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

The first examples of alkali metal selenite sulfates, namely, Na 8 (SeO 3 )(SO 4 ) 3 (1), Na 2 (H 2 SeO 3 )(SO 4 ) (2), and K 4 (H 2 SeO 3 )(HSO 4 ) 2 (SO 4 ) (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. Na 8 (SeO 3 )(SO 4 ) 3 (1) features a noncentrosymmetric structure, while Na 2 (H 2 SeO 3 )(SO 4 ) (2) and K 4 (H 2 SeO 3 )(HSO 4 ) 2 (SO 4 ) (3) are centrosymmetric. Powder second-harmonic-generation measurements revealed that Na 8 (SeO 3 )(SO 4 ) 3 (1) shows a phase-matchable SHG intensity about 1.2 times that of KDP. UV-vis-NIR diffuse reflectance spectroscopic analysis indicated that Na 8 (SeO 3 )(SO 4 ) 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 Na 8 (SeO 3 )(SO 4 ) 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.