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.