CsSbF2SO4: An Excellent Ultraviolet Nonlinear Optical Sulfate with a KTiOPO4 (KTP)‐type Structure

A novel noncentrosymmetric (NCS) polar fluoride sulfate, CsSbF₂SO₄, was obtained by ionothermal synthesis. A meticulously designed co‐substitution approach was used to successfully replace the TiO₆]^8? and PO₄]^3? functional groups in KTiOPO₄ (KTP) with SbO₄F₂]^7? and SO₄]^2? units, respectively. The structure of CsSbF₂SO₄ features a pseudo‐3D framework consisting of interconnected 1D SbF₂O₂SO₄]^5? chains of corner‐sharing SbO₄F₂]^7? octahedra and SO₄]^2? tetrahedra. The title compound exhibits a sharply enlarged band gap compared to its parent compound, KTP, benefitting from the introduction of F^? ions and the displacement of Sb³⁺ cations. Second harmonic generation (SHG) measurements manifested that CsSbF₂SO₄ is phase‐matchable and revealed a strong SHG response of about 3.0 KH₂PO₄ (KDP), which is the highest value reported for any metal sulfate reported to date. The reported fluoride sulfate is a promising near ultraviolet (UV) nonlinear optical (NLO) material.