Maximizing the second-harmonic generation response via coordination-induced localization of nonbonding electrons.

To enhance laser frequency conversion efficiency, the development of nonlinear optical (NLO) crystals with strong second-harmonic generation (SHG) responses remains a central challenge. However, for chalcogenides, atomic-level crystal design has seldom yielded materials with SHG responses exceeding 3 × AgGaS2. Previous studies suggest that modulation of nonbonding electrons can enhance both linear and NLO properties, yet strategies to localize nonbonding electrons for maximizing SHG remain underexplored. Here, we demonstrate that reducing the coordination number increases the spatial localization of nonbonding electrons, thereby boosting SHG performance. Guided by this principle, we synthesized KBiP2S6 (P21, no. 4), exhibiting the highest SHG response among sulfides to date (15 × AgGaS2). Atomic space tessellating analysis reveals that ∼75% of the SHG contribution originates from S, particularly from localized S-3p nonbonding electrons-challenging conventional stereochemically active lone-pair (SCALP)-based contribution models that overlook the dominant role of S. Moreover, symmetry analysis identifies the polar screw axis as the favorable symmetry for high-SHG SCALP-based chalcogenides. This work transitions NLO material design from structural building-unit assembly to electronic-level engineering, opening new avenues for next-generation high-performance NLO materials.