On the possibility of metal hydride formation - Part II: Geometric considerations

A simple analysis based on crystal geometry is proposed as an aid in predicting the formation of hydrides from intermetallic compounds. It is derived from a slight variation Westlake's empirical geometric criterion that an interstitial site should have a radius greater than or equal to 0.4 Angstrom to be occupied by hydrogen. For comparison this criterion has been applied to a compound which hydrides (LaNi5) and to a compound with a similar crystal structure (MgNi3B2) which does not. When applied to the LaNi5-H system, this criterion predicts the occupation of the 6m and 12n sites in the P6/mmm alpha solid solution phase LaNi5H<1, the 6m and 12n (similar to 6i) sites in the beta hydride phase LaNi5H3, and 6m(6c), 6i(6c) and 4h(2b) sites of the gamma hydride phase LaNi5H6-7 (P6(3)mc). The compound MgNi3B2, on the other hand, has a densely packed crystal structure with almost no interstitial sites satisfying the Westlake criterion. Because of its relative simplicity, this type of analysis may prove to be a useful aid in the discovery and development of new intermetallic hydrides. (C) 1998 Elsevier Science S.A.