Two Mixed-Anion Semiconductors in the Ba-Sn-Te-S System with Low Thermal Conductivity

We synthesized two mixed-anion tin chalcogenides, Ba5Sn2Te1.327S7.673 (1) and Ba7Sn3Te0.839S12.161 (2), by a typical high-temperature solid-state reaction using an evacuated silica tube for the first time in the system of Ba-Sn-Q (Q = S1-xTex; 0 < x < 1). Compound 1 crystallizes in the monoclinic space group P21/c with unit cell parameters of a = 17.501(5) angstrom, b = 8.908(2) angstrom, c = 12.508(3) angstrom, and Z = 4. Compound 2 has an orthorhombic space group Pnma with a = 12.386(5) angstrom, b = 24.17(2) angstrom, c = 8.872(4) angstrom, and Z = 4. The structures of compounds 1 and 2 are both zero dimensional (0D), and the formulas can be written as Ba5(SnIVQ4)2Q and Ba7(SnIVQ4)3Q with Sn/Q ratios of 2/9 and 3/13, respectively. Compounds 1 and 2 exhibit Q2- anion units, which is unique among Ba-Sn-Q compounds due to the [SnQ4] tetrahedra and low Sn/Q ratios. The phonon transport can be significantly scattered because the weakly bond Ba atom and highly distorted [SnQ4] tetrahedra can enhance the lattice anharmonicity. As a result, compounds 1 and 2 have low lattice thermal conductivity (kappa lat) values of-,0.3-0.4 W m-1 K-1 in the range of 300 to 773 K, which results in a design strategy as thermoelectric materials.