HIGH PRESSURE - HIGH TEMPERATURE STUDIES OF SUPERIONIC CONDUCTORS. S. Hull, D. A. Keen, N. J. G. Gardner and W. Hayes^{[[daggerdbl]]}. ISIS Science Division, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX. U.K. ^{[[daggerdbl]]}Clarendon Laboratory, Parks Road, Oxford, OX1 3PU. U.K.

The results of recent neutron diffraction investigations of the structural behaviour of model superionic compounds at elevated pressures and temperatures are presented. CuCl, CuBr, CuI and AgI form a family of tetrahedrally co-ordinated compounds which transform under hydrostatic pressure to octahedrally co-ordinated phases (rocksalt or distorted rocksalt structures) and have been widely studied at elevated temperatures because (with the exception of CuCl) they undergo structural transitions to superionic phases characterised by exceptionally high cation conductivity. The pressure induced transition from 4-fold to 6-fold co-ordination occurs via intermediate phases of lower symmetry. Indirect evidence exists in the literature suggesting that some of the high pressure phases also exhibit superionic behaviour. This paper describes the first structural investigations of these effects, using neutron diffraction studies of powder samples to ~1.2GPa and ~800K.

Specifically, the following results are presented.

A superionic phase is observed at 0.8GPa and 700K in CuCl, which has no superionic phase at ambient pressure.

The onset of disorder in the high pressure rocksalt structured phase of AgI is observed.

The high pressure rhombohedral phase of CuI shows highly anisotropic lattice expansion on increasing temperature, but little cation disorder.

Finally, a brief description is given of the high pressure high temperature cell which is capable of taking some 2cm3 of sample to 1.2GPa and 1000K and is optimised for fixed geometry, energy dispersive neutron diffraction.