Syntheses and Characterization of Ba₁₆Si₈Te_44+δ and Ba₆Si₄Te₆(Te₃)₃ Structures with Complex Te-Te Interactions

Multinary tellurides with complex structures and narrow bandgaps are potential candidates for thermoelectric applications. Herein, we report the syntheses of two new ternary polytellurides, Ba₁₆Si₈Te_44+δ and Ba₆Si₄Te₆(Te₃)₃. Both title structures adopt unprecedented structure types. Interestingly, the Ba₁₆Si₈Te_44+δ structure shows three types of polytelluride units: Te₇, Te₃, and Te₂. Each of the two crystallographically unique Si atoms of the structure forms a slightly distorted tetrahedral SiTe₄ unit. The assignment of formal charges on the atoms of the Ba₁₆Si₈Te_44+δ structure is difficult due to intermediate Te-Te interactions. The Ba₆Si₄Te₆(Te₃)₃ structure is also complex, including three Ba, two Si, and eight Te sites. The Si₂Te₆ dimers and Te₃ trimers are the primary motifs of the Ba₆Si₄Te₆(Te₃)₃ structure, forming the anionic $$[Si₄Te₁₅]^12- layers. Interestingly, the structure has two types of Te₃ trimers: a linear Te₃ unit with equidistant Te atoms and a bent Te₃ unit with two different Te-Te interactions. A polycrystalline Ba₁₆Si₈Te_44.2 sample shows a remarkably low total thermal conductivity (κ) value of 0.41 Wm^-1 K^-1 at 623 K. Interestingly, an upturn below 60 K was revealed by a resistivity study of the sample. Moreover, DFT calculations were carried out to understand the theoretical electronic structures of the phases.