Crystal structural variation and phase transition in caesium trichlorocuprate at high pressure

Single-crystal and powder X-ray diffraction experiments, together with Raman spectroscopy, show that the 'hexagonal perovskite' CsCuCl₃ undergoes a phase transition at pressures between 1.65 and 3.1 GPa. The transition pressure is strongly dependent upon the form of the sample, being lower in powder specimens than in single crystals. The transition is first order in character and is accompanied by a 1.8% volume charge and a reduction in the length of the c-axis from the six-layer repeat of the room pressure phase to a two-layer repeat. These observations, together with the change in Raman modes at the transition lead us to conclude that the high-pressure structure contains statically disordered Jahn-Teller-distorted CuCl₆ octahedra, in contrast to the known high-temperature phase, which contains dynamically disordered distorted octahedra, and the low-pressure, ambient-temperature structure, which displays an ordered array of distorted octahedra.