Two Acentric Zn-Based l-Tartrates with Moderate Second Harmonic Generation Responses and Large Birefringence

The discovery of novel organic-inorganic hybrid nonlinear optical (NLO) crystal materials holds great potential in advancing laser science and technology as they offer a wide range of benefits including easy synthesis, structural versatility, and high hyperpolarizability. Herein, the integration of l-tartaric acid (L-C₄H₆O₆) and ZnSO₄ gave rise to two acentric Zn-based organic-inorganic hybrid crystals, namely, Zn₂(H₂O)₂(C₄H₄O₆)₂·3H₂O (Zn-LT) and Zn₂(H₂O)(C₄H₄O₆)(C₄H₆O₆)(SO₄)·4H₂O (Zn-LTS). They both feature layered structures constructed by [ZnO₆] octahedron, l-C₄H₆O₆/l-C₄H₄O₆^2-, water molecule, or [SO₄] tetrahedron. Interestingly, they possess moderate second-order NLO effects of 0.28 × KH₂PO₄ (Zn-LT) and 0.57 × KH₂PO₄ (Zn-LTS), large birefringence of 0.075 (Zn-LT) and 0.069 (Zn-LTS), suggesting that the introduction of [SO₄] groups with intrinsically weak polarizability and weak optical anisotropy induces the enhancement of the NLO effect while without reducing birefringence much. In addition, both of them display UV cutoff edges near 210 nm, indicating their potential as NLO crystals applied in the UV and solar-blind region.