Achieving record-high birefringence in a 3D fluorozincoborate via coplanar alignment of π-conjugated [BO3] units

Abstract

Borates are advanced optical materials that demonstrate broad application prospects in the field of birefringence. This study employed a low-temperature flux method to synthesize two novel alkaline metal fluorozincoborates, namely, Na₆ZnF₂(B₁₂O₂₄)H₆ and KZn₃F(BO₃)₂. They crystallize in the Rc and P6₃/m space groups, respectively. Na₆ZnF₂(B₁₂O₂₄)H₆ possesses isolated high-symmetry [ZnF₂(B₁₂O₂₄)]¹²⁻ groups, based on corner sharing [BO₄]⁵⁻ tetrahedra, [BO₃]³⁻ triangles, and [ZnO₃F]⁵⁻ tetrahedra. Six-membered rings (6-MRs) are contained in the [B₁₂O₂₄]¹²⁻ groups, into which were inserted [ZnO₃F]⁵⁻ tetrahedra via sharing common vertices. In contrast, KZn₃F(BO₃)₂ features an unprecedented three-dimensional (3D) [Zn₃(BO₃)₂F]⁻ framework, which was constructed by [BO₃]³⁻ triangles, [ZnO₆]¹⁰⁻ octahedra, and [(ZnO₃)₂F]⁹⁻ dimers. Two types of 6-MRs were contained in the structure along the a- and b-axes, built by two [BO₃]³⁻ triangles and four [ZnO₃F]⁵⁻ tetrahedra. Optically, Na₆ZnF₂(B₁₂O₂₄)H₆ exhibits moderate birefringence (Δn_cal. = 0.04@546 nm, Δn_exp. = 0.05@546 nm). Remarkably, KZn₃F(BO₃)₂ exhibits the highest birefringence (Δn_cal. = 0.11@546 nm, Δn_exp. = 0.09@546 nm) within the zinc borate family, in which the π-conjugated modules, [BO₃]³⁻, are the key source of the large optical anisotropy. The synthetic processes, elemental analyses, topological structures, thermal stability, Raman spectra, UV-Vis-NIR diffuse reflectance, and birefringence properties were systematically investigated. This work not only enriches the structural chemistry of fluorozincoborates, but also provides valuable insights for the rational design of novel birefringent materials.