NEW STRUCTURAL SYSTEMATICS IN THE II-VI, III-V AND GROUP IV SEMICONDUCTORS AT HIGH PRESSURE. R.J. Nelmes, M.I.McMahon, H. Liu and S.A. Belmonte, Department of Physics and Astronomy, The University of Edinburgh, Edinburgh, EH9 3JZ, U.K.

Diffraction studies of the core II-VI, III-V and group IV semiconductors have remained at the forefront of experimental high-pressure research since the introduction of the diamond anvil cell in the early 1960's. Recently, using angle-dispersive (AD) diffraction techniques with an image-plate area detector and synchrotron radiation, we have made a detailed re-examination of the high-pressure behaviour of most of these semiconductors. This work has yielded many new results which show that the structural systematics compiled over the last 30 years must be modified quite substantially. The supposed structures of many high -pressure phases have been found to be incorrect; new high-pressure phases and structural relationships have been found; and, contrary to the accepted picture, many of the high-pressure phases have been shown to have relatively low-symmetry structures, with variable atomic coordinates. The emerging systematics will be summarised. And the most recent new results of this programme will be presented, including: an apparently classic soft-mode transition in ZnSe, with implications for GaAs; a new and unexpected high-to-low cristobalite-type transition in HgSe and HgTe; a radical revision of the P-T phase diagram of InSb, and the phenomenon of intermediate phases as a feature of all its transitions; and the apparent fact that only the group IV elements have phases with true tetragonal or hexagonal symmetry.