Ba₁₅Zr₁₄Te₄₂: a new complex ternary telluride structure with low thermal conductivity

Heavier metal-based tellurides with complex structures are of great interest for thermoelectric (TE) applications. Herein, we report the synthesis of a new telluride Ba₁₅Zr₁₄Te₄₂ using high-temperature reactions of elements. Our single-crystal X-ray diffraction study reveals that it crystallizes in the space group R3̄c of the trigonal crystal system and is isostructural to its Se analogue Ba₁₅Zr₁₄Se₄₂ complex. The unit cell of the structure accommodates 426 atoms with cell dimensions of a = b = 13.2666(10) Å, c = 96.195(9) Å, and V = 14 662(3) ų. This structure consists of 18 unique crystallographic atoms (3 × Ba, 8 × Zr, and 7 × Te). The bonding of Zr and Te atoms creates chains of ∞1[Zr₁₄Te₄₂]^30-, which are separated by the Ba²⁺ cations. Although all the Zr atoms have a coordination number of 6, they form two types of coordination polyhedra by bonding with six Te atoms: slightly distorted octahedral and trigonal prisms of ZrTe₆. We have synthesized polycrystalline Ba₁₅Zr₁₄Q₄₂ (Q = Se/Te) samples, which were characterized by optical absorption studies to reveal direct bandgaps of <0.5 eV for the Te analogue and 1.3(1) eV for the Se analogue. The lattice thermal conductivity (k) values of the samples are ultralow: ∼0.46 W mK^-1 and ∼0.30 W mK^-1 at 773 K for the Te and Se analogues, respectively. Temperature-dependent resistivity and thermopower studies were carried out for the Ba₁₅Zr₁₄Te₄₂, which showed the p-type degenerate semiconducting nature of the sample at high temperatures. The theoretical DFT studies predict a bandgap of 0.14 eV for the Ba₁₅Zr₁₄Te₄₂ phase.