The Laves phase-type compounds of Y and rare earths with Mn (RMn2) easily absorbed H which located at the interstitial sites. It was well known that the lattice expands with increasing H concentration and the volume increased up to about 30%. The increase of the unit volume was non-linear as a function of H content. To explain this behaviour a model in which the volume changes were correlated with the changes of the bulk modulus with H concentration was proposed by Hirata and co-workers. This model described properly the initial volume increase, but did not reflect the changes at higher concentrations. For the hydrides with R = Y, Gd, Er, Dy, Tb, Ho at x ≈ 3 the volume of AB3 interstitials became large enough for H to enter. It was shown that the calculated volume expansion closely fitted the experimental data for RMn2 (R = Y, Gd, Dy, Tb, Ho).

Unit Cell Expansion in Laves Phase-Type Compounds Caused by Interstitial Hydrogen. H.Figiel, S.Osuchowski, A.Paja: Physica Status Solidi A, 2007, 204[10], 3286-90