Zwitterionic Glycine-Based Antimony Fluorides with Exceptional Ultraviolet Nonlinear Optical Properties

Despite significant advancements in ultraviolet nonlinear optical (UV NLO) materials, a systematic design strategy remains elusive. This study introduces an approach to enhance NLO performance by utilizing zwitterionic units. Two antimony fluoride compounds, α–2SbF3·Gly and β–2SbF3·Gly, were synthesized by combining glycine zwitterions with Sb3+ cations and highly electronegative F– anions. The structure of α–2SbF3·Gly belongs to the polar noncentrosymmetric space group, Ia (No. 9), while β–2SbF3·Gly adopts the centrosymmetric space group, P21/c (No. 14). The metastable α–2SbF3·Gly undergoes an irreversible phase transition to the thermodynamically stable polymorph, β–2SbF3·Gly, at 160 °C. Notably, α–2SbF3·Gly exhibits an optimized arrangement of glycine zwitterions and SbF3 polyhedra, interconnected through hydrogen bonding. This structural configuration imparts exceptional optical properties, including a strong second-harmonic generation intensity 3.3 times that of KH2PO4, a wide band gap of 4.78 eV, and a suitable birefringence of 0.146 at 546 nm, highlighting its potential as an advanced UV NLO material. This work underscores the promise of zwitterionic units as a powerful design tool for UV NLO materials, providing a foundation for the development of next-generation NLO technologies.