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

The structural properties of copper (I) iodide have been investigated at el evated pressures and temperatures using the neutron powder diffraction tech nique, to probe the effects of pressure on the superionic properties of thi s compound. On increasing temperature at a pressure of p = 1.30(8) GPa, thr ee 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 increas es 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 sub sequent transition to the disordered f.c.c. structured phase CuI-I (alpha-C uI) occurs at T = 694(5) K. This phase shows complete cation disorder at ai l measured pressures and temperatures. Finally, CuI undergoes a further pha se transition at a temperature of T = 920(15) K. The first diffraction stud ies of this high pressure phase (labelled CuI-VII) are presented, which ind icate that this phase is a body-centred cubic (b.c.c.) superionic with comp lete disorder of the cation sublattice. The cations are found to preferenti ally occupy the tetrahedral sites, in a manner similar to that in isostruct ural (ambient pressure) superionic phases such as (alpha-AgI and alpha-CuBr . The structural systematics of the superionic binary halide compounds and their thermally induced disorder are briefly summarized.