Synthesis, Crystal Structure, and Optical Properties of the First Alkali Metal Rare-Earth Iodate Fluoride: Li₂Ce(IO₃)₄F₂

The first alkali metal rare-earth iodate fluoride, Li₂Ce­(IO₃)₄F₂, was synthesized using a hydrothermal method. This compound features a novel [CeI₄O₁₂F₂]_∞ infinite layer with each [CeO₆F₂] dodecahedron connected to six parallel umbrella-like [IO₃] groups. Each layer is connected by [Li₂I₂O₈F₂] rings to form a condensed 3D structure. The first-principles calculations indicate that Li₂Ce­(IO₃)₄F₂ possesses moderate birefringence (Δn = 0.054, λ = 589 nm), which is mainly originated from the umbrella-like [IO₃] group and the [CeO₆F₂] polyhedrons. Li₂Ce­(IO₃)₄F₂ is possibly a birefringence crystal for visible light.