Stepwise Substitution Strategy for Beryllium-Based Selenites with Enhanced SHG Response, Moderate Birefringence, and Short UV Cutoff Edges

Designing nonlinear optical (NLO) materials that simultaneously exhibit enhanced second-harmonic generation (SHG), moderate birefringence (Δn), and short-wavelength ultraviolet (UV) transparency remains a significant challenge. This study presents a stepwise structural regulation strategy using the International Zeolite Association (ABW)-type zeolite NH4BePO4 (I) as the parent material. By systematically modulating the anionic units, we successfully synthesized two compounds: KBe2(HPO3)2F (II) and NH4Be2(OH)(SeO3)2·H2O (III). Compound III represents a breakthrough, demonstrating an SHG intensity 4.0 times greater than that of KH2PO4, a birefringence of 0.080 at 546 nm, and a UV cutoff at 203 nm. The enhanced optical anisotropy and NLO efficiency of compound III arise from the stereochemically active lone pairs in the [SeO3] groups. By optimizing SHG intensity, short-wavelength UV transparency, and birefringence simultaneously, compound III addresses the key challenge of designing high-performance short-wavelength UV NLO materials. This work not only validates the stepwise regulation strategy but also provides a design framework for achieving balanced NLO performance, paving the way for the development of next-generation optical materials.