Gold valence transition and phase diagram in the mixed-valence complexes, M-2[(AuX2)-X-I][(AuX4)-X-III] (M = Rb, Cs; X = Cl, Br, and I)

In this paper, I review the structural and physical properties of the gold mixed-valence complexes, M-2[(AuX2)-X-I][(AuX4)-X-III] (M = Rb, Cs; X = Cl, Br, and I), having perovskite-type structure. In spite of the three dimens ional perovskite-type structure, the charge transfer interaction between Au -I and Au-III in these complexes is two-dimensional. These complexes underg o the pressure-induced Au valence transition from the mixed valence state o f Au-I,Au-III to the single valence state of Au-II, which is coupled with a structural phase transition. In the pressure region below the Au valence t ransition, these complexes show a metallic behavior, which would be attribu ted to a dynamic two-electron exchange between the Au-I and Au-III states ( i.e. highly mobile bipolarons). The metallic cubic phase appearing commonly for these complexes under high pressure and high temperature can be obtain ed as a metastable phase even at ambient pressure and room temperature. Mor eover, as a recent topic, I report the photo-induced Au valence transition for Cs-2[(AuBr2)-Br-I] [(AuBr4)-Br-III].