NaYFPO and KYFPO: [YOF] Polyhedra Inducing Birefringence Enhancement in Non-π-Conjugated Phosphate Systems.

In this paper, two novel compounds NaYFPO and KYFPO were synthesized via high-temperature solid-state reactions. NaYFPO and KYFPO respectively crystallize in space group 2/ and 1̅. These two compounds both feature [YOF] polyhedra and [PO] tetrahedra. Theoretical calculations show that NaYFPO and KYFPO 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 APO (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 [SO] tetrahedra. This study advances structural chemistry within the domain of rare-earth phosphates and offers a novel approach for designing phosphates with moderate birefringence.