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| Home > Publications database > Twinned $CsLn_{2}F_{7}$ compounds (Ln=Nd,Gd,Tb,Er,Yb): the role of a highlysymmetrical cation lattice with an arrangement analogous to the Lavesphase $MgZn_{2}$ |
| Home > Publications database > Twinned $CsLn_{2}F_{7}$ compounds (Ln=Nd,Gd,Tb,Er,Yb): the role of a highlysymmetrical cation lattice with an arrangement analogous to the Lavesphase $MgZn_{2}$ |
| Journal Article | PUBDB-2016-04574 |
; ;
2016
De Gruyter
Berlin
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Please use a persistent id in citations: doi:10.1515/zkri-2016-1972
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 CsLn$_2$F$_7$ is no exception. We refined the structures of the twinned compounds with Ln=Nd, Gd, Tb, Er, Yb in space group P112$_1$/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 MgZn$_2$. Several other compounds ALn$_2$F$_7$, which have been described in the literature, show a similar cation array. The diversity of different space groups which have been reported for ALn$_2$F$_7$ compounds can be better understood using group-subgroup relationships assuming the hypothetical structure of the cation array with space group P6$_3$/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 CsLn$_2$F$_7$ compounds.
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