Substitution effects on superconducting and normal state properties in (Mo_1–xT_x)₃Sb₇, where T = Ru and Fe

The effects of Ru- and Fe-doping on the physical properties of Mo₃Sb₇ were studied by measuring ac-susceptibility, dc-magnetization, electrical resistivity and specific heat. It is found that single-phased (Mo${}_{1-x}$Ru_x)₃Sb₇ and (Mo${}_{1-x}$Fe_x)₃Sb₇ samples are obtained for the concentrations $x⩽$ 0.1. In this concentration range, the lattice parameter shrinks with increasing dopant content. The substitution effects on the superconducting and normal state properties are completely different between two investigated systems. In the Ru-based solid solution alloys, the substitution enhances the superconducting parameters T _c and H _c2 and also benefits the spin-gap opening at $T^{\ast }\sim$ 50 K. In contrast, in the Fe-substituted samples, there is a depression of both T _c and H _c2, accompanied by vanishing spin-gap and low-dimensional magnetic correlation. The substitution of the Mo atoms by Ru and Fe atoms suppresses the cubic-tetragonal distortion, but presumably favours the magnetic field to induce a magnetic order below T ^*. (paper)