Hexagonal lattice parameters a and c of the NiAs-type NiS1−ySey expand with increasing y below the solubility limit yL=0.20. The first-order transition temperature Tt(y) [between the antiferromagnetic less-conductive and the Pauli-paramagnetic high-conductive states] decreases with increasing y, and is suppressed down to liq. He temperature at the critical concentration yc=0.133. Just above Tt(y) the temperature slope of the susceptibility has positive sign at low y, and shows a y-dependence. The isothermal susceptibility of the Pauli-paramagnetic state decreases with increasing y. These experimental results are discussed on the basis of the strongly correlated features of d and p bands in NiS.
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