Metal-semiconductor transition in single crystal hexagonal nickel sulphide

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Abstract
Hexagonal nickel sulphide shows a metal to semiconductor first order phase transition at a temperature TN 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-NiS1.02 in the temperature range 82-300 K. The results suggest that below TN 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 TN 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 TN.

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