Ba₁₄Si₄Sb₈Te₃₂(Te₃): hypervalent Te in a new structure type with low thermal conductivity

Heavier pnictogen (Sb, Bi) containing chalcogenides are well known for their complex structures and semiconducting properties for numerous applications, particularly thermoelectric materials. Here, we report the syntheses of single crystals and polycrystalline phases of a new complex quaternary polytelluride, Ba₁₄Si₄Sb₈Te₃₂(Te₃), via a high-temperature reaction of elements. A single-crystal X-ray diffraction study showed that it crystallizes in an unprecedented structure type with monoclinic symmetry (space group: P2₁/c). The crystal structure of Ba₁₄Si₄Sb₈Te₃₂(Te₃) consists of one-dimensional ∞1[Si₄Sb₈Te₃₂(Te₃)]^28- stripes, which are separated by the Ba²⁺ cations. Its complex structure features linear polytelluride units of Te₃^4- having intermediate Te⋯Te interactions. A polycrystalline Ba₁₄Si₄Sb₈Te₃₂(Te₃) sample shows a direct narrow bandgap of 0.8(2) eV, which indicates its semiconducting nature. The electrical resistivity of a sintered pellet of the polycrystalline sample exponentially decreases from ∼39.3 Ωcm to ∼0.57 Ωcm on heating it from 323 K to 773 K, confirming the sample's semiconducting nature. The sign of Seebeck coefficient values is positive in the 323 K to 773 K range confirming the p-type nature of the sintered sample. Interestingly, the sample attains an extremely low thermal conductivity of ∼0.32 Wm^-1K^-1 at 773 K, which could be attributed to the lattice anharmonicity caused by the lone pair effect of Sb³⁺ species in its complex pseudo-one-dimensional crystal structure. The electronic band structure of the title phase and the strength of chemical bonding of pertinent atomic pairs have been evaluated theoretically using the DFT method.