Phase transition and structure of silver azide at high pressure

Silver azide (AgN 3 ) was compressed up to 51.3 GPa. The results reveal a reversible second-order orthorhombic-to-tetragonal phase transformation starting from ambient pressure and completing at 2.7 GPa. The phase transition is accompanied by a proximity of cell parameters a and b , a 3° rotation of azide anions, and a change of coordination number from 4-4 (four short, four long) to eight fold. The crystal structure of the high pressure phase is determined to be in I 4 /mcm space group, with Ag at 4a , N 1 at 4d , and N 2 at 8h Wyckoff positions. Both of the two phases have anisotropic compressibility: the orthorhombic phase exhibits an anomalous expansion under compression along a -axis and is more compressive along b -axis than c -axis; the tetragonal phase is more compressive along the interlayer direction than the intralayer directions. The bulk moduli of the orthorhombic and tetragonal phases are determined to be K OT =39±5 GPa with K OT '=10±7 and K OT =57±2 GPa with K OT '=6.6±0.2, respectively.