Hydrogen site energetics in LaNi₅H_n and LaCo₅H_n: Toward predicting hydrides

We have investigated the energetics of hydrogen site occupation in LaNi₅H_n (hexagonal P63mc structure) and LaCo₅H_n (orthorhombic Cmmm structure) via calculation of the site-dependent enthalpies of hydride formation ΔH. For each structure ΔH was determined for a broad variety of hydrogen configurations. In LaNi₅H_n (LaCo₅H_n) we find that the minimum ΔH occurs for hydrogen filling of the 2b6c16c2 (4e4h) sites, precisely those identified by neutron diffraction. Hydrogen-richer hydrides are predicted for both structures, in qualitative agreement with experiments performed at higher pressures.