Intrinsic extremely low thermal conductivity in BaIn2Te4: Synthesis, crystal structure, Raman spectroscopy, optical, and thermoelectric properties

Abstract The single crystals of BaIn 2 Te 4 have been synthesized at 1173 K using the sealed tube solid-state method. The polycrystalline sample of the compound was also synthesized by stoichiometric reaction of elements with intermittent grinding and annealing at 1123 K. The crystal structure of BaIn 2 Te 4 has been established by single crystal X-ray diffraction study at 298(2) K. It crystallizes in space group D 2 h 20 −Cccm of the orthorhombic crystal system with unit cell dimensions of a  = 7.1417(14) A, b  = 12.034(2) A, and c  = 12.107(2) A with four formula units. The structure of BaIn 2 Te 4 is related to the tetragonal binary TlSe structure (space group: I 4/ mcm ). The crystal structure of BaIn 2 Te 4 features one-dimensional chains of 1 ∞ [In 2 Te 4 2− ] that are separated by filling of Ba atoms. Indium atoms in these chains are surrounded by four Te atoms that form distorted tetrahedron geometry around In atoms. This compound does not show any homoatomic bonding and is having a closed shell electronic configuration. Thus, it can be easily charge balanced as Ba 2+ (In 3+ ) 2 (Te 2− ) 4 . The phase purity of the polycrystalline BaIn 2 Te 4 sample was established by the powder X-ray diffraction study. The direct band gap of 1.36(2) eV was estimated from the optical absorption study at room temperature. This ternary compound is an n -type semiconductor as suggested by the negative sign of the Seebeck coefficients ( S ). The absolute value of S was found to be decreasing on heating the polycrystalline BaIn 2 Te 4 sample. The temperature-dependent resistivity study confirmed the semiconducting nature of the BaIn 2 Te 4 sample. The thermal conductivity (κ tot ) values of polycrystalline BaIn 2 Te 4 were found to be extremely low and were decreasing with raising the temperature with the lowest value of 0.30 Wm −1 K −1  at 965 K. Hence, BaIn 2 Te 4 and its analogs with related structures could be promising materials for thermoelectric applications by further optimization of carrier concentrations to improve electrical transport properties.