Large Second-Harmonic Response and Giant Birefringence of CeF₂(SO₄) Induced by Highly Polarizable Polyhedra

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.