Cuboctahedral anion clusters in fluorite-related superstructures: the structure of Ca2YbF7. II

The structure of the compound Ca 2 YbF 7 has been determined ab initio from a single-crystal X-ray analysis: the R-factor was 0.039. The unit cell is tetragonal, space group I4/m, with a=8.680(1) A, c=16.567(2) A and z=10. The result has confirmed the correctness of the polyhedral model, based on M 6 X 37 square-antiprismclusters, proposed previously. The refinement leads to a Ca/Yb ratio of 19.77/10.23 instead of the ideal value 20.00/10.00: this discrepancy is attributed to contamination by oxygen. The excess Yb, however, is not incorporated in the clusters at the expense of Ca there, as might have been expected, but in a cubic site which, ideally, would be fully occupied by Ca atoms only