A2BeS2O8 (A = NH4, K, Rb, Cs) Deep Ultraviolet Nonlinear Optical Crystals

A series of new noncentrosymmetric beryllium sulfates, A2BeS2O8 (A = NH4, K, Rb, Cs), were rationally designed and synthesized based on the parent borophosphates KMBP2O8 (M = Sr, Ba). Owing to the rigid three-dimensional frameworks, A2BeS2O8 (A = NH4, K, Rb, Cs) showed the high thermodynamic stabilities. Among them, A2BeS2O8 (A = K, Rb, Cs) exhibited the highest thermodynamic stabilities among all the reported nonlinear (NLO) sulfates. In addition, benefiting from the flexible Be–O bonds within [BeO4], A2BeS2O8 (A = NH4, K, Rb, Cs) exhibited the enhanced second harmonic generation (SHG) responses, which were about 3 to 5 times that of parent borophosphates. Furthermore, they possessed a moderate birefringence of 0.013–0.024 and could be transparent down to the deep ultraviolet (DUV) region below 200 nm. This work indicated that the basic building blocks in borophosphates and beryllium sulfates were potentially interchangeable, which provided the possibility for a further study of new NLO crystals.