New superconducting ternary molybdenum chalcogenides InMo6Se8,T1Mo6S8, and T1Mo6Se8

Abstract

We present the synthesis and the physical properties of new ternary molybdenum chalcogenides $M{\mathrm{Mo}}_6{X}_8 (M=\mathrm{I}\mathrm{n},\mathrm{T}1\mathrm{};X=\mathrm{S},\mathrm{S}\mathrm{e})$ that cannot be made by the usual high-temperature techniques (except for In${\mathrm{Mo}}_6$ ${\mathrm{S}}_8$). These materials were obtained by diffusion of the ternary element into the binary phase ${\mathrm{Mo}}_6{X}_8$ at a relatively low temperature (430°C and 520°C for the sulfides and selenides, respectively). They crystallize in the rhombohedral space group $R3\mathrm{}$ ($C_3^4$) with ($a_r=6.78\mathrm{}$ Å, ${\alpha }_r=89.86\mathrm{}°$) when $M=\mathrm{In}$ and $X=\mathrm{Se}$ and with ($a_r=6.57\mathrm{}$ Å, ${\alpha }_r=88.45\mathrm{}°$) and ($a_r=6.836\mathrm{}$ Å, ${\alpha }_r=88.35\mathrm{}°$) when $M=\mathrm{T}1$ and $X=\mathrm{S}$ and Se, respectively. T1${\mathrm{Mo}}_6$ ${\mathrm{Se}}_8$ presents a large temperature-dependent paramagnetism and begins superconducting at 12.2 K, the highest value ever observed in the selenide Chevrel phases, while that of T1${\mathrm{Mo}}_6$ ${\mathrm{S}}_8$ is lower, 8.7 K. The susceptibility of In${\mathrm{Mo}}_6$ ${\mathrm{Se}}_8$ reveals the existence of two first-order transitions that have considerable hysteresis, while only one transition without hysteresis is observed for In${\mathrm{Mo}}_6$ ${\mathrm{S}}_8$. The absence of superconductivity down to 1.5 K has been confirmed for this last compound, while the homolog selenide begins superconducting at 8.2 K.