Relationships between intermetallic compound structure and hydride formation

Abstract

Listen

Translate article

The structures of four families of intermetallic compounds are analyzed in terms of the number, sizes, positions and symmetry properties of tetrahedral interstitial holes. The structure types considered are hexagonal AB 5 (D2 d), cubic AB 2 (C15), hexagonal AB 2 (C14) and orthorhombic AB (B f). Holes and clusters of holes are considered in terms of their suitability for hydrogen occupancy in the formation of intermetallic compound hydrides. In the cases of the hydrides LaNi 5H 6 and LaCo 5H 4, the parent hexagonal structure P6/ mmm is changed on hydriding to trigonal P31 m in the former case and to orthorhombic Cmmm in the latter case. The symmetry changes that correspond to these transformations are shown to be directly related to the amount of hydrogen absorbed; i.e. some potential sites in the parent compound are retained and others are not, depending on the structural change. In those cases where structural data for the hydride phases are not available or where the parent structure does not change on hydriding, it is shown that reasonable estimates of the extent of hydrogen absorption can be made based on the number and the sizes of available holes or hole clusters in the parent compound.