Twinned CsLn2F7 compounds (Ln=Nd, Gd, Tb, Er, Yb): the role of a highly symmetrical cation lattice with an arrangement analogous to the Laves phase MgZn2

Abstract

Abstract The occurrence of twinning can often be related to higher symmetrical structures. Fluorides are frequently twinned due to their close relation to high symmetry structures like fluoride, tysonite or pyrochlores. The series of compounds CsLn2F7 is no exception. We refined the structures of the twinned compounds with Ln=Nd, Gd, Tb, Er, Yb in space group P1121/b. An analysis of the pseudosymmetry of the resulting structures shows a highly symmetrical cation partial structure with a cation distribution similar to the one in the hexagonal Laves phase MgZn2. Several other compounds ALn2F7, which have been described in the literature, show a similar cation array. The diversity of different space groups which have been reported for ALn2F7 compounds can be better understood using group-subgroup relationships assuming the hypothetical structure of the cation array with space group P63/mmc as aristotype. Furthermore, the twinning is easily understood on the basis of the lost symmetry operations in the symmetry reduction from point group 6/mmm, e.g. to 2/m in the case of the CsLn2F7 compounds.