High-pressure spectroscopic study of silver azide

Abstract

The pressure-induced structural phase transition in AgN₃ was investigated using Raman and infrared spectroscopy. At ambient pressure, the Raman-active bending and asymmetric stretch modes of NNN 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 was present due to the increasing symmetrical element. The softening of the v₂(B_2u) mode and hardening of the v₂(B_3u) 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. The perpendicular arrangement of N₃⁻ in the tetragonal structure is energetically favorable and beneficial for structural stability.