Second-harmonic generation (SHG) response and birefringence are two critically important properties of nonlinear optical (NLO) materials. However, the simultaneous optimization of these two key properties remains a major challenge because of their contrasting microstructure requirements. Herein, we report the first tetravalent rare-earth metal fluorinated sulfate, CeF2(SO4). Its structure features novel net-like layers constructed by highly distorted [CeO4F4] polyhedra, which are further interconnected by [SO4] tetrahedra to form a three-dimensional structure. CeF2(SO4) exhibits the strongest SHG effect (8 times that of KH2PO4) and the largest birefringence for sulfate-based NLO materials, the latter exceeding the birefringent limit for oxides. Theoretical calculations and crystal structure analysis reveal that the unusually large SHG response and giant birefringence can be attributed to the introduction of the highly polarizable fluorinated [CeO4F4] polyhedra as well as the favorable alignment of [CeO4F4] polyhedra and [SO4] tetrahedra. This research affords a new paradigm for the designed synthesis of high-performance NLO materials.

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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.