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 Ni1-xMxS are observed at the binding energies of M in ''M2S3'' rather than those of ''MS'' below and above the nonmetal-metal transition temperature (T(t)), indicating that the M atom are ionized to M3+ and acts as shallow donor impuritites. Below T(t) the T1/2 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 (x(c)). The value of x(c) 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 E(F) within the narrow charge-transfer sap of the parent NiS. Above T(t) 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.