Achieving balanced UV SHG responses, optical band gaps and birefringence in rare earth compounds Ln(IO3)(SO4)·3H2O (Ln = Y, Gd, Er, Ho, Dy, Eu)†

A series of rare earth iodate sulfate UV compounds, Ln(IO₃)(SO₄)·3H₂O (Ln = Y, Gd, Er, Ho, Dy, Eu), have been successfully synthesized by the hydrothermal method at 200 °C. These isostructural compounds all crystallize in the noncentrosymmetric space group P2₁2₁2₁ (no. 19) and feature a neutral three-dimensional Ln(IO₃)(SO₄) framework which is composed of 2D cationic Ln[SO₄]⁺ layers bridged by anionic [IO₃]⁻ trigonal pyramids through sharing corner oxygen atoms. Under 1064 nm laser irradiation, Y(IO₃)(SO₄)·3H₂O exhibits a second-harmonic generation (SHG) with an efficiency of 0.7 × KDP@1064 nm. Furthermore, Y(IO₃)(SO₄)·3H₂O has a moderate birefringence (0.118@532 nm) and a large band gap (4.60 eV) and may be a potential UV nonlinear optical material. For Eu(IO₃)(SO₄)·3H₂O, it emits intense photoluminescence peaks at 594 nm and 617 nm when excited under 393 nm near-ultraviolet light, showing promising applications as red phosphors of white-LEDs. The current study elucidates that the incorporation of highly anisotropic lone-paired (IO₃)⁻ units into highly isotropic (SO₄)²⁻ sulfate groups can achieve balanced SHG responses, optical band gaps and birefringence, facilitating the development of novel iodate sulfate crystals for UV nonlinear optical applications.