high-pressure behaviors of NNN bending vibrations of CsN 3 have not yet been conducted so far, which have been found to play a significant role for exploring the evolution of azide ions under pressure.…”

“…17,25 (2) The pressure-induced evolution of T(E g ) mode has been found to trigger structural phase transitions in the tetragonal KN 3 , RbN 3 , and TlN 3 ; 14,16,27 however, it was absent in recent high-pressure study of CsN 3 due to the limitation of measuring range. 25 (3) The high-pressure behaviors of NNN bending vibrations of CsN 3 have not yet been conducted so far, which have been found to play a significant role for exploring the evolution of azide ions under pressure. 16,28 Therefore, the significance and absence of high-pressure spectroscopic study of CsN 3 prompted our endeavor to present more in-depth investigations to explore its structural behavior, which may bring in new structures and properties for the highly energetic polymeric nitrogen.…”