SYNTHESIS AND STRUCTURE OF MIXED BA12F19CL-DELTA-BR5-DELTA CRYSTALS

In the structure Ba12F19Cl5 [hexagonal space group <P(6)over bar 2m>] the two chlorides on the sites Cl(1) and Cl(2) can partially be replac ed by bromide ions. Single crystals of the type Ba12F19CldeltaBr5-delt a with a chloride to bromide ratio up to 2:3 could be obtained by cool ing a flux of 75 mol% BaF2 and 25 mol% BaX(2) with X = Cl, Br. The cry stal quality decreases with increasing bromide concentration. Structur al parameters of five selected single crystals with different chloride /bromide ratio were studied by single crystal X-ray diffraction method s. The refined total Cl-/Br- population ratio in the crystals is close to the one of the flux. The lattice parameters and interatomic distan ces change in various ways, when the smaller chloride ion is replaced by the bigger bromide ion. The refinements show a statistical disorder on the halide sites with preferential bromide substitution on site Cl (1).