STRUCTURAL MODIFICATIONS WITHIN COPPER(I) IODIDE AT PRESSURES UP TO 27 KBAR

Powder neutron diffraction measurements of copper(I) iodide at a temperature of 293 K and pressures up to 27 kbar are presented. A structural phase transition is observed at a pressure of 16.3(1) kbar and involves a volume change of 4.1(1)%. Above the transition CuI is found to adopt a rhombohedral structure, space group R3mBAR with Z = 2, alpha = 7.236(3) angstrom and alpha = 33.37(2)-degrees at p = 17.4(1) kbar. The I- ions essentially retain the cubic close-packed array adopted in the ambient pressure zincblende phase, whilst the smaller Cu+ ions are rearranged over the available tetrahedral voids. No structural evidence has been found for the transitions around 4 and 5 kbar observed by other workers.