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.