ErFe2-H-2 system I. The interstitial-site occupancy by hydrogen atoms and model predictions

The validity of various models used for predicting the interstitial-site oc cupancy by H or D atoms in metal hydride systems is reviewed. The geometric al parameters of the various types of tetrahedral interstitial site have be en calculated using the Goldschmidt radii as well as compressed radii for t he constituent metal atoms. Explicit expressions for the interstitial-site coordinates, the hole radii and the intersite separations are presented. Ne utron diffraction studies have been carried out to find the D atom coordina tes and the site occupancy in the ErFe2Dx system. These findings are compar ed, vis-ri-vis, with the predictions based on the semiempirical models, wit h a particular emphasis on the composition range with 0 < x less than or eq ual to 2. It is concluded that for the system under investigation the choic e of Goldschmidt radii is more appropriate and, in view of the comparable h ole radii of the relevant interstitial sites, the relative affinity of H wi th the metal atoms (forming the interstitial hole) decides the H atom occup ancy.