The High-pressure Studies Of Alkali Metal Azides And Silver Azide

Recently,azides,as the important members of nitrogen-rich materials,have become the precursor for the synthesis of the high energy density material-polymeric nitrogen.High-pressure studies about these compounds will provide new methods and experimental data for synthetic of the polymeric nitrogen.In this paper,we present the high-pressure studies of rubidium azide?RbN₃?,silver azide?AgN₃?,cesium azide?Cs N₃?and sodium azide?NaN₃?using the X-ray diffraction,Raman scattering spectroscopy,Infrared absorption spectroscopy,and theoretical simulation methods.The main achievements are as follows:1.We present the in-situ X-ray diffraction studies of RbN₃ up to 42.0 GPa at room temperature.Two pressure-induced phase transitions of?-RbN₃ ? ?-RbN₃? ?-RbN₃ were revealed at 6.5 and 16.0 GPa,respectively.During the phase transition of ?-RbN₃ ? ?-RbN₃,lattice symmetry decreases from a fourfold to a twofold axis accompanied by a rearrangement of azide anions.The ?-RbN₃ was identified to be a monoclinic structure with C2/m space group.Upon further compression,an orthogonal arrangement of azide anions becomes energetically favorable for ?-RbN₃.The?-RbN₃ was identified to be a orthogonal structure with P222/Pmm2/Pmmm space group.The compressibility of ?-RbN₃ is anisotropic due to the orientation of azide anions.The bulk modulus of ?-RbN₃ is 18.4GPa,quite close to those of KN₃ and Cs N₃.By comparing the phase transition pressures of alkali azides,their ionic character is found to play a key role in their pressure-induced phase transitions.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 experimental and calculated methods.Detailed spectroscopic analyses reveal two phase transitions at 6.5 and ¹6.0 GPa,respectively.Upon compression,the shearing distortion of the unit cell induces the displacive structural transition of phase ? ? ?.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 ?????.Additionally,the pressure-induced non-linear/asymmetric existence of N=N=N and the two crystallographically nonequivalent sites of N₃-were observed in phase ?.2.In this work,we present the effects of high pressure on the structure and stability of cesium azide?Cs N₃?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.The 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 stretchn1mode 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.3.The high-pressure Raman and IR measurements of NaN₃ were performed at room temperature with pressure up to 35 and 26 GPa,respectively.All of the fundamental vibrational modes were analyzed based on experimental and theoretical methods.Three phase transitions of ?-NaN₃ ? ?-NaN₃ ? ?-NaN₃ ? ?-NaN₃ were revealed around 0.5,14.0,and 27.6 GPa.The phase transition from ?-NaN₃ to ?-NaN₃ is accompanied with the shearing distortion of unit cell and the rotation of the azide ions with increasing pressure.The azide ions evolved into crystallographically nonequivalent sites upon compression revealed by the splitting of the internal modes.The abnormal symmetric evolution of bending modes in IR measurements reveals rotational behavior of azide groups upon compression.Moreover,the azide ions probably evolve into an energetically favorable perpendicular arrangement under higher pressure.4.The pressure-induced structural phase transition in AgN₃ was investigated using Raman and infrared spectroscopy with the pressure up to 24.0 and 13.0 GPa,respectively.At ambient pressure,the Raman-active bending and asymmetric stretch modes of N=N=N indicate the N₃-is nonlinear and/or asymmetrical.Upon compression,a reversible orthorhombic-to-tetragonal phase transition was observed at ².7 GPa,while the degeneration of the lattice modes were present due to the increasing symmetric element.Thesoftening of the v 2?B2u?mode and hardening of the v 2?B3u?mode demonstrate the rotation of the N₃-during the phase transition,which reveals the essential source of the a-axis expansion and the structural phase transition.Moreover,the perpendicular arrangement of N₃-in the tetragonal structure is energetically favorable and beneficial for structural stability.