Unit cell expansion in Laves phase-type compounds caused by interstitial hydrogen

The Laves phase-type compounds of yttrium and rare earths with manganese (RMn2) easily absorb hydrogen which locates at the interstitial sites. It is well known that the lattice expands with increasing hydrogen concentration and the volume increases up to about 30%. The increase of the unit volume is nonlinear as a function of hydrogen content. To explain this behaviour a model in which the volume changes were correlated with the changes of the bulk modulus with hydrogen concentration was proposed by Hirata and co-workers. This model describes properly the initial volume increase, but does 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 becomes large enough for hydrogen to enter. According to this observation we extended this model. We show that the calculated volume expansion fits very well to the experimental data for RMn2 (R=Y,Gd,Dy,Tb,Ho). (copyright 2007 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)