Rational Design of the Nonlinear Optical Response in a Tin Iodate Fluoride Sn(IO₃)₂F₂

A new tin iodate fluoride, Sn­(IO₃)₂F₂, was meticulously designed based on Sn­(IO₃)₄ with an intriguing chemical tailoring transformation from the centrosymmetric to noncentrosymmetric structure. Sn­(IO₃)₂F₂ crystallizes in polar space group P2₁ and features an one-dimensional chain constructed by IO₃ and SnO₄F₂. It has an enlarged band gap (4.08 eV) compared to the Sn­(IO₃)₄ (3.96 eV) because of the introduction of strong electronegative F^–. Also, powder second-harmonic generation (SHG) measurements revealed that Sn­(IO₃)₂F₂ is a phase-matchable nonlinear crystal with a strong SHG response about 3 times that of KH₂PO₄. Theoretical investigations elaborated that its large nonlinearity originates from the synergistic effect of lone pairs on IO₃ and flexible dipole moments of SnO₄F₂ polyhedra.