Electronic structure and crystallography of MoTe2and WTe2

Calculations of electronic structure have been undertaken using ab initio tight-binding (TB) method comparing the group VIb ditellurides WTe2 and MoTe2 in their various guises. The group VIb ditellurides show deviation from a simple band model which predicts semiconducting behaviour due to a trigonal prismatic crystal-field splitting of a filled nonbonding dz2 orbital. For WTe2 and beta -MoTe2 the metal atom is displaced from the centre of an octahedron of Te atoms, and metal-metal chains with bond lengths only slightly longer than the elemental metals run along the layers. The reduced Madelung term for this configuration compensates for loss of the lone dz2-based band and thus results in semi-metallic crystals. Mo but not W occurs with a trigonal prismatic coordination ( beta -MoTe2). beta -MoTe2 differs from WTe2 only slightly-in the stacking of layers; a low-temperature polymorph is believed to stack identically to WTe2.