Multicentered p-s-p* Antibonding Interaction Induces Strong Anharmonicity and High Thermoelectric Performance in Complex Metal Chalcohalide Tl5Te2I.

Metal chalcohalides are a fascinating new class of crystalline solids with a unique chemical bonding hierarchy which combines the notable stability of metal chalcogenides along with the enhanced electronic tunability of metal halides. Metal chalcohalides with their complex structures are promising candidates for thermoelectrics if they can exhibit halide-like low thermal conductivity alongside chalcogenide-like enhanced electrical conductivity. However, most metal chalcohalides mimic a wide band gap electronic structure similar to metal halides, limiting overall electrical transport and thus reducing their applicability in thermoelectrics. Here, we present a metal chalcohalide, Tl5Te2I, with a narrow band gap and degenerate semiconductor-like significant electrical conductivity. We explore the structural and chemical bonding attributes of Tl5Te2I and demonstrate it to be suitable for low thermal conductivity arising from its complex crystal structure with significant bonding hierarchy. Tl5Te2I, in its octahedral Tl sublattice, exhibits a multicentric bonded structural framework. This bonding feature allows delocalization of electrons, which permits symmetry-allowed three-center antibonding pz-s-pz and px/y-s-px/y interactions of I-Tl-I and Te-Tl-Te, respectively. These antibonding interactions at the top of the valence band near the Fermi level make interatomic force constants extremely soft and reduce the lattice thermal conductivity to the glass limit. With the combined effects of these features, Tl5Te2I exhibits an extraordinarily high p-type thermoelectric figure of merit of ∼1.2 at ∼650 K in its pristine form. Our investigations highlight the role of multicentric antibonding features to realize record-high thermoelectric performance in mixed anionic chalcohalides.