Thermal Expansion, Thermoelectric Power, and XPS Study of the Nonmetal-Metal Transition in Ni_1-xS_1-ySe_y

Magnetic susceptibility χ, electrical resistivity ρ, thermoelectric power S, XPS spectra and hexagonal lattice parameters c and a are measured on the NiAs-type Ni1_x S1-ySey (x≦0.04;. y≦ 1.00). We find the followings (1) the nonmetal-metal transition temperature Tt, the phonon-drag coefficient C below Tt and dχ(T> Tt)/dTdecrease and they are suppressed at xc and yc with increase in x and y, respectively. (2) The nearest neighboring S1-ySey(Z)-Z, Ni-Z and Ni-Ni distances just below Tt (Dz_z, Z)Ni_z, DNl-Nl, respectively) are depressed with increase in x. [formula omitted] and [formula omitted] only slightly depend on y, while [formula omitted] increases with increasing y. (3) No change in the d8L peak (L: hole in the Z network) in the valence band region is observed among Ni1-x S1Sey, and Ni0.93Te. (4) The Ni 2p3/2d9L core level peak shifts to lower binding energy EB with increasing y, while it hardly changes with x. (5) The EB of S 2s and S 2p core levels decrease with increasing x; the EB of S 2p one decreases with increasing y. From the view point of the charge-transfer model, the pressure and the Ni-vacancy substitution effects on Tt 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.