XPS study of layered oxysulfides (La1-xCaxO)Cu1-yNiyS (y=0 and y=x)

LaOCuS is a semiconductor with a layered structure in which the LaOLa and SCuS layers are stacked on one other. In (La1-xCaxO)CuS, in which La is substituted by Ca, the electrical resistivity decreases drastically with x, but metallic electrical conductivity is not observed. In (La1-xCaxO)Cu1-xNixS, in which Cu and La are simultaneously substituted by Ni and Ca, respectively, the conductivity increases remarkably with x, and a metal-insulator transition occurs at x=0.03. In the Cu 2p XPS spectra, very weak peaks corresponding to the satellite peaks of Cu2+ are observed in LaOCuS in addition to the main peaks. The intensity of these satellite peaks increases with x for (La1-xCaxO)Cu1-xNixS, while it hardly changes with x for (La1-xCaxO)CuS. In the S 2p XPS spectra, marked changes have not been observed for either system. This indicates that Cu2+ is formed by Ni substitution. The d holes thus introduced becomes mobile and the metal-semiconductor transition occurs at an appropriate hole concentration. For (La1-xCaxO)CuS, the main site where the change of the electronic state is introduced by the substitution has not been clarified. (C) 1998 Elsevier Science S.A.