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

## TL;DR

Scientists designed a new material that significantly boosts laser frequency conversion efficiency by localizing nonbonding electrons through coordination control.

## Contribution

A novel strategy for maximizing SHG response by reducing coordination numbers to localize nonbonding electrons in chalcogenide crystals.

## Key findings

- KBiP2S6 achieves a SHG response of 15 × AgGaS2, the highest among sulfides.
- Localized S-3p nonbonding electrons contribute ~75% to SHG, challenging existing SCALP-based models.
- Polar screw axis symmetry is identified as favorable for high-SHG chalcogenides.

## Abstract

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.

By leveraging coordination-induced modulation to enhance nonbonding-electron localization, KBiP₂S₆ achieves 15 times of AgGaS₂ SHG response, benefiting from the advantageous 2₁-screw-axis alignment of its SCALP units.