CsSbF₂ SO₄ : An Excellent Ultraviolet Nonlinear Optical Sulfate with a KTiOPO₄ (KTP)-type Structure

A novel noncentrosymmetric (NCS) polar fluoride sulfate, CsSbF₂ SO₄ , 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 [SbO₄ F₂ ]^7- and [SO₄ ]^2- units, respectively. The structure of CsSbF₂ SO₄ features a pseudo-3D framework consisting of interconnected 1D [SbF₂ O₂ SO₄ ]^5- chains of corner-sharing [SbO₄ F₂ ]^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 CsSbF₂ SO₄ is phase-matchable and revealed a strong SHG response of about 3.0 KH₂ PO₄ (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.