Characterization of the metallic phases in BaCo0.9Ni0.1S1.87

The temperature dependence of the resistivity along the c axis and in the a-b plane of layered BaCo0.9Ni0.1S1.87 has been measured on a single crystal under hydrostatic pressures. The monoclinic metallic (LTM) phase stable at T<~150K is suppressed under a pressure P>1.5kbar; it has a nearly isotropic conductivity. Pressure induces an insulator-metal (IM) transition at P>10kbar within the tetragonal phase. A change from a nearly isotropic conduction to a highly anisotropic conduction is associated with the IM transition. Isotropic conduction in the LTM phase is consistent with its crystallographic structure; the IM transition in the tetragonal phase has been interpreted to be due to a high-spin to low-spin transition of Co(II) ions. The resistivity and thermal conductivity of the LTM metallic phase are typical of those of a normal metal.