Heteroanionic [VOxS4-x] groups: tetrahedral units with large birefringence for mid-infrared nonlinear optical crystals

Tetrahedral groups can possess a wide bandgap and large second harmonic generation (SHG) response in nonlinear optical (NLO) materials; however, they usually exhibit a small optical birefringence owing to the low structural anisotropy, which would deteriorate the NLO performance. Herein, the [VOxS4-x] (x = 0-4) tetrahedra that combine the V5+ cation with oxygen-sulfur hybrid anions are highlighted as a type of good structural group to overcome the above problem. Using theoretical calculations, we systematically investigate the optical and NLO properties of all reported V5+-based thiovanadates. The calculated results reveal the largest birefringence (similar to 0.37 in Ba5V2O4S8) among the pure tetrahedron-based compounds and indicate the potential of [VOxS4-x] units in enhancing the birefringence and well-balanced NLO performance suitable for the mid-infrared (mid-IR) region. Furthermore, Na3VOS3 in this family is selected for experimental synthesis and measurements to verify our strategy and calculations. This work demonstrates that the [VOxS4-x] (x = 0-4) tetrahedral groups, which had been overlooked for a long time, are a good type of birefringent and NLO unit for the exploration of mid-IR NLO materials.