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.

Wide ultraviolet (UV) transparency, strong second‐harmonic generation (SHG) response, and sufficient optical birefringence for phase‐matching (PM) at short SHG wavelengths are vital for practical UV nonlinear optical (NLO) materials. However, simultaneously optimizing these properties is a major challenge, particularly for metal phosphates. Herein, we report a non‐traditional π‐conjugated cation‐based UV NLO phosphate [C(NH2)3]6(PO4)2⋅3 H2O (GPO) with a short UV cutoff edge. GPO is SHG active at 1064 nm (3.8 × KH2PO4 @ 1064 nm) and 532 nm (0.3 × β‐BaB2O4 @ 532 nm) and also possesses a significant birefringence (0.078 @ 546 nm) with a band gap >6.0 eV. The PM SHG capability of GPO can extend to 250 nm, indicating GPO is a promising UV solar‐blind NLO material. Calculations and crystal structure analysis show that the rare coexistence of wide UV transparency, large SHG response, and optical anisotropy is due to the introduction of π‐conjugated cations [C(NH2)3]+ and their favorable arrangement with [PO4]3− anions.

Second-harmonic-generation (SHG) efficiency and birefringence are two crucial parameters for nonlinear optical (NLO) crystals. However, the simultaneous optimization of these two key parameters remains a great challenge due to their contrasting microstructure prerequisites. In this paper, the first example of tetravalent rare-earth iodate-sulfate Ce­(IO3)2(SO4) (CISO) was designed by a defluorinated homovalent substitution strategy based on the parent compound CeF2(SO4)·H2O (CFSO) from the centrosymmetric to the noncentrosymmetric structure, which shows a large SHG effect of 3.5 × KDP in metal sulfates and a significant birefringence (0.259 at 546 nm) in hetero-oxyanion tetrahedral-group-based optical crystals. In the structure of CISO, both [IO3] and [SO4] groups are in the cis-position to each other, while [CeO8] polyhedra are highly distorted due to the different coordination environments of two kinds of oxyanions in one structure, which displays optimized optical anisotropy and polarizability attributed to their synergistic effect, leading to the strong SHG efficiency and sufficient birefringence. First-principles simulations for CISO have been employed to rationalize the correlations between the molecular structure and optical properties.