Giant Optical Anisotropy in the UV-Transparent 2D Nonlinear Optical Material Sc(IO₃)₂(NO₃)

Birefringence is a fundamental optical property for linear and nonlinear optical (NLO) materials. Thus far, it has proved to be very difficult to engineer large birefringence in optical crystals functioning in the UV region. Herein, we report the first 2D rare-earth iodate–nitrate crystal Sc(IO₃)₂(NO₃) (SINO), which is shown to exhibit giant optical anisotropy. Air-stable SINO possesses a short UV absorption edge (298 nm), a strong NLO response (4.0 times that of benchmark KH₂PO₄) for the nitrate family, and the largest birefringence (Δn=0.348 at 546 nm) of inorganic oxide optical crystals. The unusually large birefringence and NLO response can be attributed to an optimized 2D layered structure, combined with highly polarizable and anisotropic building units [IO₃]⁻ and [NO₃]⁻. These findings will facilitate the development of UV linear and NLO materials with giant optical anisotropy and promote their potential application in optoelectronic devices.