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FeS and NiS show first order phase transitions with a change in electronic structure at Ta = 415 K and Tt = 265 K respectively, the latter depending sensitively on non-stoichiometry in the form of nickel vacancies. Extensive experimental data on NiS is reviewed which shows that the low-temperature phase is a semimetallic itinerant-electron antiferromagnet and the high temperature phase a weakly-correlated Pauli paramagnetic metal. Less complete data on FeS suggests the low temperature phase is a degenerate antiferromagnetic semiconductor and the hight-emperature phase an antiferromagnetic poor metal where the iron retains its local moment. In both compounds the increase in lattice entropy is an important contribution to the increase in entropy on heating through the transitions. In NiS, the electronic contribution provides approximately one fourth of the total entropy of transition due to the eight fold increase in density of states at the Fermi level at Tt whereas in FeS, there is a large magnetic contribution due to weakening of the exchange at Tα. Ni0-95S is a weakly-correlated paramagnetic metal at all temperatures and iron impurities show Kondo-like behaviour at a concentration below 1 %, and dilute antiferromagnetic or spin-glass-like order at higher concentrations. A preliminary T : x : δ phase diagram for the system (Ni1-xFex)1-δS shows three distinct regions : I. antiferromagnetic order of both Ni and Fe, II. paramagnetic Ni and Fe, Ni being a Pauli paramagnet and Fe a Curie-Weiss paramagnet or Kondo-like, III. Fe magnetically ordered while Ni remains Pauli paramagnetic

METAL-NON METAL TRANSITIONS IN TRANSITION METAL COMPOUNDS.THE ELECTRONIC PHASE TRANSITIONS IN FeS AND NiS