Syntheses and Characterization of Ba16Si8Te44+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 2∞$\genfrac{}{}{0pt}{}{2}{\infty }$$\genfrac{}{}{0pt}{}{2}{\infty }$[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_tot) 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.