Insulator-metal transition in BaCo0.9Ni0.1S2-ySey

Compounds of the Se-doped layered transition-metal sulfide series BaCo0.9Ni 0.1S2-ySey, (y less than or equal to 0.4) were prepared and structural, mag netic, and electrical properties were investigated. These compounds exhibit the unusual first-order antiferromagnetic semiconductor to paramagnetic me tal transition with decreasing temperature that was previously seen in the sulfur-deficient BaCo1-xNixS2-y series. This transition is associated with a major structural change. The previously unknown crystal structure of the low-temperature phase is reported. Electrical resistance and magnetic susce ptibility measurements are used to construct the phase diagram for BaCo0.9N i0.1S2-ySey. Doping with Se increases the insulator-metal transition temper ature, but has very little effect on other properties since it substitutes for S only in the BaS layers.