Crystal chemistry of the G-phases in the {Ti, Zr, Hf}-Ni-Si systems

摘要

Ternary compounds M6Ni16Si7 (M = Ti, Zr, Ho have been investigated by X-ray powder/single crystal and neutron powder diffraction. Compounds with Zr and Hf crystallize in the ordered Th6Mn23 type Mg6Cu16Si7 -type, space group Fm (3) over barm), whereas Ti6Ni16.7Si7 contains an additional Ni atom partially occupying the 24e site (M2 site, x = 0.4637,0,0; occ. = 0.119) inside a Ti octahedron; Ti atoms occupy a split position. Ti6Ni16.7Si7 represents a new variant of the filled Th6Mn23 type structure. Ab initio calculations confirm the structural difference: additional Ni atoms favour the 24e site for Ti6Ni16.7Si7, however, for the Zr and Hf-based compounds the unoccupied site renders an energetically lower ground state. Enthalpies of formation of Ti6Ni17Si7, Zr6Ni16Si7, and Hf6Ni16Si7 were calculated to be -68.65, -74.78, and -78.59kJ/(mol of atoms), respectively. (c) 2006 Elsevier Inc. All rights reserved.