MAlB3O6F2 (M = Ca and Pb): Two New Fluoroaluminoborates Featuring π-Conjugated [BO3] and [AlO4F2] Units with Large Birefringence

Current birefringent crystals (e.g., MgF₂, α-BBO, and YVO₄) are limited by either small birefringence or insufficient deep-UV transparency. Achieving both properties simultaneously remains a key challenge. To address this issue, recent research based on the “confined π-conjugated” strategy has turned its attention to fluoroaluminoborates. In this structural design, non-π-conjugated [Al−O/F] units connect and isolate planar π-conjugated groups, effectively broadening the bandgap while maintaining optical anisotropy. Building on this concept and incorporating different cations, this study reports two new fluoroaluminoborate compounds: CaAlB₃O₆F₂ (CABOF) and its isostructural analogue, PbAlB₃O₆F₂ (PABOF). In PABOF, the introduction of Pb²⁺ induces local structural distortion, thereby enhancing the optical anisotropy. Both CABOF and PABOF exhibit birefringence values exceeding 0.1 at 546 nm, with PABOF demonstrating the largest birefringence among the known fluoroaluminoborates. In addition, both compounds exhibit excellent thermal stability. This work identifies two potential UV-birefringent materials, further enriching the fluoroaluminoborate family.