Crystal Chemistry of the Ti3Sn-D, Nb4MSi-D and Pd-Ni-P Systems

Abstract: Future energy systems based on hydrogen as energy carrier require reliable ways for storing hydrogen gas in safe, clean and efficient ways. Metal hydrides absorb hydrogen gas reversibly, making them suitable for storage applications. Investigations of the crystal structures of these materials contribute to an understanding of the factors which can influence the absorption. Three systems, Ti3Sn-D, Nb4MSi-D (M=Co or Ni) and Pd-Ni-P, have been investigated in this thesis. Various solid state synthesis techniques have been used for sample preparation. The crystal structures have been studied using x-ray and neutron diffraction techniques.Three metal hydride phases were found in the Ti3Sn-D system upon hydrogenation. Deuterium occupies titanium octahedra and the applied deuterium pressure induces the phase transitions. The distances between the deuterium atoms increase from 2.47 Å in orthorhombic Ti3SnD0.80 to 4.17 Å in cubic Ti3SnD.The Nb4MSi-D system (M=Co or Ni) readily absorbs deuterium at room temperature and 90 kPa deuterium pressure to give a deuterium content of Nb4MSiD~2.5. Two interstitial voids, both coordinated by four niobium atoms arranged in a tetrahedral configuration, accommodate deuterium atoms.Two ternary phases and a solid solution of nickel in Pd3P have been synthesised and the crystal structures determined. PdNi2P is orthorhombic and crystallises in the MgCuAl2-type structure: an ordered derivative of the Re3B-type structure. Pd8Ni31P16 is a tetragonal high-temperature phase stable at 700°C with 110 atoms in the unit cell. Pd2.7Ni0.3P0.94 has the cementite-type structure with mixed occupancy of palladium and nickel at one of the two non-equivalent crystallographic metal positions.