Centrosymmetric RbSnF2NO3vs. noncentrosymmetric Rb2SbF3(NO3)2

Abstract

Two fluoride nitrates named RbSnF₂NO₃ and Rb₂SbF₃(NO₃)₂ have been successfully synthesized using a rapid evaporation concentration method by introducing Sn²⁺/Sb³⁺ cations with stereochemically active lone pairs into the nitrate system. The discrepancy in the cation sizes and coordination modes induces the different symmetries of these two title compounds, RbSnF₂NO₃ crystallizing in a centrosymmetric (CS) space group of C2/m, while Rb₂SbF₃(NO₃)₂ in a noncentrosymmetric (NCS) space group of P2₁. In particular, NCS Rb₂SbF₃(NO₃)₂ exhibits a strong second harmonic generation (SHG) response of approximately 2.7 times that of benchmark KH₂PO₄ (KDP) and an appropriate birefringence (0.06@1064 nm) for phase matching, indicating that Rb₂SbF₃(NO₃)₂ is a promising UV nonlinear optical material. Detailed structural and theoretical analysis confirmed that the large nonlinear optical coefficient was derived from the synergistic effect of the π-conjugated planar triangular NO₃ groups and the Sb³⁺ cations with stereochemically active lone pairs.