Ab initio determination and Rietveld refinement of the crystal structure of Ba_7Cl_2F_12

摘要

The structure of Ba_7Cl_2F_12 has been determined ab initio form conventional X-ray powder diffraction data by the "heavy atom" method. The cell is hexagonal (sapce group P6 Z=1), with a=10.6373(2) A and c=4.1724(2) A. Refinement of 38 parameters by the Rietveld method, using 278 reflections, leads to cR_wp=0.173, cR_p=0.135, and R_B=0.054. The structure has common characteristics with that of the other BaF_2-rich fluorochloride, Ba_12Cl_5F_19. In both phases Ba~2+ ions lie in tricapped trigonal prisms formed by nine halide ions, and Cl~- ions occupy the Center of trigonal prisms of Ba~2+ ions. F~- ions are located in cationic tetrahedra or square pyramids.