Single crystal X-ray diffraction study of a mixed-valence gold compound, Cs2AuIAuIIICl6 under high pressures up to 18 GPa: Pressure-induced phase transition coupled with gold valence transition

We performed the single-crystal X-ray diffraction study of a perovskite-type gold mixed-valence compound, Cs2AuIAuIIICl6, under high pressures up to 18 GPa by using a diamond-anvil-cell with helium gas as an ideal hydrostatic pressure-transmitting medium. The lattice parameters and the variable atomic positional parameters were obtained with reasonable accuracy at various pressures. A structural phase transition at ca. 12.5 GPa from I4/mmm to Pm3m was found. The lattice parameters a0 and c0, denoted in the tetragonal cell setting, result in the relationship 21/2a0=c0, and the superstructure reflections h k l (l is odd), caused by the shift of the Cl ions from the midpoint of the Au ions, disappeared at pressures above the phase transition. Both elongated [AuIIICl6] and compressed [AuICl6] octahedra in the low-pressure phase smoothly approach regular octahedra with increasing pressure. Above the structural phase transition at 12.5 GPa, all the [AuCl6] octahedra are crystallographically equivalent, which shows that the tetragonal-to-cubic phase transition accompanies the valence transition from the AuI/AuIII mixed-valence state to the AuII single-valence state.