Effect of Early Transition-Metal (M = Ti, V and Cr) Doping on the Electronic Structure of Charge-Transfer Type Compound NiS Studied by Thermoelectric Power and X-Ray Photoemission Measurements

The M 2p core-level XPS peaks of doped elements in N1_xMxS are observed at the binding energies of M in "M2S3" rather than those of "MS" below and above the nonmetal-metal transition temperature (Tt), indicating that the M atom are ionized to M3+ and acts as shallow donor impuritites. Below Tt the Tl/2 contribution with negative sign to the thermoelectric power (S) is superimposed on the usual degenerate semiconductive contribution with positive sign. It overcomes the latter contribution above a concentration (xc). The value of xc increases in the order of M=Ti, V and Cr. These results indicate that the T1/2 contribution to S comes from variable range hopping through the localized states formed near EP within the narrow charge-transfer gap of the parent NiS. Above Tt the sign of S is negative in the electron-doped Ni1-xMXS as well as in the hole-doped Ni1-xS, suggesting that they have the Fermi surface topology.