Thermal Expansion, Thermoelectric Power, and XPS Studyof the Nonmetal-Metal Transition in Ni1-xS1-ySey

Magnetic susceptibility χ, electrical resistivity ρ, thermoelectric power S , XPS spectra and hexagonal lattice parameters c and a are measured on the NiAs-type Ni 1- x S 1- y Se y ( x ≤0.04; y ≤1.00). We find the followings (1) the nonmetal-metal transition temperature T t , the phonon-drag coefficient C below T t and dχ( T > T t )/d T decrease and they are suppressed at x c and y c with increase in x and y , respectively. (2) The nearest neighboring S 1- y Se y (Z)-Z, Ni-Z and Ni-Ni distances just below T t ( D Z-Z N , D Ni-Z N , D Ni-Ni N , respectively) are depressed with increase in x . D Z-Z N and D Ni-Ni N only slightly depend on y , while D Ni-Z N increases with increasing y . (3) No change in the d 8 L peak ( L : hole in the Z network) in the valence band region is observed among Ni 1- x S 1- y Se y and Ni 0.93 Te. (4) The Ni 2p_3/2 d 9 L core level peak shifts to lower binding energy E B with increasing y , while it hardly changes with x . (5) The E B of S 2 s and S 2 p core levels decrease with increasing ; the E B of S 2 p one decreases with increasing y . From the view point of the charge-transfer model, the pressure and the Ni-vacancy substitution effects on T t are mainly explained by the increase in the Z( p ) band width, and the Se-substitution effect by the decrease in the charge-transfer energy. The Ni-vacancies also introduce extra holes in the Z( p ) band.