Structural Phase Transformation in the Cluster Chalcogenides Eu${\mathrm{Mo}}_6$${\mathrm{S}}_8$ and Ba${\mathrm{Mo}}_6$${\mathrm{S}}_8$

Low-temperature x-ray-diffraction analysis at ambient pressure shows that the rhombohedral chalcogenides Eu${\mathrm{Mo}}_6$ ${\mathrm{S}}_8$ and Ba${\mathrm{Mo}}_6$ ${\mathrm{S}}_8$ transform at 110(5) and 175(10) K, respectively, into a triclinic distorted low-temperature structure. The transitions are presumably due to a Jahn-Teller-type electronic instability arising from the octahedral ${\mathrm{Mo}}_6$ clusters. The absence of superconductivity is attributed to the triclinic deformation of these clusters which leads to a significant splitting of the nearly half-filled Mo $d$-like conduction band.