Centrosymmetric Rb2Sb(C2O4)2.5(H2O)3 and Noncentrosymmetric RbSb2(C2O4)F5: Two Antimony (III) Oxalates as UV Optical Materials

Herein, we have successfully synthesized two rubidium antimony (III) oxalates, namely, Rb₂Sb(C₂O₄)_2.5(H₂O)₃ and RbSb₂(C₂O₄)F₅, utilizing a low-temperature hydrothermal method. These two compounds share a similar chemical composition, consisting of Sb³⁺ cations with active lone pair electrons, alkali metal Rb⁺ ions, and planar π-conjugated C₂O₄^2- anions. However, they exhibit different symmetries, Rb₂Sb(C₂O₄)_2.5(H₂O)₃ is centrosymmetric (CS), while RbSb₂(C₂O₄)F₅ is noncentrosymmetric (NCS), which should be caused by the presence of F^- ions. Notably, the NCS compound, RbSb₂(C₂O₄)F₅, demonstrates a moderate second-harmonic generation (SHG) response, approximately 1.3 times that of KH₂PO₄ (KDP), and exhibits a large birefringence of 0.09 at 546 nm. These characteristics indicate that RbSb₂(C₂O₄)F₅ holds promising potential as a nonlinear optical material for ultraviolet (UV) applications. Detailed structural analysis and theoretical calculations confirm that the excellent optical properties arise from the synergistic effects between Sb³⁺ cations with SCALP and planar π-conjugated [C₂O₄]^2- groups.