Ba2.1Bi0.9(O, F)6.8−δ: A new ordered anion-excess fluorite

The anion-excess ordered fluorite-related phase Ba2.1Bi0.9(O, F)6.8−δ has been synthesized by a solid state reaction of BaF2, BiF3 and Bi2O3 at 873 K with subsequent short annealing at 573 K. The crystal structure of the new phase has been solved using electron diffraction and X-ray powder diffraction (a = 9.5372(1) Å, c = 18.1623(3) Å, space group I4/m, RI = 0.025, RP = 0.029). Interstitial anions in the fluorite-based structure are considered to form isolated cuboctahedral 8:12:0 clusters. The structural relationship between the oxyfluoride phase Ba2.1Bi0.9(O, F)6.8−δ and similar rare-earth-based fluorides is discussed.