Enhancement of Birefringence in Alkali Metal Sulfates Induced by Rare Earth Metal-Centered Polyhedra and Structure Regulation via Cations

Sulfates have remained promising candidates for deep-ultraviolet (DUV) optical crystals owing to the inherently wide bandgaps induced by the [SO 4 ] tetrahedral groups. However, the structural rigidity of [SO 4 ] tetrahedra imposes severe restrictions on the polarization anisotropy, fundamentally preventing sulfates from achieving sufficient birefringence required for regulating light polarization. Herein, by incorporating f 0 rare earth metal cation Y 3+ into alkali metal sulfates, we synthesized six compounds, AY(SO 4 ) 2 and A 3 Y(SO 4 ) 3 (A = K, Rb, Cs). They exhibit dramatically enhanced birefringence between 0.013–0.056 at 546 nm, with up to 25 times that of their parent alkali metal sulfates, which originate from the highly aberrant Y–O polyhedra and the dimensionality reduction of the Y–O–S network regulated by A-site cations. This study provides valuable insights into the further exploration of sulfate-based optical materials.