Na₃Y₂F₃P₂O₈ and K₄Y₃F₅P₄O₁₄: [YOF] Polyhedra Inducing Birefringence Enhancement in Non-π-Conjugated Phosphate Systems

In this paper, two novel compounds Na₃Y₂F₃P₂O₈ and K₄Y₃F₅P₄O₁₄ were synthesized via high-temperature solid-state reactions. Na₃Y₂F₃P₂O₈ and K₄Y₃F₅P₄O₁₄ respectively crystallize in space group P2₁/c and P1̅. These two compounds both feature [YOF] polyhedra and [PO₄] tetrahedra. Theoretical calculations show that Na₃Y₂F₃P₂O₈ and K₄Y₃F₅P₄O₁₄ exhibit moderate band gaps (5.211 and 5.208 eV, respectively) and birefringence values (0.027 and 0.023 at 1064 nm, respectively). Compared with the single alkali metal phosphate A₃PO₄ (A = Na, K), the birefractive indices are significantly enhanced, whereas the band gaps are slightly reduced. The calculation shows that the [YOF] polyhedron is the main reason for the enhancement of the birefractive index. Similarly, the [YOF] polyhedron also exhibits a significant birefringent gain in compounds containing [SO₄] tetrahedra. This study advances structural chemistry within the domain of rare-earth phosphates and offers a novel approach for designing phosphates with moderate birefringence.