Structural Phase Transition and Au Valence State of the Perovskite-Type Au Mixed-Valence Compounds M2Au2X6(M = Rb, Cs; X = Cl, Br, I) Under High Pressure

Abstract We have investigated the structural P-T phase diagram, the Au valence state and the physical properties of M2Au2X6 (M= Rb, Cs; X= Cl, Br, I) under high pressures. When the pressure is applied at room temperature, Cs2Au2X6(X=Cl, Br, I) and Rb2Au2I6 undergo a tetragonal-to-tetragonal phase transition at 11 GPa, 9 GPa, 5.5 GPa and 8.5 GPa, respectively. This transition is regarded as a band Jahn-Teller transition driven by the AuI,III→ AuII transition, which was proven by the 197Au Mössbauer spectros-copy under high pressure. The metallic cubic perovskite structure appearing under high pressures and high temperatures for all the title compounds, M2Au2X6(M=Rb, Cs; X=Cl, Br, I) could be obtained as a metastable state at room temperature and ambient pressure. The Au valence states in the cubic phase are concluded to be AuII.