In general, fluorides are widely recognized as a distinguished class of optical materials, playing a critical role in various optical applications, including fluorescent, birefringent, and nonlinear optical materials. However, the high-symmetry and isotropic [MF₆] (M = metal atoms) octahedra significantly mitigate the development of large Δa and birefringence. Thus, addressing the challenges and developing birefringent fluorides is an urgent priority in the field of optical materials science.

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

The 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 result in the displacement of interlayer Cl⁻ anions along the c direction, and further break the symmetry from Fm−3m to P4/nmm and create anisotropy from isotropy.

Then, the researchers introduced the S-S bond to this system to further upgrade the anisotropy and birefringence, and it finally succeeded in obtaining a salt-inclusion chalcogenide BaFS with large birefringence (0.238@546 nm) achieved by overall considering experimental observations with theoretical analysis.

The researchers revealed that the decisive structural units for birefringence are interlayer units. To compare the three different interlayer units, Gaussian calculation results indicated that the polarizability anisotropy of [Ba₄F]⁷⁺ tetrahedra, [Ba₅Cl]⁹⁺ tetragonal pyramids, and [Ba₅S₂]⁸⁺ units are 0, 5.72 and 31.78, respectively. These calculation results demonstrate 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 exploration and development of potential birefringent materials in laser field.

The birefringence enhancement from BaF2 to BaFCl to BaFS via interlayer anion substitution (Image by Prof. GUO’s group)

Contact:

Prof. GUO Guocong

Fujian Institute of Research on the Structure of Matter

Chinese Academy of Sciences

Email: gcguo@fjirsm.ac.cn‬‬‬