Antibonding Lone Pairs as Hidden Drivers of Bonding, Dynamics, and Energy Performance

Stereochemically active lone pairs (LPs) modulate the properties of energy materials, while the so-called “silent” (i.e., stereochemically inactive) ones are often dismissed as inert. Contrary to this conventional view, our calculations show that in the cubic rock-salt phase of IV-chalcogenides, the hidden ns2 LPs play a more decisive role than their “silent” reputation suggests, rivaling or even exceeding the impact of the active LPs. In the cubic phase, the ns2 LPs, when hybridized with anion p-orbtials, lift filled antibonding states to the top of the valence band, lowering the bond order, softening the lattice, and driving the system toward Peierls instability. In rock-salt SnSe, a mere 4% volume expansion─comparable to that of thermal expansion at 600 K─triggers LP ovalization and off-centering. The ovalization causes a shallow double-well potential that drives a specific form of temperature-induced dynamical off-centering, known as emphanisis, whose magnitude scales with the antibonding ns2 weight. The silent LPs enhance optical absorption through their hybridization with chalcogen p-states, producing a large antibonding contribution from the cation ns2 LP that makes the bonds highly polarizable (so-called metavalent) and responsive to external stimuli. Therefore, even when seemingly quenched, ns2 LPs─through their hybridization with anion p states─constitute a common electronic thread linking several exotic and otherwise independently discussed phenomena in IV-chalcogenides, including metavalency, emphanisis, hypervalency, and cation off-centering, while preserving the conceptual and mechanistic distinctions between these frameworks. Thus, exploiting ns2 LPs provides a route to advanced energy materials design, where local LP activation and distortions tune the electronic, phononic, and optical properties without sacrificing the softness and polarizability found in rock-salt group IV chalcogenides.