Metavalent or Hypervalent Bonding: Is There a Chance for Reconciliation?

Area of Science:

• Materials Science
• Solid-State Chemistry
• Quantum Chemistry

Background:

• Certain solids, including phase change materials (GeTe, Sb2Te3), topological insulators (Bi2Se3), and halide perovskites (CsPbI3), exhibit unusual properties not explained by traditional bonding models (ionic, metallic, covalent).
• These materials are characterized by half-filled p-bands, involving a balance between electron localization and delocalization.

Purpose of the Study:

• To investigate the bonding mechanisms in these unconventional solids.
• To clarify the role of quantum chemical bonding descriptors and resolve disagreements regarding electron-deficient (metavalent) versus electron-rich (hypervalent) bonding.

Main Methods:

• Analysis of bonding in representative materials like XeF2 (electron-rich) and GeTe (electron-deficient).
• Focus on the contribution of p-electrons versus s-electrons to bonding.

Main Results:

• Independent of the quantum chemical approach, electron-deficient bonds are found to be dominant in the studied solids.
• p-electrons are the primary drivers of bonding in both XeF2 and GeTe, with s-electrons playing a minor role.
• Despite similarities in p-electron bonding, the properties of electron-deficient crystals differ significantly from molecular crystals.

Conclusions:

• The unique properties of phase change materials and related solids arise from an extended system of half-filled bonds.
• The term 'metavalent bonding' is appropriate and necessary to describe this distinct bonding mechanism.