Nature of chemical bond and phase stability of hydrogen storage compounds

Our recent investigations on the nature of the chemical bond and phase stability of hydrogen storage compounds and their hydrides are reviewed in search of a new method for materials design. A series of electronic structure calculations has been performed using the DV-Xα molecular orbital method. It is found that hydrogen interacts more strongly with hydride non-forming elements, B, (e.g. Ni, Mn, Fe) than hydride forming elements, A, (e.g. La, Zr, Ti, Mg) in every hydrogen storage compound, despite that there is a larger affinity of A element for hydrogen than B element in the binary metal-hydrogen system. The stability of hydrides can be elucidated in terms of the nature of the chemical bond between atoms in a small polyhedron where hydrogen is stored and also of the crystal structural evolution in the course of hydrogenation. In addition, it is shown that the A/B compositional ratio of hydrogen storage compounds is predictable using a simple parameter, 2×Bo(AB)/[Bo(AA)+Bo(BB)], where Bo(AB), Bo(AA) and Bo(BB) are the bond orders between atoms given in the parentheses.