P-T phase diagram and Au valence state of the perovskite-type Au mixed-valence complexes M2[AuIX2][AuIIIX4] (M = K, Rb, Cs; X = Cl, Br, I)

M-2[(AuX2)-X-I][(AuX4)-X-111] (M = K, Rb, Cs; X = Cl, Br, I) are perovskite-type gold mixed-valence complexes, In this review, we describe their structural P-T phase diagram, the pressure induced Au valence transition from the Au-I,Au- III state to the Au-II one, which is coupled with a structural phase transition. The metallic cubic phase appearing under high pressure and high temperature can be obtained as a metastable phase even at ambient pressure and room temperature. In the pressure region below the Au valence transition, Cs-2[(AuX2)-X-1][(AuX4)-X-III] (X = Cl, Br, I) show metallic behavior, which is attributed to the Au valence-fluctuation. (C) 2000 Elsevier Science S.A. All rights reserved.