High-pressure spectroscopic study of silver azide

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 N=N=N indicate the N-3(-) is nonlinear and/or asymmetrical. Upon compression, a reversible orthorhombic-to-tetragonal phase transition was observed at similar to 2.7 GPa, while the degeneration of the lattice modes was present due to the increasing symmetrical element. The softening of the v(2)(B-2u) mode and hardening of the v(2)(B-3u) mode demonstrate the rotation of the N-3(-) during the phase transition, which reveals the essential source of the a-axis expansion and the structural phase transition. The perpendicular arrangement of N-3(-) in the tetragonal structure is energetically favorable and beneficial for structural stability.