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 (TX₂) layer compounds, including materials with the C6(1T-HfS₂,1T-TaS₂), C27(2H-TaS₂,2H-NbSe₂), and C7(2H-MoS₂) structure types. These calculations involve crystal potentials that are derived from neutral-atom charge densities. The results of these calculations confirm that the group-IVB (1T-HfS₂) and group-VIB (2H-MoS₂) 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-VB compounds 1T-TaS₂, 2H-TaS₂, and 2H-NbSe₂. A novel feature of the metal d bands in the 2H-TX₂ compounds is the occurence of a 1-eV hybridization gap within the d_z² and dxy, d_x²-y² manifolds. This splits off a pair of hybridized d bands which are half-filled in 2H-TaS₂ and 2H-NbSe₂ and completely filled in 2H-MoS₂. As a result of this hybridization gap, the valence or conduction bandwidths in each of these 2H-TX₂ compounds are reduced to about 1 eV.