NH4Y(SO4)2·H2O and NH4YSO4F2: Two New Ammonium-Rare Earth Metal Sulfates with Enhanced Optical Anisotropy and Deep Ultraviolet Transmission

Tetrahedral oxygenated groups with large highest occupied molecular orbital-lowest unoccupied molecular orbital (H gaps such as [SO₄] are beneficial for deep ultraviolet (DUV) transmission but usually make against generating sufficient birefringence due to the small polarizability anisotropy. Thus, it is extremely difficult to obtain DUV transmission and large birefringence simultaneously in the search for DUV birefringent materials in sulfates. Herein, two new ammonium-rare earth metal sulfates, NH₄Y(SO₄)₂·H₂O and NH₄YSO₄F₂, with DUV transmission are presented. Meanwhile, both exhibit greatly elevated birefringence through the involvement of NH₄⁺ units, compared to Y₂(SO₄)₃·8H₂O. Their optical properties are further investigated by theoretical calculations, and the effect of the introduction of NH₄⁺ into yttrium sulfate on optimizing the structures and properties is discussed. This work may provide a new perspective for further exploration of DUV birefringent materials in tetrahedral oxygenated group sulfates.