Metavalent Bonding-Mediated Dual 6s² Lone Pair Expression Leads to Intrinsic Lattice Shearing in n-Type TlBiSe₂

Abstract

Metavalent bonding has attracted immense interest owing to its capacity to impart a distinct property portfolio to materials for advanced functionality. Coupling metavalent bonding to lone pair expression can be an innovative way to propagate lattice anharmonicity from lone pair-induced local symmetry-breaking via the soft p -bonding electrons to achieve long-range phonon dampening in crystalline solids. Motivated by the shared chemical design pool for topological quantum materials and thermoelectrics, we based our studies on a three-dimensional (3D) topological insulator TlBiSe 2 that held prospects for 6 s 2 dual-cation lone pair expression and metavalent bonding to investigate if the proposed hypothesis can deliver a novel thermoelectric material. Herein, we trace the inherent phononic origin of low thermal conductivity in n-type TlBiSe 2 to dual 6 s 2 lone pair-induced intrinsic lattice shearing that strongly suppresses the lattice thermal conductivity to a low value of 1.1–0.4 Wm –1 K –1 between 300 and 715 K. Through synchrotron X-ray pair distribution function and first-principles studies, we have established that TlBiSe 2 exists not in a monomorphous R- 3 m structure but as a distribution of distorted configurations. Via a cooperative movement of the constituent atoms akin to a transverse shearing mode facilitated by metavalent bonding in TlBiSe 2, the structure shuttles between various energetically accessible low-symmetry structures. The orbital interactions and ensuing multicentric bonding visualized through Wannier functions augment the long-range transmission of atomic displacement effects in TlBiSe 2 . With additional point-defect scattering, a κ latt of 0.3 Wm –1 K –1 was achieved in TlBiSeS with a maximum n-type thermoelectric figure of merit ( zT ) of ∼0.8 at 715 K.

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