Valence Transition and Phase Diagram of Mixed Valence Complexes under High Pressure

Abstract

Cs₂Au₂X₆ (X=Cl, Br, I) are mixed valence complexes with Perovskite-type structure. Recently, we have elucidated the structural P-T phase diagram of these complexes under high pressures and high temperatures for the first time. When the pressure is applied at room temperature, Cs₂Au₂X₆ (X=Cl, Br, I) undergo a tetragonal-to-tetragonal phase transition at 11 GPa, 9 GPa and 5. 5 GPa, respectively. This transition is regarded as a band Jahn-Teller transition driven by the Au^{I, III}→Au^II transition. The cubic perovskite structure appears under high pressures and high temperatures for all the three complexes. The cubic phase is obtained as a metastable state at room temperature and ambient pressure. The Au valence states in the second tetragonal phase and the cubic phase are considered to be Au^II.