Structural phase transition and Au valence state of the perovskite-type Au mixed-valence compounds M(2)Au(2)X(6) (M=Rb, Cs; X=Cl, Br, I) under high pressure

We have investigated the structural P-T phase diagram, the Au valence state and the physical properties of M(2)Au(2)X(6) (M=Rb, Cs; X=Cl, Br, I) under high pressures. When the pressure is applied at room temperature, Cs(2)Au(2)X(6) (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 Au-I,Au-III --> Au-II transition, which was proven by the Au-197 Mossbauer spectroscopy under high pressure. The metallic cubic perovskite structure appearing under high pressures and high temperatures for all the title compounds, M(2)Au(2)X(6) (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 Au-II.