To improve the key properties of nonlinear optical crystals assembled with tetrahedral functional building units

Nonlinear optical (NLO) crystals assembled with conventional non-π-conjugated tetrahedral functional building units (FBUs), generally referring to [PO₄] and [BO₄], usually exhibit weak nonlinearity and poor birefringence. It is currently proposed that partially substituting oxygen atoms with fluoride atoms in these FBUs could enhance these crucial properties. Hence, we investigated for the first time the NLO-related properties of NH₄BAsO₄F (ABAF), which was constructed from tetrahedral [BO₃F] and [AsO₄] FBUs, and enhancements of these properties were observed in this material, that is large second-harmonic generation (SHG) response (2 × KDP) and improved birefringence (0.03 at 1064 nm). Notably, both SHG coefficient and birefringence of ABAF exceeded those of a great majority of phosphates, sulfates, or boron phosphates and achieved a preferable balance. It is interesting that ABAF shows vast structural similarities to the typical NLO crystals Sr₂Be₂B₂O₇ (SBBO) and KBe₂BO₃F₂ (KBBF), which might be the partial reason why it showed improvement in these vital properties. This work may afford some inspiration for enhancing the key performances of NLO crystals assembled with non-π-conjugated tetrahedra.