A research team from the Xinjiang Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences, has achieved progress in the development of sulfate-based deep-ultraviolet (DUV) birefringent crystals. By incorporating rare earth metal Y³⁺ cations into alkali metal sulfates, the team synthesized six new sulfate crystal compounds, improving the birefringence for this material system. The study was recently published in Chemistry of Materials.

Birefringence, a key optical property of crystalline materials, refers to their ability to exhibit anisotropic refractive indices when interacting with polarized light. This property is critical to modern optoelectronic technologies, including integrated optical modulators, electro-optical switches, and nonlinear optical frequency conversion. However, materials that simultaneously demonstrate high birefringence and DUV transmission remain relatively scarce.

Sulfates are widely regarded as ideal candidates for DUV optical materials, thanks to the wide energy gap of their [SO₄] tetrahedral groups. Yet the low polarizability anisotropy of the [SO₄] tetrahedral structure has long hindered efforts to further enhance birefringence in sulfate-based systems.

To address this challenge, the team introduced rare earth metal Y³⁺ cations—characterized by an f⁰ electronic configuration—into traditional alkali metal sulfates. This approach enabled the successful design and synthesis of six new compounds: AY(SO₄)₂ and A₃Y(SO₄)₃, where A represents alkali metals (K, Rb, or Cs).

Structural analysis revealed that Y³⁺ forms highly distorted [YOn] polyhedra (with n = 7 or 8). These polyhedra, when combined with [SO₄] tetrahedra, create a Y–O–S network structure with adjustable dimensionality. Notably, as the radius of the alkali metal cation increases, the Y–O–S framework gradually transitions from a three-dimensional structure to a near two-dimensional layered structure— a shift that enhances optical anisotropy.

Theoretical calculations confirmed that the birefringence of these new compounds ranges from 0.013 to 0.056 at 546 nm. The highest birefringence value recorded is 25 times that of the corresponding parent alkali metal sulfates, outperforming most previously reported sulfate-based DUV crystals. Additionally, all six compounds maintain an absorption edge below 200 nm, satisfying the technical requirements for DUV applications.

This study not only achieves a substantial enhancement in birefringence for DUV sulfate crystals but also provides an approach for the future design of high-performance sulfate-based DUV birefringent crystals and nonlinear optical crystals, the researchers noted.

This work was supported by funding from the Tianshan Innovation Team Program, the National Natural Science Foundation of China, and other institutional sources.