New superconducting ternary molybdenum chalcogenides In Mo 6 Se 8 ,T1 Mo 6 S 8 , and T1 Mo 6 Se 8

We present the synthesis and the physical properties of new ternary molybdenum chalcogenides $M{\mathrm{Mo}}_{6}{X}_{8} (M=\mathrm{I}\mathrm{n},\phantom{\rule{0ex}{0ex}}\mathrm{T}1;X=\mathrm{S},\phantom{\rule{0ex}{0ex}}\mathrm{S}\mathrm{e})$ that cannot be made by the usual high-temperature techniques (except for In${\mathrm{Mo}}_{6}$${\mathrm{S}}_{8}$). These materials were obtained by diffusion of the ternary element into the binary phase ${\mathrm{Mo}}_{6}{X}_{8}$ at a relatively low temperature (430\ifmmode^\circ\else\textdegree\fi{}C and 520\ifmmode^\circ\else\textdegree\fi{}C for the sulfides and selenides, respectively). They crystallize in the rhombohedral space group $R3$ (${C}_{3}^{4}$) with (${a}_{r}=6.78$ \AA{}, ${\ensuremath{\alpha}}_{r}=89.86\ifmmode^\circ\else\textdegree\fi{}$) when $M=\mathrm{In}$ and $X=\mathrm{Se}$ and with (${a}_{r}=6.57$ \AA{}, ${\ensuremath{\alpha}}_{r}=88.45\ifmmode^\circ\else\textdegree\fi{}$) and (${a}_{r}=6.836$ \AA{}, ${\ensuremath{\alpha}}_{r}=88.35\ifmmode^\circ\else\textdegree\fi{}$) when $M=\mathrm{T}1$ and $X=\mathrm{S}$ and Se, respectively. T1${\mathrm{Mo}}_{6}$${\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.