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 R m with Z = 2, a = 7.236(3) Å and α = 33.37(2)° 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.