Band Structures of Transition-Metal-Dichalcogenide Layer Compounds

The nonrelativistic augmented-plane-wave (APW) method is applied to calculate the electronic band structures of several transition-metal-dichalcogenide ($T{X}_2$) layer compounds, including materials with the $C6(1T-\mathrm{Hf}{\mathrm{S}}_2,1T-\mathrm{Ta}{\mathrm{S}}_2)$, $C27(2H-\mathrm{Ta}{\mathrm{S}}_2,2H-\mathrm{Nb}{\mathrm{Se}}_2)$, and $C7(2H-\mathrm{Mo}{\mathrm{S}}_2)$ structure types. These calculations involve crystal potentials that are derived from neutral-atom charge densities. The results of these calculations confirm that the group-$\mathrm{IV}B$ ($1T-\mathrm{Hf}{\mathrm{S}}_2$) and group-$\mathrm{VI}B$ ($2H-\mathrm{Mo}{\mathrm{S}}_2$) compounds are semiconductors; the calculated indirect band gaps of 2.7 and 1.2 eV are in reasonable agreement with the observed values of 2.0 and 1.4 eV, respectively. Metallic behavior is predicted for the intermediate group-$\mathrm{V}B$ compounds $1T-\mathrm{Ta}{\mathrm{S}}_2$, $2H-\mathrm{Ta}{\mathrm{S}}_2$, and $2H-\mathrm{Nb}{\mathrm{Se}}_2$. A novel feature of the metal $d$ bands in the $2H-T{X}_2$ compounds is the occurence of a 1-eV hybridization gap within the $d_{z^2}$ and $d_{xy}$, $d_{x^2-y^2}$ manifolds. This splits off a pair of hybridized $d$ bands which are half-filled in $2H-\mathrm{Ta}{\mathrm{S}}_2$ and $2H-\mathrm{Nb}{\mathrm{Se}}_2$ and completely filled in $2H-\mathrm{Mo}{\mathrm{S}}_2$. As a result of this hybridization gap, the valence or conduction bandwidths in each of these $2H-T{X}_2$ compounds are reduced to about 1 eV.