MAGNETIC-PROPERTIES AND SPECIFIC-HEAT STUDIES OF THE METAL-INSULATOR-TRANSITION IN BACO0.9NI0.1S2-Y

We have investigated the BaCo0.09Ni0.1S2-y system which crystallizes i n the tetragonal BaNiS2-type structure, by several complementary exper imental techniques. The insulating stoichiometric compound (y=0) is an tiferromagnetically ordered T-N=265 K. The sulfur-deficient compounds undergo a first-order metal-insulator (MI) transition at T-s=220 K. Mo ssbauer studies on Fe-57-doped materials show that the magnetic struct ure observed in the insulting state persists in the metallic state wit hout any discontinuity in the magnetic properties. In addition, this M I transition is accompanied by (i) a slight distortion in the tetragon al BaNiS2 structure, and by (ii) a valence transition of divalent Ni2 ions to monovalent Ni. Specific-heat measurements show an anomaly at the MI transition, and the entropy associated with this transition is 1.6 (J/mol K) and is associated with both crystallographic and valence transitions.