Synthesis and structure of Ba7F12Cl2

Crystals of ordered and disordered Ba7F12Cl2 were prepared by flux growth a nd solid state reactions. These new structures were characterized by single crystal and powder X-ray diffraction, The disordered variant which shows d isorder on one of the cation sites was obtained from BaF2 + BaCl2 + NaCl/Na F flux. It has hexagonal space group P6(3)/m (176) with one formula unit pe r unit cell. The lattice constants are a = b = 1059.55(5) pm and c = 420.10 (4) pm (at 21 degrees C). The structure was refined to R(R-w) = 0.026(0.030 ) for 346 independent reflections and 26 parameters. Slow cooling of a mixture of BaF2 and LiCl yields the ordered variant. This one crystallizes in the hexagonal space group P (6) over bar (174) with on e formula per unit cell. Lattice constants at 21 degrees C are a=b=1063.46( 2)pm and c = 417.52(1) pm. The structure was refined to R(R-w) = 0.017 (0.0 17) for 638 independent reflections and 45 parameters. The structural arrangement and the interatomic distances of the two variant s are mutually similar. The barium atoms have coordination number nine. Pro peller-type arrangements with a chloride ion on the axis and the fluoride i ons as blades are observed. These latter ones are interconnected into 'chan nels' of tricapped fluoride prisms. Occupation disorder of the barium sites in the channels of the disordered variant makes the main difference betwee n the two. An unexpectedly high X-ray density obtained for both variants of Ba7F12Cl2 can be correlated to the density of other barium fluorohalides h aving a coordination number of nine for the barium ion.