Structural and Electronic Properties of Magnesium Hydride Surfaces.

The Mg--MgH$_2$ system is of considerable interest in hydrogen technology, though the hydride forms very slowly. The microscopic mechanisms of hydriding/dehydriding Mg are not yet fully understood. In particular, transmission electron microscopy experiments\footnote{B. Bokhonov {\it et al.}, Mat. Lett. {\bf 5}, 218 (1987); A. Altmann and T. Schober, Scripta Met. {\bf 25}, 723 (1991).} have shown differences in the relative orientation between the MgH$_2$ lattice and the (hexagonal) Mg substrate during the formation process $[ \, (100)_{\rm hydride} \; \| \; (0001)_{\rm Mg} \, ]$, and the decomposition one $[ \, (110)_{\rm hydride} \; \| \; (0001)_{\rm Mg} \, ]$. In order to contribute to the understanding of the above orientation differences, and in general of the reactions occurring at the MgH$_2$ surface, we have performed a comparative study of the unrelaxed and relaxed (100) and (110) surfaces. The electronic structure has been calculated using the LAPW method, and the chemical bonding has been obtained from the corresponding Wannier functions. Bonding at the surface differs from that of the bulk, and strongly depends on surface orientation and relaxation.