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Chemical bonding is a chemist’s strongest tool to tailor and tune material properties, especially
when optimizing material attributes like the thermoelectric performance that is composed of
conflicting correlations between electrical and thermal transport in crystalline solids. Metavalent
bonding has recently attracted immense attention owing to its capability to impart distinct
property portfolio in crystalline solids. It is an exemplary and distinct bonding, characterized by
the fine tuning between the localization (covalency) and delocalization (metallicity) of the
bonding electrons. Thus, the unique features of metavalent bonding can directly address the
thermoelectric challenge.
While searching for materials with excellent electrical properties, our attention was
drawn by quantum materials such as topological insulators. They are an exotic family of
compounds that have metallic surface states but semiconducting bulk states. Due to their
special electronic band structure, they show high carrier mobility harbouring intriguing electrical
transport. This class of materials share many design features with thermoelectric materials, like
having heavy atoms, low band gap, spin-orbit coupling etc. Due to the shared chemical design
pool, topological materials are also potential thermoelectric materials and many of them show
metavalent bonding.
A change of bonding type from covalent to metavalent was realized in GeSe by alloying
it with AgBiTe 2. Our strategy increased the zT from ~0.2 in pristine GeSe to ~1.35 in
(GeSe) 0.9 (AgBiTe 2 ) 0.1 at 627 K. 1 Further, guided by the unique properties offered by metavalent
bonding and lone pair expression, our recent work on quantum materials, TlBiSe 2 demonstrated
that employing metavalent bonding could be an elegant way to devise a configurational under-
constraint in crystalline solids to allow degenerate manifold of ground states that can be easily
accessed through slight structural distortions induced by lone pair expression. The soft p-
bonding network in TlBiSe 2 facilitates long-range propagation of the dual lone-pair induced
lattice anharmonicity, revealing a new channel to intrinsically lower thermal conductivity. 2 |