The Nature of Metavalent Bonding in Crystals—The Origin of Emergence of Unusual Properties

Phase change chalcogenide materials exhibit a remarkable ability to transform reversibly from a covalently bonded amorphous structure to rocksalt crystalline forms considered to exhibit a distinct type of metavalent bonding. An unusual portfolio of anomalous functional properties emerge in the metavalent rocksalt forms, and their origin and electronic mechanisms are yet to be understood. Here, we present first-principles theoretical analysis of the evolution of metavalent bonding along continuous paths in the structural and chemical composition space, starting from distinct covalent, ionic and metallic states. We show that metavalent bonding arises in rocksalt chalcogenides stabilized in weakly broken symmetry states of the parent covalent metal, a simple cubic crystal of Group V metalloid. High electronic degeneracy at the nested Fermi surface of the parent metal drives spontaneous breaking of its translational symmetry with structural and chemical changes, which mediate strong coupling between conduction and valence bands separated by a small energy gap making metavalent crystals highly conductive, polarizable and sensitive to bond-lengths. Stronger changes is structural and chemical fields, however, evolve them discontinuously to covalent and ionic semiconducting states respectively. We predict them to exhibit anomalous second order Raman scattering, adding to the set of their unusual finger-printing properties. Wannier function analysis reveals multi-centre, mixed bonding and antibonding pp and sp orbital interactions supporting fractional bond-order and high coordination numbers. Our precise picture of metavalent bonding will guide in design of new metavalent materials with improved thermoelectric, ferroelectric and nontrivial electronic topological properties.