K2S4O6: improving birefringence and nonlinear optical properties with the [O3S–S–S–SO3]2− group

Abstract

Sulfates have gradually become a hot research topic of NLO materials due to their generally large band gaps and can be easily grown by aqueous methods. However, they have small anisotropy and hyperpolarizability, which tend to result in a weak second harmonic generation (SHG) effect and small birefringence. Herein, the tetrathionate anionic group (S₄O₆)²⁻ caught our attention, and we synthesized potassium tetrathionate K₂S₄O₆ by a simple aqueous method. Compared with most reported UV NLO sulfates, it has a better SHG effect of 1.8 × KDP and a large birefringence (0.066@1064 nm), which resolves contradictions between the birefringence and SHG effects of sulfates. More importantly, the discovery of this compound will provide more ideas for the exploration of NLO crystals with novel structures.