ErFe2-H2 system I. The interstitial-site occupancy by hydrogen atoms and model predictions

Abstract The validity of various models used for predicting the interstitial-site occupancy by H or D atoms in metal hydride systems is reviewed. The geometrical parameters of the various types of tetrahedral interstitial site have been calculated using the Goldschmidt radii as well as compressed radii for the constituent metal atoms. Explicit expressions for the interstitial-site coordinates, the hole radii and the intersite separations are presented. Neutron diffraction studies have been carried out to find the D atom coordinates and the site occupancy in the ErFe2Dx, system. These findings are compared, vis-a-vis, with the predictions based on the semiempirical models, with a particular emphasis on the composition range with 0 < x ⩽ 2. It is concluded that for the system under investigation the choice of Goldschmidt radii is more appropriate and, in view of the comparable hole radii of the relevant interstitial sites, the relative affinity of H with the metal atoms (forming the interstitial hole) decid...