Symmetry Breaking via Anion-Molecule Synergy: Fluorine and Crystalline Water Directed Construction of Noncentrosymmetric Sr(IOF)·2HO.

Noncentrosymmetric (NCS) compounds are pivotal for advancing functional materials in nonlinear optics (NLO), ferroelectrics, and piezoelectrics. Herein, we report the first NCS alkaline-earth metal fluorooxoiodate, Sr(IOF)·2HO, synthesized via hydrothermal methods. This compound crystallizes in the polar space group 222 and features a wide bandgap of 4.52 eV and a short ultraviolet cutoff edge of 240 nm. Structural evolution from centrosymmetric (CS) Sr(IO)·HO and Ba(IOF) to NCS Sr(IOF)·2HO was achieved through strategic fluorination and hydration. Experimental and computational analyses reveal that Sr(IO)·HO exhibits exceptional birefringence of 0.126 at 1064 nm and a wide bandgap of 4.53 eV, positioning it as a promising UV birefringent optical material. In contrast, Sr(IOF)·2HO demonstrates moderate birefringence of 0.04 at 1064 nm and a nonlinear response of 0.64 × KDP by calculated. This work highlights the fluorine-water synergy as a powerful strategy for designing functional crystals with tailored optical properties.