Metavalent Bonding Origins of Unusual Properties of Group IV Chacogenides*

A distinct type of metavalent bonding (MVB) was recently proposed to explain unusual set of anomalous functional properties of group IV chalcogenide crystals. However, electronic mechanisms of MVB and emergent properties are yet to be understood. Through theoretical analysis of evolution of MVB along continuous paths in structural and chemical composition space, we show that MVB arises in rocksalt chalcogenides stabilized as weakly broken symmetry states of the parent metal of simple cubic crystal of Group V metalloid. Symmetry-breaking structural and chemical polar fields couple with its degenerate frontier states opening up a gap resulting in MVB state with high polarizability and sensitivity to bond-lengths. It transforms discontinuously to covalent and ionic semiconducting states with stronger symmetry breaking structural and chemical fields respectively. Wannier function analysis reveals mixed, long-range bonding and antibonding pp, sp orbital interactions, supporting high coordination numbers. MVB involves bonding and antibonding pairwise interactions alternating along linear chains of at least five atoms, which facilitate long range electron transfer in response to polar fields causing unusual properties. Our precise picture of MVB predicts anomalous second order Raman scattering as an addition to set of their unusual finger-printing properties, and will guide in design of new metavalent materials with improved thermoelectric, ferroelectric and nontrivial electronic topological properties.