Synthesis and peculiar properties of ${\mathrm{InMo}}_6$${\mathrm{S}}_{8\mathrm{-}\mathrm{x}}$${\mathrm{Se}}_{\mathrm{x}}$, ${\mathrm{TlMo}}_6$${\mathrm{S}}_{8\mathrm{-}\mathrm{x}}$${\mathrm{Se}}_{\mathrm{x}}$, and ${\mathrm{Hg}}_{\mathrm{y}}$${\mathrm{Mo}}_6$${\mathrm{S}}_{8\mathrm{-}\mathrm{x}}$${\mathrm{Se}}_{\mathrm{x}}$

We report the low-temperature synthesis and properties of the rhombohedral Chevrel phases ${\mathrm{InMo}}_6$ ${\mathrm{S}}_{8\mathrm{-}\mathrm{x}}$ ${\mathrm{Se}}_{\mathrm{x}}$, ${\mathrm{TlMo}}_6$ ${\mathrm{S}}_{8\mathrm{-}\mathrm{x}}$ ${\mathrm{Se}}_{\mathrm{x}}$, and ${\mathrm{Hg}}_{\mathrm{y}}$ ${\mathrm{Mo}}_6$ ${\mathrm{S}}_{8\mathrm{-}\mathrm{x}}$ ${\mathrm{Se}}_{\mathrm{x}}$. In contrast to all other known Chevrel phases, ${\mathrm{InMo}}_6$ ${\mathrm{S}}_{8\mathrm{-}\mathrm{x}}$ ${\mathrm{Se}}_{\mathrm{x}}$ shows a large change in rhombohedral angle near x=6 at 300 K. We show that this anomalous behavior is associated with temperature-induced structural phase transitions that exist for all x. We present crystallographic evidence that In and Tl do not adopt their usual valence (+1 or +3) but rather have an intermediate valence. These are the first Chevrel phases in which a significant contribution of ternary metal character to the wave functions at the Fermi surface has been proposed. While a single-phase product is formed for ${\mathrm{Hg}}_{\mathrm{y}}$ ${\mathrm{Mo}}_6$ ${\mathrm{S}}_8$ (0<y<1), the amount of Hg that can be inserted in ${\mathrm{Mo}}_6$ ${\mathrm{S}}_{8\mathrm{-}\mathrm{x}}$ ${\mathrm{Se}}_{\mathrm{x}}$ decreases rapidly with increasing x. This behavior is consistent with a simple statistical model in which Hg will occupy an insertion site only if none or one of the eight near-neighbor chalcogen atoms is selenium.