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

Abstract

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 [VO_xS_4−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 (∼0.37 in Ba₅V₂O₄S₈) among the pure tetrahedron-based compounds and indicate the potential of [VO_xS_4−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 [VO_xS_4−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.