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 6s 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 6s 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-3m 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.

Chemical Bonding Tuned Lattice Anharmonicity Leads to a High Thermoelectric Performance in Cubic AgSnSbTe3

“…Previously such non‐classical chemical bonding mechanism in few metal chalcogenides was identified as electron deficient or multicentered hypervalent bonding, [67–69] however, many of the property descriptors still to be verified and it needs further investigations on several extended structures. Moreover, the existence of such non‐classical chemical bonding in a solid is proven to amplify the thermoelectric performance to a significant extent [21–22,70] . Metavalent bonded solids are also demonstrated to exhibit high S with large Fermi surface complexity factor which further validates our findings of high S in AgSnSbTe 3 [35] …”

“…Chemical bonding and lattice periodicity play a decisive role in the phonon dynamics which necessitates a deep understanding of the crystal structure of solids [25–31] . Recently, it has been demonstrated that generation of extensive antibonding states below the Fermi level by the interaction of different orbitals of cation and anion, soften the lattice remarkably leading to an ultra‐low κ lat [22,25,32–33] . Interestingly, few IV–VI metal chalcogenides and Pb‐based halide perovskites have also been studied exhibiting contributions from metal and ligand orbitals just below the Fermi level which further appends to the aforementioned statement [26–28,34] .…”

“…Interestingly, few IV–VI metal chalcogenides and Pb‐based halide perovskites have also been studied exhibiting contributions from metal and ligand orbitals just below the Fermi level which further appends to the aforementioned statement [26–28,34] . Metavalent bonding in a crystalline solid provides softening of chemical bonds due to high chemical polarizability along with high electrical conductivity, which are an essential criteria for thermoelectrics [21–22,28,35–36] . An in‐depth knowledge of chemical bonding in solids is thus a key to understand the electronic and thermal transport properties.…”

Hg Doping Induced Reduction in Structural Disorder Enhances the Thermoelectric Performance in AgSbTe2

“…In terms of electronic origin, the inherent instabilities of AgSbTe 2 may be attributed to the electron-filled antibonding states contributed by the Sb-5s and Te-5p orbital in the valence band maxima near the Fermi level ( E F ) (Figure S5, SI). A similar type of antibonding interaction near the Fermi level has been observed in AgSbSe 2 and TlBiSe 2 as well. Hole doping via divalent ion reduces the electron density in such an antibonding state by pushing E F inside the valence band and enhances the stability of the system.…”

Microstructural iterative reconstruction toward excellent thermoelectric performance in MnTe

“…This was mainly attributed to the introduced layer structures of MnSb 2 Te 4 . According to the literature, 5,45 these types of micro structures will always contribute to low thermal conductivity. The refined calculation, which provided plentiful local bonding asymmetries and the lone-pair electrons of Sb and Te, was presented directly by the electron localization function (ELF) image, as illustrated in Fig.…”