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.