P-T PHASE-DIAGRAM AND GOLD VALENCE STATE OF THE PEROVSKITE-TYPE MIXED-VALENCE COMPOUNDS CS(2)AU(2)X(6) (X=CL, BR, AND I) UNDER HIGH-PRESSURES

We have elucidated the structural P-T phase diagram of Cs(2)Au(2)X(6) (X = Cl Br, and I) under high pressures and high temperatures for the first time. When the pressure is applied at room temperature, the Cs(2)Au(2)X(6) (X = Cl, Br, and I) compounds undergo a tetragonal-to-tetragonal phase transition at 11, 9, and 5.5 Gpa, respectively. This transition is regarded as a band Jahn-Teller transition driven by the Au-I,Au-III --> Au-II transition. The cubic perovskite structure appears under high pressures and high temperatures for all three compounds. For Cs2Au2Br6 and CS2-Au2I6, the cubic phase could be obtained as a metastable state at room temperature and ambient pressure. The Au valence states in the second tetragonal phase and the cubic phase are considered to be Au-II.