Energy bands and bonding in LaB6and YB6

Energy bands of the "covalent metal" La${\mathrm{B}}_6$ have been calculated by a discrete variational method in the Hartree-Fock-Slater model. We find that the basic topology of the bands and the predicted Fermi surface are rather insensitive to the atomic configuration assumed in constructing the potential. The proposed Fermi surface is consistent with the basic features of published experimental de Haas-van Alphen data; it is not consistent with the hypothesis of magnetic breakdown which has been invoked to explain the finer details. A study of selected valence-band and conduction-band wave functions supports the conclusion that La-B bonding is more important than La-La bonding in explaining the metallic behavior of La${\mathrm{B}}_6$. The total density of states and the interband joint density of states for several bands are calculated and the results are compared to x-ray and optical data. Preliminary calculations of the energy bands of Y${\mathrm{B}}_6$ are presented; the similarity of these bands to those of La${\mathrm{B}}_6$ indicates that the relatively high superconducting transition temperature of Y${\mathrm{B}}_6$ cannot be explained by simple density-of-states arguments.