Metavalent Bonding in Solids: Characteristic Representatives, Their Properties, and Design Options

Heavier chalcogenides display a surprisingly wide range of applications enabled by their unconventional properties. Herein, recent studies of three groups of chalcogenides from a chemical bonding perspective are reviewed to reveal the underlying reason for their wide range of applications. For IV-VI materials (GeTe, SnTe, PbTe, PbSe, and PbS), the unique property portfolio and bond-breaking behavior are related to a novel chemical bonding mechanism termed "metavalent bonding" (MVB). The same phenomena are also found for several V2VI3 solids (Bi2Te3, Bi2Se3, Sb2Te3, and beta-As2Te3) and some ternary chalcogenides including crystalline (GeTe)(1-x)(Sb2Te3)(x) alloys. This provides evidence for the prevalence of MVB in these compounds. Subsequently, a quantum-chemistry-based map is presented. Using the transfer and sharing of electrons between adjacent atoms as its two coordinates, materials using MVB are all found in a well-defined region of the map, characterized by sharing about one electron between adjacent atoms and only small charge transfer. This also implies that the degree of MVB is tailored either via Peierls distortions (electron sharing) or charge transfer (electron transfer), leading to the transition toward covalent bonding and ionic bonding, respectively. The tailoring of MVB provides a new approach for materials design.