Metavalent Bonding in Crystalline Solids: How Does It Collapse?

The chemical bond is one of the most powerful, yet much debated concepts in chemistry, explaining property trends in solids. Recently, a novel type of chemical bonding was identified in several higher chalcogenides, characterized by a unique property portfolio, unconventional bond breaking, and sharing of about one electron between adjacent atoms. This metavalent bond is a fundamental type of bonding in solids, besides covalent, ionic, and metallic bonding, raising the pertinent question as to whether there is a well-defined transition between metavalent and covalent bonds. Here, three different pseudo-binary lines, namely, GeTe_{1- x} Se_x , Sb₂ Te_{3(1- x )} Se_{3 x} , and Bi_{2-2 x} Sb_{2 x} Se₃ , are studied, and a sudden change in several properties, including optical absorption ε₂ (ω), optical dielectric constant ε_∞ , Born effective charge Z*, electrical conductivity, as well as bond breaking behavior for a critical Se or Sb concentration, is evidenced. These findings provide a blueprint to experimentally explore the influence of metavalent bonding on attractive properties of phase-change materials and thermoelectrics. Particularly important is its impact on optical properties, which can be tailored by the amount of electrons shared between adjacent atoms. This correlation can be used to design optoelectronic materials and to explore systematic changes in chemical bonding with stoichiometry and atomic arrangement.