Acta Crystallographica Section B

Structural Science

Volume 64, Part 2 (April 2008)

research papers

Acta Cryst. (2008). B64, 160-171 [ doi:10.1107/S0108768108001870 ]

KSm(MoO₄)₂, an incommensurately modulated and partially disordered scheelite-like structure

A. Arakcheeva, P. Pattison, G. Chapuis, M. Rossell, A. Filaretov, V. Morozov and G. Van Tendeloo

Abstract: The incommensurately modulated scheelite-like KSm(MoO₄)₂ structure has been refined in the monoclinic superspace group I2/b( [alpha] [beta] 0)00 by the Rietveld method on the basis of synchrotron radiation powder diffraction data. The systematic broadening of satellite reflections has been accounted for by applying anisotropic microstrain line-broadening. The microstructure has been studied by transmission electron microscopy (TEM). The partial disorder of the K and Sm cations in the A position is best approximated by a combination of harmonic and complex crenel functions with (0.952Sm + 0.048K) and (0.952K + 0.048Sm) atomic domains. This combination yields a compositional wave distribution from {KMoO₄} to {SmMoO₄} observed in the ab structure projection along q. The specific features of KSm(MoO₄)₂ and degree of the A-cation ordering are discussed in comparison with the previously reported structure of KNd(MoO₄)₂.

Keywords: incommensurately modulated structure; occupation modulation; degree of ordering; scheelite-like structures; anisotropic microstrain line-broadening.