Electronic structure and thermoelectric properties of Sn_{1.2- x}Nb _x Ti_0.8S₃ with a quasi-one-dimensional structure

We report the electronic structure and thermoelectric properties of a tin titanium trisulfide, Sn_1.2Ti_0.8S₃. The crystal structure is composed of infinite "ribbons" of double edge-sharing (Sn⁴⁺/Ti⁴⁺)S₆ octahedra capped by Sn²⁺. First-principles calculations predict a nearly unidirectional transport of electrons along the ribbon axis for a single crystal and the existence of lone-pair electrons on Sn²⁺. Experiments on polycrystalline pressed samples demonstrate that Sn_1.2Ti_0.8S₃ exhibits semiconducting temperature dependence of electrical resistivity and a large negative Seebeck coefficient at room temperature. Substitution of Nb⁵⁺ for Sn⁴⁺ at the octahedral sites increases the electron carrier concentration, leading to an enhancement of the thermoelectric power factor. Anisotropy in the electronic properties is weak because of a weak orientation of the ribbon axis of crystallites in the pressed sample. The lattice thermal conductivity is less than 1 W K^-1 m^-1 for the pristine and substituted samples, which is attributed to weak bonding between the ribbons via the lone-pair electrons of Sn²⁺ and to random occupation of Sn⁴⁺, Ti⁴⁺, and Nb⁵⁺ at the octahedral sites.