The Origin of the Metal-insulator Transitions in Non-stoichiometric TlCu3-xS2 and α-BaCu2-xS2

The structure-property relations of ternary copper chalcogenides, TlCu 3-x S 2 and α-BaCu 2-x S 2 are examined. The density of states, band dispersions, and Fermi surfaces of these compounds are investigated to verify the reason of the metal-insulator transitions by extended Huckel tight-binding band calculations. The origin of the metal-insulator transitions of non-stoichiometric TlCu 3-x S 2 and α-BaCu 2-x S 2 is thought to be the electronic instability induced by their Fermi surface nesting.