Insulator-metal transition inBaCo0.9Ni0.1S2−ySey

Compounds of the Se-doped layered transition-metal sulfide series ${\mathrm{BaCo}}_{0.9}{\mathrm{Ni}}_{0.1}{\mathrm{S}}_{2\ensuremath{-}y}{\mathrm{Se}}_{y} (yl~0.4)$ were prepared and structural, magnetic, and electrical properties were investigated. These compounds exhibit the unusual first-order antiferromagnetic semiconductor to paramagnetic metal transition with decreasing temperature that was previously seen in the sulfur-deficient ${\mathrm{BaCo}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}{\mathrm{S}}_{2\ensuremath{-}y}$ 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 susceptibility measurements are used to construct the phase diagram for ${\mathrm{BaCo}}_{0.9}{\mathrm{Ni}}_{0.1}{\mathrm{S}}_{2\ensuremath{-}y}{\mathrm{Se}}_{y}.$ Doping with Se increases the insulator-metal transition temperature, but has very little effect on other properties since it substitutes for S only in the BaS layers.