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 observ ed at the binding energies of M in ''M2S3'' rather than those of ''MS' ' below and above the nonmetal-metal transition temperature (T(t)), in dicating 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 th e thermoelectric power (S) is superimposed on the usual degenerate sem iconductive contribution with positive sign. It overcomes the latter c ontribution 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 c ontribution to S comes from variable range hopping through the localiz ed states formed near E(F) within the narrow charge-transfer sap of th e parent NiS. Above T(t) the sign of S is negative in the electron-dop ed Ni1-xMxS as well as in the hole-doped Ni1-xS, suggesting that they have the Fermi surface topology.