Ba 15 Zr 14 Te 42 : 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 Ba 15 Zr 14 Te 42 using high-temperature reactions of elements. Our single-crystal X-ray diffraction study reveals that it crystallizes in the space group R 3̄ c of the trigonal crystal system and is isostructural to its Se analogue Ba 15 Zr 14 Se 42 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) Å 3 . 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[Zr 14 Te 42 ] 30- , which are separated by the Ba 2+ 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 ZrTe 6 . We have synthesized polycrystalline Ba 15 Zr 14 Q 42 ( 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 lat ) 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 Ba 15 Zr 14 Te 42 , 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 Ba 15 Zr 14 Te 42 phase.