Tl0.6mo3s5, An Original Large Tunnel-Like Molybdenum Sulfide With Mo Zigzag Chains And Disordered Tl Cations

We report on the crystal structure and physical properties of Tl0.6Mo3S5, which belongs to a novel family of materials with large tunnels, reminiscent of those observed in the romanechite structure type. Tl cations are partially filling these tunnels delimited by the Mo-S cluster framework in which the Mo atoms form infinite zigzag chains. Single-crystal X-ray diffraction data indicate that this compound crystallizes in the monoclinic, non-centrosymmetric space group P2(1) (no. 4; a = 9.344(2) angstrom, b = 3.234(2) angstrom, c = 11.669(2) angstrom and beta = 113.09(2)degrees at 293 K). While electron diffraction performed on single crystals further evidences a commensurate modulation running solely along the b axis with a modulation wave vector further experiments carried out on a polycrystalline sample suggest a compositional dependence of q on the Tl content. Low-temperature transport properties measurements (5-300 K) reveal that Tl0.6Mo3S5 behaves as a narrow-band-gap n-type semiconductor. The strongly non-linear temperature dependence of the thermopower further suggests the presence of several electronic bands contributing to the electrical transport. Consistent with the extended electronic distribution in the tunnels that requires a description using two adjacent Tl sites, specific heat data measured down to 0.35 K evidence the presence of a Schottky-type anomaly that may be associated with the tunneling of the Tl cations between several equivalent, off-centered equilibrium sites. This inherent disorder contributes to suppression of the low-temperature Umklapp peak in the lattice thermal conductivity kappa(L), the temperature dependence of which mimics that observed in strongly-disordered compounds. In spite of this characteristic, high kappa(L) values of up to 3.8 W m(-1) K-1 are reached at 300 K, due to the covalent Mo-S network that contributes to maintaining high sound velocities.