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, Rint =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.