Superionic behaviour in copper(I) chloride at high pressures and high temperatures

Neutron powder diffraction studies of copper(I) chloride have been performed at pressures to 7.3 kbar and temperatures to 740 K. The presence of a solid phase at pressures greater than similar to 2 kbar and temperatures greater than similar to 700 K, identified in previous differential thermal analysis studies, is confirmed. This phase, labelled CuCl-III, has a cubic structure, space group IM(3) over bar m. The chlorine anions form a body-centred cubic sublattice and the copper cations are randomly distributed over the 12(d) tetrahedral sites at (1/4, 0, 1/2) etc. Both the basic structure and the nature of the thermally induced disorder appear similar to those shown by the ambient pressure superionic phases alpha-AgI and alpha-CuBr. As a result, CuCl can no longer be considered an anomalous (non-superionic) member of the family of binary, tetrahedrally coordinated halide compounds.