The validity of the various models which were used for predicting the H or D interstitial-site occupancies in metal hydrides was reviewed. The geometrical parameters of the various types of tetrahedral interstitial site were calculated by using the Goldschmidt radii, as well as compressed radii for the constituent metal atoms. Explicit expressions were presented for the interstitial-site coordinates, the hole radii and the inter-site separations. Neutron diffraction studies were carried out in order to find the D atom coordinates and the site occupancy in the Fe2ErDx system. These findings were compared with those based upon semi-empirical models, with particular emphasis being placed on the x-value range of 0 to 2. It was concluded that, for the present system, the choice of Goldschmidt radii was more appropriate. Because of the comparable hole radii of the relevant interstitial sites, the relative affinity of H for the metal atoms forming the interstitial hole decided the H-atom occupancy.

ErFe2-H2 System I. The Interstitial-Site Occupancy by Hydrogen Atoms and Model Predictions. P.Raj, K.Shashikala, A.Sathyamoorthy, V.Siruguri, S.K.Paranjpe: Philosophical Magazine B, 1999, 79[8], 1185-94