Cationic Coordination Modification Drives Birefringence and Nonlinear Effect Double Lifting in Sulfate

As nonlinear optical (NLO) crystals, sulfates have the superiority of transparency for ultraviolet (UV) light, but they are often troubled by small nonlinear coefficients and birefringence owing to the high symmetry of the [SO₄]^2- group. By introducing two neutral diethylenetriamine (DETA) molecules to replace the six coordinated water molecules of the [Zn(H₂O)₆]²⁺ complex cation in [Zn(H₂O)₆](SO₄)(H₂O), a new sulfate with an acentric structure, namely, [Zn(DETA)₂](SO₄)(H₂O)₃, has been designed and synthesized. Structural investigation reveals that the coordination modification of Zn²⁺ ion tremendously enhances its intraoctahedral distortion. The formed distorted [Zn(DETA)₂]²⁺ cations and the [SO₄]^2- groups feature an optimized arrangement, endowing [Zn(DETA)₂](SO₄)(H₂O)₃ with enhancements in both second harmonic generation (SHG) intensity, from undetectable to 0.25 × KH₂PO₄ (KDP), and birefringence, from 0.014 to 0.042 at 1064 nm. Despite the slight compromise made in the light transmission range, [Zn(DETA)₂](SO₄)(H₂O)₃ still possesses a short absorption edge of 220 nm, retaining the majority of the light transmission range in the solar-blind region. Our work provides a novel and applicable approach of cationic coordination modification to improve the nonlinear optical coefficient and birefringence of sulfates.