Tl0.6Mo3S5, an original large tunnel-like molybdenum sulfide with Mo zigzag chains and disordered Tl cations

Abstract

We report on the crystal structure and physical properties of Tl_0.6Mo₃S₅, 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 P2₁ (no. 4; a = 9.344(2) Å, b = 3.234(2) Å, c = 11.669(2) Å and β = 113.09(2)° 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 on the Tl content. Low-temperature transport properties measurements (5–300 K) reveal that Tl_0.6Mo₃S₅ 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 κ_L, the temperature dependence of which mimics that observed in strongly-disordered compounds. In spite of this characteristic, high κ_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.