  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.
