Unlocking Strong SHG Response and Large Birefringence in Hybrid Antimony Halides via a Three-in-One Strategy

The design and synthesis of outstanding nonlinear optical (NLO) crystals continue to represent a pivotal challenge in advanced laser technologies. Herein, two new zero-dimensional (0D) organic-inorganic hybrid antimony halides, C₄H₉N₅SbX₅·H₂O (X = Cl, Br) featuring [SbX₅] square pyramids, were successfully synthesized by three-in-one integrating a Sb³⁺ ion with a stereochemically active lone pair (SCALP), π-conjugated organic (C₄H₉N₅)²⁺ cation, and halide anion. They adopt trigonal noncentrosymmetric (NCS) space groups P3₂ and P3₁, respectively. Both exhibit outstanding comprehensive NLO properties, featuring phase-matchable second-harmonic generation (SHG) intensities around 3.8 and 3.9 times that of KDP, respectively, broad optical bandgaps of 3.17 and 2.68 eV, and adequate birefringence of 0.12 and 0.10 at 546 nm, demonstrating their significant potential as nonlinear optical materials. Theoretical studies and structural analyses reveal that their excellent optical properties originate from the synergistic effect of organic cations and inorganic [SbX₅] square pyramids. This work establishes a rational design framework for guiding the development of advanced NLO crystals.