UV-Transparent SHG Material Explored in an Alkali Metal Sulfate Selenite System

The first examples of alkali metal selenite sulfates, namely, Na₈(SeO₃)(SO₄)₃ (1), Na₂(H₂SeO₃)(SO₄) (2), and K₄(H₂SeO₃)(HSO₄)₂(SO₄) (3), were successfully synthesized by hydrothermal reactions. Their structures display three different zero-dimensional configurations composed of isolated sulfate tetrahedra and selenite groups separated by alkali metals. Na₈(SeO₃)(SO₄)₃ (1) features a noncentrosymmetric structure, while Na₂(H₂SeO₃)(SO₄) (2) and K₄(H₂SeO₃)(HSO₄)₂(SO₄) (3) are centrosymmetric. Powder second-harmonic-generation measurements revealed that Na₈(SeO₃)(SO₄)₃ (1) shows a phase-matchable SHG intensity about 1.2 times that of KDP. UV-vis-NIR diffuse reflectance spectroscopic analysis indicated that Na₈(SeO₃)(SO₄)₃ (1) has a short UV cutoff edge and a large optical band gap, which makes it a possible UV nonlinear optical material. Theoretical calculations revealed that the birefringence of Na₈(SeO₃)(SO₄)₃ (1) is 0.041 at 532 nm, which is suitable for phase-matching condition. This work provides a good experimental foundation for the exploration of new UV nonlinear crystals in an alkali metal selenite sulfate system.