Metal-insulator transitions in ${\mathrm{BaCo}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ni}}_{\mathit{x}}$${\mathrm{S}}_{2\mathrm{-}\mathit{y}}$

We report on the electrical and magnetic properties of the layered ternary sulfide system ${\mathrm{BaCo}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ni}}_{\mathit{x}}$ ${\mathrm{S}}_{2\mathrm{-}\mathit{y}}$. Ni doped samples of ${\mathrm{BaCoS}}_2$ that contain sulfur vacancies exhibit a first-order pahse transition from a high temprature antiferromagnetic, semiconducting phase to a low temperature metallic phase. Increasing the sulfur vacancy concentration increases the Néel temperature of the two-dimensional antiferromagnetic phase and decreases the resistivity in the metallic phase. As the Ni content is increased, the system undergoes a smooth transition to a paramagnetic metal at all temperatures.