Germanium Antimony Bonding in Ba 4 Ge 2 Sb 2 Te 10 with Low Thermal Conductivity.

A new quaternary telluride, Ba 4 Ge 2 Sb 2 Te 10 , was synthesized at high temperature via the reaction of elements. A single-crystal X-ray diffraction study shows that the title compound crystallizes in its own structure type in the monoclinic P 2 1 / c space group having cell dimensions of a = 13.984(3) Å, b = 13.472(3) Å, c = 13.569(3) Å, and β = 90.16(3)° with four formula units per unit cell ( Z = 4). The pseudo-one-dimensional crystal structure of Ba 4 Ge 2 Sb 2 Te 10 consists of infinite 1 ∞ [Ge 2 Sb 2 Te 10 ] 8- stripes, which are separated by Ba 2+ cations. Each of the Ge(1) atoms is covalently bonded to four Te atoms, whereas the Ge(2) atom is covalently bonded with one Sb(2) and three Te atoms in a distorted tetrahedral geometry. The title compound is the first example of a chalcogenide that shows Ge-Sb bonding. The Sb(1) atom is present at the center of the seesaw geometry of four Te atoms. In contrast, the Sb(2) atom forms a seesaw geometry by coordinating with one Ge(2) and three Te atoms. Condensation of these Ge and Sb centered polyhedral units lead to the formation of 1 ∞ [Ge 2 Sb 2 Te 10 ] 8- stripes. The temperature-dependent resistivity study suggests the semimetallic/degenerate semiconducting nature of polycrystalline Ba 4 Ge 2 Sb 2 Te 10 . The positive sign of Seebeck coefficient values indicates that the predominant charge carriers are holes in Ba 4 Ge 2 Sb 2 Te 10 . An extremely low lattice thermal conductivity of ∼0.34 W/mK at 773 K was observed for polycrystalline Ba 4 Ge 2 Sb 2 Te 10 , which is presumably due to the lattice anharmonicity induced by the stereochemically active 5s 2 lone pair of Sb. The electronic structure of Ba 4 Ge 2 Sb 2 Te 10 and the bonding of atom pairs in the structure have been analyzed by means of ELF analysis and crystal orbital Hamilton population (COHP) analysis.