Theoretical study of lithium graphite. I. Band structure, density of states, and Fermi-surface properties

The results of a detailed band-structure calculation for first-stage lithium graphite (Li${\mathrm{C}}_{6}$) are presented. In addition to the band dispersion, the density of states near the Fermi level, the shape of the Fermi surface, plasma frequencies, optical masses, de Haas-van Alphen frequencies and masses, and interband optical transitions are obtained. It is found that the occupied bands of Li${\mathrm{C}}_{6}$ are essentially those of graphite with $A\ensuremath{-}A$ layer stacking and $\frac{1}{6}$ excess electron per C atom. Except for some hybridization with the Li $2s$ states, the dispersion of the occupied bands in a layer plane is in quantitative agreement with the corresponding dispersion for two-dimensional graphite, as calculated by previous workers. The Fermi level of Li${\mathrm{C}}_{6}$ corresponds to an energy near a saddle point in the $\ensuremath{\pi}$ bands of two-dimensional graphite. The Li $2s$ states are found to hybridize with a bonding C $\ensuremath{\pi}$ band \ensuremath{\sim} 7-9 eV below ${E}_{F}$ and to form a metal-like band having a minimum \ensuremath{\sim} 1.7 eV above ${E}_{F}$. Hybridization of the Li $2s$ states with the Fermi-level bands is weak, so that the metallic properties of Li${\mathrm{C}}_{6}$ are derived from partially filled bands which have primarily C $\ensuremath{\pi}$ character. The present results are found to be consistent with experimental measurements of the Fermi-level density of states and of the plasma frequencies.