Electronic Phase Diagram of Mixed-Valence Gold Chloride

A comprehensive electronic phase diagram is derived for mixed- valence gold chloride Cs₂Au₂Cl₆ by means of high-pressure Raman spectroscopy. At all the temperatures investigated (100 K ≤ T ≤ 300 K), applications of pressure induce a phase transition from the mixed-valence (MV) state to a single-valence (SV) state. In the SV state, a broad Au-Cl stretching band appears at ～ 400cm^-1 below ～ 200 K. We have interpreted the appearance of the band in terms of formation of the AuX^-₂-like local lattice distortion, or the bipolarons.