Metavalent Bonding in 2D Chalcogenides: Structural Origin and Chemical Mechanisms

Area of Science:

• Materials Science
• Condensed Matter Physics
• Quantum Chemistry

Background:

• Anomalous properties in Group IV chalcogenides were linked to metavalent bonding (MVB), challenging the 8-N rule.
• The precise mechanisms of MVB and the role of lone pairs in Group IV cations remain under investigation.
• Two-dimensional (2D) materials offer unique platforms to study bonding due to dimensionality constraints.

Purpose of the Study:

• To theoretically analyze the bonding nature in five distinct 2D lattices of Group IV chalcogenides (MX).
• To systematically explore the interplay between lone pair expression and in-plane bonding in these 2D materials.
• To elucidate the conditions and characteristics of metavalent bonding in low-dimensional systems.

Main Methods:

• First-principles theoretical calculations.
• Analysis of bonding in honeycomb, square, and orthorhombic 2D lattices of MX compounds (M: Sn, Pb, Ge; X: S, Se, Te).
• Investigation of structural puckering, Born-effective charges, dielectric constants, and Grüneisen parameters.

Main Results:

• Honeycomb lattices exhibit covalent bonding, adhering to the 8-N rule.
• Square and orthorhombic lattices display in-plane metavalent bonding, influenced by the cationic lone pair.
• Anomalous properties (Born-effective charges, dielectric constants, Grüneisen parameters) are confined to the in-plane behavior of MVB structures.
• Structural puckering is activated by the lone pair, affecting relative stability.

Conclusions:

• Metavalent bonding in Group IV chalcogenides is confirmed to be a strictly 2D phenomenon.
• MVB originates from p-p orbital interactions and is modulated by the cationic lone pair.
• This study provides a foundation for the chemical design of MVB-based 2D materials and heterostructures.