TRENDS IN THE ELECTRONIC-STRUCTURE OF INTERMETALLIC HYDRIDES - M2MH6 (M = MG, CA, SR, M = FE, RU, OS)

The electronic structure of the recently synthetized and characterized intermetallic hydrides M2M'H6 (M = Mg, Ca, Sr, M' = Fe, Ru, Os) is investigated by means of an ab initio determination of their energy bands and densities of states using the augmented plane wave method. These systems which are isoelectronic and isostructural to Mg2FeH6 are all found to be non-conducting in agreement with experimental observations. These compounds are characterized by an octahedral coordination of the transition metal (TM) element with hydrogen which results in a strong covalent interaction between the TM-d orbitals of e(g) symmetry with the H-s states while the TM-d orbitals of t2g symmetry lead to weakly interacting narrow d states. The presence of d states from the divalent metals Ca and Sr modifies the nature of the conduction bands. We discuss the trends in the values of the energy gaps and the ordering of the states in the series of compounds in terms of differences in metal-hydrogen distances and atomic energy levels.