CRYSTAL STRUCTURAL VARIATION AND PHASE-TRANSITION IN CESIUM TRICHLOROCUPRATE AT HIGH-PRESSURE

Single-crystal and powder x-ray diffraction experiments, together with Raman spectroscopy, show that the 'hexagonal perovskite' CsCuCl3 unde rgoes a phase transition at pressures between 1.65 and 3.1 GPa. The tr ansition pressure is strongly dependent upon the form of the sample, b eing lower in powder specimens than in single crystals. The transition is first order in character and is accompanied by a 1.8% volume charg e and a reduction in the length of the c-axis from the six-layer repea t 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 disord ered Jahn-Teller-distorted CuCl6 octahedra, in contrast to the known h igh-temperature phase, which contains dynamically disordered distorted octahedra, and the low-pressure, ambient-temperature structure, which displays an ordered array of distorted octahedra.