Superionic behaviour in copper (I) iodide at elevated pressures and temperatures

The structural properties of copper (I) iodide have been investigated at elevated pressures and temperatures using the neutron powder diffraction technique, to probe the effects of pressure on the superionic properties of this compound. On increasing temperature at a pressure of p = 1.30(8) GPa, three structural phase transitions are observed. The first is from the ambient temperature zincblende structured phase CuI-III to rhombohedral CuI-IV at T = 444(6) K. There is only limited cation disorder in CuI-IV which increases gradually with temperature. The preferred locations of the interstitial cations are sites between the tetrahedral and octahedral interstices within the slightly distorted face-centred cubic (f.c.c.) anion sublattice. A subsequent transition to the disordered f.c.c. structured phase CuI-I occurs at T = 694(5) K. This phase shows complete cation disorder at all measured pressures and temperatures. Finally, CuI undergoes a further phase transition at a temperature of T = 920(15) K. The first diffraction studies of this high pressure phase (labelled CuI-VII) are presented, which indicate that this phase is a body-centred cubic (b.c.c.) superionic with complete disorder of the cation sublattice. The cations are found to preferentially occupy the tetrahedral sites, in a manner similar to that in isostructural (ambient pressure) superionic phases such as and . The structural systematics of the superionic binary halide compounds and their thermally induced disorder are briefly summarized.