AYSO₄F₂ (A = K, Rb): [YO₄F₄] Polyhedra Enhancement of Birefringence in Non-π-Conjugated Sulfate Systems

A mild hydrothermal method was employed to successfully synthesize two new sulfate fluorides, namely, AYSO₄F₂ (A = K, Rb). They are isomorphic, and both contain [YO₄F₄] polyhedra and [SO₄] tetrahedra in the structure. Theoretical calculations and experimental tests show that AYSO₄F₂ (A = K, Rb) have large band gaps (7.79 and 7.82 eV) and moderate birefringence (0.015 and 0.02 @ 546.1 nm), with significantly enhanced birefringence and band gaps as compared to that of the single alkali metal sulfates A₂SO₄ (A = K, Rb). Furthermore, theoretical calculations show that [YO₄F₄] polyhedra are the main reason for the band gap and birefringence enhancement. This work contributes to the advancement of structural chemistry in the field of rare-earth sulfates, offering a novel approach for the design of sulfates characterized by large birefringence.