ON THE POSSIBILITY OF METAL HYDRIDE FORMATION - PART II - GEOMETRIC CONSIDERATIONS

A simple analysis based on crystal geometry is proposed as an aid in p redicting the formation of hydrides from intermetallic compounds. It i s derived from a slight variation Westlake's empirical geometric crite rion that an interstitial site should have a radius greater than or eq ual to 0.4 Angstrom to be occupied by hydrogen. For comparison this cr iterion has been applied to a compound which hydrides (LaNi5) and to a compound with a similar crystal structure (MgNi3B2) which does not. W hen applied to the LaNi5-H system, this criterion predicts the occupat ion of the 6m and 12n sites in the P6/mmm alpha solid solution phase L aNi5H<1, the 6m and 12n (similar to 6i) sites in the beta hydride phas e LaNi5H3, and 6m(6c), 6i(6c) and 4h(2b) sites of the gamma hydride ph ase LaNi5H6-7 (P6(3)mc). The compound MgNi3B2, on the other hand, has a densely packed crystal structure with almost no interstitial sites s atisfying the Westlake criterion. Because of its relative simplicity, this type of analysis may prove to be a useful aid in the discovery an d development of new intermetallic hydrides. (C) 1998 Elsevier Science S.A.