Ce₃F₄(SO₄)₄: cationic framework assembly for designing polar nonlinear optical material through fluorination degree modulation

Noncentrosymmetric (NCS) structure is the paramount precondition for producing second-harmonic generation (SHG) of crystalline materials. Herein, a fluorine-enriched-induced centrosymmetric-to-noncentrosymmetric transformation strategy was employed in CS Ce₂F₂(SO₄)₃·2H₂O, leading to the discovery of new NCS cerium fluoride sulfate, Ce₃F₄(SO₄)₄. Ce₃F₄(SO₄)₄ crystallizes in polar space group C2, and features a 2D-layered framework constructed from [CeO₅F₃] and [CeO₆F₂] polyhedra, extending to a final 3D framework linked via two crystallographically independent [SO₄] units. Ce₃F₄(SO₄)₄ displays a phase-matchable SHG effect of about 1.0 times that of KDP and sufficient experimental birefringence (0.141@546 nm) in fluorine-containing sulfate-based NLO materials. Structural analysis and first-principle calculations suggest that the ordered arrangement of fluorinated cerium-centered polyhedra ([CeO₅F₃] and [CeO₆F₂]) as well as [SO₄] tetrahedra play a significant role in SHG activity and birefringence. Our study illustrates that the introduction of metal-centered polyhedra with a high degree of fluorination is conducive to the formation of NCS structure and is a valuable method for designing novel nonlinear optical materials.