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In the structure Ba12F19Cl5 [hexagonal space group P2m] the two chlorides on the sites Cl(1) and Cl(2) can partially be replaced by bromide ions. Single crystals of the type Ba12F19ClBr5- with a chloride to bromide ratio up to 2: 3 could be obtained by cooling a flux of 75 mol % BaF2 and 25 mol % BaX2 with X = Cl, Br. The crystal quality decreases with increasing bromide concentration. Structural parameters of five selected single crystals with different chloride/bromide ratio were studied by single crystal X-ray diffraction methods. The refined total Cl-/Br- population ratio in the crystals is close to the one of the flux. The lattice parameters and interatomic distances 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). KUBEL, Frank, HAGEMANN, Hans-Rudolf, BILL, Hans. Synthesis and structure of mixe