The First Alkaline-Earth Fluorooxoborate Ba[B₄ O₆ F₂ ]-Characterisation and Doping with Eu²

The very first alkaline-earth fluorooxoborate Ba[B₄ O₆ F₂ ] was synthesised by solid state methods starting from Ba(BF₄ )₂ , β-BaB₂ O₄ , and B₂ O₃ . The crystal structure derived from single-crystal X-ray diffraction (P2₁ /n, a=6.6384(2) Å, b=7.6733(3) Å, c=11.3385(4) Å, β=91.281(2)°, Z=4, R_int =0.0269, R₁ =0.018, wR₂ =0.034) comprises layers of BO₃ F tetrahedra condensed through triangular BO₃ units according to the descriptor 2Δ2□:<Δ2□>Δ. The extraordinary thirteen-fold coordination of barium by oxygen and fluorine leads to interesting optical properties of a sample doped with divalent europium, where a 4f→4f emission was recorded around 359 nm together with a broad emission band of a 5d→4f emission peaking at 366 nm. The compound is further characterised by IR-, Raman-, and solid-state NMR-spectroscopic methods. Moreover, DFT calculations as well as TGA and DSC measurements were performed.