HIGH PRESSURE X-RAY STUDY OF AlH3 AND AlD3: ANOMALOUS ISOTOPE EFFECT. Stanislav P. Besedin and Andrew P. Jephcoat, Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX1 3PR, UK

The metal hydrides are of interest owing to a number of phenomena in which the quantum character of a light hydrogen atom in a metallic matrix is exhibited. We report synchrotron powder x-ray diffraction measurements at room temperature for the predominantly ionic (partly covalent) hydrogen-rich metal hydrides AlH3 and AlD3 in a diamond- anvil cell to maximum pressures near 53 GPa. Aluminium hydride has a rhombohedral unit cell (R-3c space group) with a=4.4493 A and c=11.8037 A in hexagonal axes. It can be regarded as "a three-dimensional polymer" all atoms of which are linked by Al-H-Al bonds. The structure can also be described in terms of a distorted hcp matrix of hydrogen atoms, in which one third of the octahedral interstices are occupied by aluminium atoms. The pressure-volume data obtained show an unexpected difference in the compressibility between isotopes, with AlD3 more compressible at all pressures. No structural phase transition from the rhombohedral aluminium lattice is observed in either isotope, which could be associated with the previous prediction of an insulator-to-metal transition at 46 GPa. On the basis of our data for ALH3, we expect that the pressure for which the atomic hydrogen volume equals the 2.8-2.9 A₃/H atom characteristic of the transition metal hydrides, is near 80 GPa, higher than suggested by previous data. The difference between the compressibilities of the isotopic modifications is interpreted in terms of zero point dynamics.