Electronic structure of theLiMgH3class of compounds: Cluster calculations

The lattice spacing and the electronic structure of ${\mathrm{LiMgH}}_3$, ${\mathrm{NaMgH}}_3$, and ${\mathrm{LiCaH}}_3$ have been calculated by modeling these hydrides as clusters with modified perovskite structure and using the self-consistent-field linear-combination-of-atomic-orbitals molecular-orbitals theory. This yields lattice constants of 3.47, 3.53, and 4.07 Å for ${\mathrm{LiMgH}}_3$, ${\mathrm{NaMgH}}_3$, and ${\mathrm{LiCaH}}_3$, respectively. The electronic structure of ${\mathrm{LiMgH}}_3$ is characteristic of an insulating material. The gap at the Fermi energy in the density of states of ${\mathrm{LiMgH}}_3$ is, however, significantly less than that of ${\mathrm{LiBeH}}_3$ and is insensitive to moderate changes in the interatomic spacing. On the other hand, ${\mathrm{LiCaH}}_3$ and ${\mathrm{NaMgH}}_3$ are both found to exhibit metallic character with the Fermi energy lying in regions of high electron density of states.