La₂Zn₃(SeO₃)₆: A UV Selenite Crystal with Second-Harmonic Generation and Large Birefringence

Inorganic selenites are promising candidates for nonlinear optical and birefringent crystals. However, selenites that can effectively transmit in the UV region are still relatively scarce. In this study, we successfully synthesized the first Zn-based UV selenite with birefringent and nonlinear optical properties using a hydrothermal method by introducing Zn²⁺ and La³⁺ ions, which can cause a blue shift in the compound's bandgap, into the selenite system. The compound crystallizes in the chiral space group P3₁12 and presents a unique three-dimensional structure composed of two-dimensional zinc selenite layers bridged by LaO₁₀ polyhedra. Notably, the compound possesses a wide bandgap (5.0 eV), high birefringence (0.092 at 546 nm), excellent thermal stability (∼540 °C), a broad transparency window (0.21-6 μm), and apparent SHG efficiency (0.2 × KDP). Our work not only demonstrates the great potential of Zn-based selenite as UV optical crystals but also expands the research scope of UV optical materials.