Metavalent bonding in crystalline solids: how does it collapse?

The chemical bond is one of the most powerful, yet controversial concepts in chemistry, explaining property trends in solids. Recently, a novel type of chemical bonding has been identified in several higher chalcogenides, characterized by a unique property portfolio, unconventional bond breaking and sharing of about one electron between adjacent atoms. Metavalent bonding is a fundamental type of bonding besides covalent, ionic and metallic bonding, raising the pertinent question, if there is a well-defined transition between metavalent and covalent bonding. For three different pseudo-binary lines, namely GeTe1-xSex, Sb2Te3(1-x)Se3x and Bi2-2xSb2xSe3, a sudden drop in several properties, including the optical dielectric constant, the Born effective charge, the electrical conductivity as well as the bond breaking is observed once a critical Se or Sb concentration is reached. This finding provides a blueprint to explore the impact of metavalent bonding on attractive properties utilized in phase change materials and thermoelectrics.