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Chromium(II) diphosphate, Cr~2~P~2~O~7~, has an incommensurately modulated structure at ambient conditions with __a__ = 7.05, __b__ = 8.41, __c__ = 4.63 Å, β = 108.71° and q = (−0.361, 0, 0.471). It undergoes a phase transition towards a commensurate structure with a commensurate q vector, q = (−{1\over 3}, 0, 1⁄2), at __T~c~__ = 285 K. The incommensurate structure has been solved by the charge-flipping method, which yielded both the basic positions of the atoms and the shapes of their modulation functions. The structure model for the commensurate structure was derived directly from the incommensurate structure. The structure analysis shows that the modulation leads to a change of the coordination of the Cr^2+^ ions from distorted octahedra in the average structure towards a sixfold coordination in the form of a more regular octahedron and a fivefold coordination in the form of a square pyramid. The fivefold and sixfold coordination polyhedra alternate along the lattice direction a with the pattern 5-6-5 5-6-5 in the commensurate structure. In the incommensurate structure this pattern is occasionally disturbed by a 5-6-5-5 motif. Both structures can be described in superspace using

Author

Lukáš Palatinus; Michal Dušek; Robert Glaum; Brahim El Bali

Publisher

International Union of Crystallography; Blackwell Publishing Inc.; International Union of Crystallography (IUCr) (ISSN 0108-7681)

Published

2006

Language

EN