Gold Valence Transition and Phase Diagram in the Mixed-Valence Complexes, M2[AuIX2][AuIIIX4] (M = Rb, Cs; X = Cl, Br, and I)

In this paper, I review the structural and physical properties of the gold mixed-valence complexes, M2[AuIX2][AuIIIX4] (M = Rb, Cs; X = Cl, Br, and I), having perovskite-type structure. In spite of the three dimensional perovskite-type structure, the charge transfer interaction between AuI and AuIII in these complexes is two-dimensional. These complexes undergo the pressure-induced Au valence transition from the mixed valence state of AuI,III to the single valence state of AuII, which is coupled with a structural phase transition. In the pressure region below the Au valence transition, these complexes show a metallic behavior, which would be attributed to a dynamic two-electron exchange between the AuI and AuIII states (i.e. highly mobile bipolarons). The metallic cubic phase appearing commonly for these complexes under high pressure and high temperature can be obtained as a metastable phase even at ambient pressure and room temperature. Moreover, as a recent topic, I report the photo-induced Au valence transition for Cs2[AuIBr2][AuIIIBr4].