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

An unusual set of anomalous functional properties of rocksalt crystals of Group IV chalcogenides were recently linked to a kind of bonding termed as metavalent bonding (MVB) which necessarily involves violation of the 8-N rule. Precise mechanisms of MVB and the relevance of lone pair of group IV cations are still debated. With restrictions of low dimensionality on the possible atomic coordination, 2D materials provide a rich platform for exploration of MVB and understanding its orbital mechanisms. Here, we present first-principles theoretical analysis of the nature of bonding in five distinct 2D lattices of group IV chalcogenides MX (M: Sn, Pb, Ge and X: S, Se, Te), in which the natural out-of-plane expression of the lone pair versus in-plane bonding can be systematically explored.  While their honeycomb lattices respecting the 8-N rule are shown to exhibit covalent bonding, their square and orthorhombic structures exhibit MVB only in-plane, with cationic lone pair activating the out-of-plane structural puckering that controls their relative stability. Anomalies in Born effective charges, dielectric constants, Grüneisen parameters occur only in their in-plane behaviour, confirming that MVB here is confined strictly to 2D and originates mostly from p-p orbital interactions. Our work opens up directions for chemical design of MVB based 2D materials and their heterostructures exhibiting thermo- and ferro-electricity, and electronic topology important to quantum technologies.