Metallic bipolaron phase and its re-entrant phenomenon in the Au mixed-valence compounds Cs2Au2X6(X=Cl, I) under high pressure

Cs2[AuIX2][AuIIIX4] (X=C1, Br, I) are three-dimensional halogen-bridged Au mixed-valence compounds having perovskite type structure. These compounds undergo the pressure induced Au valence transition from the AuI,III mixed-valence state to the homogeneous AuII state, which is coupled with a structural phase transition. In the pressure region below the Au valence transition, these compounds show a metallic behavior, which is attributed to a dynamic two-electron exchange between AuI and AuIII (i.e. highly mobile bipolarons). In the case of Cs2Au2I6, the electrical resistivity as a function of temperature shows a re-entrant phenomenon of the metallic bipolaron phase in the vicinity of the Au valence transition.