Fluorides are widely recognized as a distinguished class of optical materials, playing a critical role in fluorescent, birefringent, and nonlinear optical materials. The high-symmetry and isotropic [MF₆] (M=metal atoms) octahedra significantly limit the development of large Δa and birefringence. It is urgent to address these issues and develop birefringent fluorides.

In a study published in Small, a research group led by Prof. GUO Guocong from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences proposed a novel strategy that transforms optical isotropy to anisotropym, and achieves large birefringence through interlayer anion substitution.

Researchers first proposed a two-step interlayer anion substitution strategy to address the low birefringence of fluorides. After editing the interlayer anions and replacing F⁻ with Cl⁻ anions, 5-fold coordination modes of [Ba₅Cl]⁹⁺ tetragonal pyramids different from [Ba₄F]⁷⁺ tetrahedra resulted in the displacement of interlayer Cl⁻ anions along the c direction, breaking the symmetry from Fm−3m to P4/nmm and creating anisotropy from isotropy.

Then, researchers introduced the S-S bond to this system to upgrade the anisotropy and birefringence, and obtained a salt-inclusion chalcogenide BaFS with large birefringence (0.238@546 nm) combining experimental observations with theoretical analysis.

The decisive structural units for birefringence were revealed to be interlayer units. Three different interlayer units were compared through Gaussian calculation. It was found that the polarizability anisotropy of [Ba₄F]⁷⁺ tetrahedra, [Ba₅Cl]⁹⁺ tetragonal pyramids, and [Ba₅S₂]⁸⁺ units are 0, 5.72 and 31.78, respectively, which demonstrates the significance of homoatomic S-S bond in generating large birefringence of BaFS.

This study aims to introduce a novel design paradigm that combines templates with linear optical building units, paving the way for the development of potential birefringent materials in laser field.