Phase transition and structure of silver azide at high pressure

Abstract

<p>ilver 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 <em>a</em>and <em>b</em>, 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<em>I</em>4<em>/mcm</em> space group, with Ag at <em>4a</em>, N₁ at <em>4d</em>, and N₂ at <em>8h</em> Wyckoff positions. Both of the two phases have anisotropic compressibility: the orthorhombic phase exhibits an anomalous expansion under compression along <em>a</em>-axis and is more compressive along <em>b</em>-axis than <em>c</em>-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.</p>