YSO₄F·H₂O: A Deep-Ultraviolet Birefringent Rare-Earth Sulfate Fluoride with Enhanced Birefringence Induced by Fluorinated Y-Centered Polyhedra

Birefringent crystals can modulate and detect the polarization of light and are important optical functional materials. The birefringence is positively correlated to the anisotropy of the structure. By partially substituting sulfate anion with large electronegative fluorine in the parent compound Y₂(SO₄)₃·8H₂O, a new fluorinated rare-earth sulfate YSO₄F·H₂O with enhanced anisotropy was achieved. YSO₄F·H₂O features a dense 3D structure constructed by the polarizable [YOF] polyhedra and [SO₄] tetrahedra. The diffuse reflectance spectrum reveals that it has a short UV absorption edge of below 200 nm. The substitution of the F^- ion enhances the optical anisotropy, making the material exhibit an enhanced birefringence (0.0357 at 546 nm), which is 5.1 times that of the parent compound and is also larger than most deep-UV birefringent sulfates. It is expected that this work may shed useful insights in the exploration of deep-UV birefringent materials with enhanced optical performances..