Metavalent Bonding Origins of Unusual Properties of Group IV Chalcogenides

• Published in: Advanced materials (Weinheim) 2023-02, Vol.35 (7), p.e2208724-n/a
• Main Authors: Arora, Raagya, Waghmare, Umesh V., Rao, C. N. R.
• Format: Article
• Language: English
• Subjects: Base metal; Broken symmetry; Chalcogenides; charge transfer; Chemical composition; Crystals; Electron transfer; Energy gap; Fermi surfaces; Ferroelectricity; ferroelectrics; Materials science; Metalloids; metavalent bonding; phase change materials; Raman spectra; Symmetry; Thermoelectric materials; thermoelectrics; Valence band
• ISSN: 0935-9648; 1521-4095
• DOI: 10.1002/adma.202208724

A distinct type of metavalent bonding (MVB) is recently proposed to explain an unusual combination of anomalous functional properties of group IV chalcogenide crystals, whose electronic mechanisms and origin remain controversial. Through theoretical analysis of evolution of bonding along continuous paths in structural and chemical composition space, emergence of MVB in rocksalt chalcogenides is demonstrated as a consequence of weakly broken symmetry of parent simple‐cubic crystals of Group V metalloids. High electronic degeneracy at the nested Fermi surface of parent metal drives spontaneous breaking of its translational symmetry with structural and chemical fields, which open up a small energy gap and mediate strong coupling between conduction and valence bands making metavalent crystals highly polarizable, conductive, and sensitive to bond‐lengths. Stronger symmetry‐breaking structural and chemical fields, however, transform them discontinuously to covalent and ionic semiconducting states. MVB involves bonding‐antibonding pairwise interactions alternating along linear chains of at least five atoms, which facilitate long‐range electron transfer in response to polar fields causing unusual properties. The precise picture of MVB predicts anomalous second‐order Raman scattering as an addition to set off their unusual properties, and will guide in design of new metavalent materials with improved thermoelectric, ferroelectric and nontrivial electronic topological properties. Metavalent bonding (MVB) is defined in the property space and its origin and distinction from resonant bonding and hyperconjugation are unclear. The essential electronic mechanisms of MVB are uncovered that involve bonding and antibonding pairwise interactions alternating along linear chains of at least five atoms, facilitating long‐range electron transfer in response to polar fields that cause exotic properties.