Preparation and structure of the compounds Ba2Pb4F10Br2-xIx (x=0-2) with related structure motifs of the fluorites and matlockites

Colourless single crystals of Ba2Pb4F10Br2-xIx (x = 0-2) have been obtained under hydrothermal conditions (T = 250 degreesC, 10 d), starting from stoi chiometric amounts of BaF2, PbF2, PbBr2 and PbI2. The compounds crystallize in the tetragonal space group P4/nmm (No. 129). A complete miscibility in the region x = 0-2 has been observed. The mixed crystals follow Vegard's ru le. For the compounds with the composition Ba2Pb4F10Br2 (a = 5.9501(2) Angs trom, c = 9.6768(10) Angstrom, R[F-2 > 2 sigma (F-2)] = 0.022, wR(F-2 all r eflections) = 0.059), Ba2Pb4F10Br1.1I0,9 (a = 5.9899(3) Angstrom, c = 9.784 8(5) Angstrom, R[F-2 > 2 sigma (F-2)] = 0.014, wR(F-2 all reflections) = 0. 035) and Ba2Pb4F10I2 (a = 6.6417(3) Angstrom, c = 9.9216(10) Angstrom, R[F- 2 > 2 sigma (F-2)] = 0.023, wR(F-2 all reflections)= 0.049) complete struct ure ana lyses have been performed on the basis of single crystal diffractom eter data. Microcrystalline single phase compounds Ba2Pb4F10Br2-xIx (x = 0- 2) have been obtained by coprecipitation from aqueous solutions of KF KBr ( KI) Ba(CH3COO)(2), Pb(NO3)(2) in acetic acid medium. For Ba2Pb4F10Br1.5I0.5 and Ba2Pb4F10Br0.5I1.5 powder data of microcrystalline samples were used f or the Rietveld analyses (R-Bragg = 0.077 for Ba2Pb4F10Br1,5I0,5 and R-Brag g = 0.065 for Ba2Pb4F10Br1.5I0.5) The crystal structure comprises alternati ng structural features of fluorite related type (CaF2) around Ba and matloc kite related type (PbFCl) around Pb1 and Pb2 along the c axis. Barium shows a {8 + 4} cuboctahedral coordination of fluorine. The coordination polyhed ron around the two crystallographically independent lead atoms is a monocap ped quadratic antiprism built of {4 + 1} fluorine and (4) bromine or iodine atoms, respectively.