Metal-semiconductor transition in single crystal hexagonal nickel sulphide

Hexagonal nickel sulphide shows a metal to semiconductor first order phase transition at a temperature T_N which depends on sulphur content. Thermoelectric power, conductivity, Hall and magnetic susceptibility measurements have been made on five single crystals of compositions ranging from NiS-NiS_1.02 in the temperature range 82-300 K. The results suggest that below T_N the Ni 3d states are essentially localized or form a narrow band. The predominant mechanism of conduction is by holes in a wide band. The discontinuity in conductivity at T_N is most likely due primarily to a change in mobility. Impurity band conduction occurs at low temperatures. The susceptibility curves have a markedly different shape from that normally expected for an antiferromagnet. This results from the first order character of the phase transition at T_N.