Electronic structure and formation energies of nonstoichiometric dichalcogenides M x X2–y (М = Nb, Mo, W; X = Se, Te)

Using density functional theory the electronic structure, chemical bond parameters, phase formation energies, and intrinsic defects in metal and non-metal sublattices of chalcogenides MX2 (M = Nb, Mo, W; X = Se, Te) are determined. For compounds with X = Te a monotonic decrease in charges on metal and non-metal atoms occurs with increasing atomic number, however, for compounds with X = Se this order is violated. With increase in the metal atomic number for both selenides and tellurides, the formation energies increases, i.e. the stability of these phases decreases. The formation energies of vacancies in both sublattices of these systems have a non-monotonic character. For MX2 (M = Mo, W; X = Se, Te) systems the formation of vacancies in the chalcogen sublattice results in the semiconductor–metal transition, and vacancies in the metal sublattice decrease the band gap.