Cubic MgH2 stabilized by alloying with transition metals : a density functional theory study

The stability of two crystallographic modifications of Mg-transition metal (Sc, Ti, Zr, Hf) dihydrides was studied using density functional theory. Beyond a certain transition metal content, the rutile structure characteristic of pure MgH2 is no longer stable, and the hydride transforms into a fluorite-type structure, similar to that of the transition metal dihydride. The transition point for both Mg–Sc and Mg–Ti hydrides is estimated to be at 20 at.% Sc/Ti, which is in very good agreement with previous experimental studies. For Mg–Zr and Mg–Hf, no experimental data are available for comparison, so the calculations have a predictive value for these systems. For Zr and Hf, the transition point is predicted to be at lower transition metal content than for Sc and Ti, at 13 at.%. This means that the Mg–Zr hydride is also of practical importance, because a fluorite structured hydride is predicted with a hydrogen content in excess of 6 wt.%.