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Electron microscopic evidence is presented for the presence of discommensuration walls in incommensurate ferro-electric-ferro-elastic Ba2NaNb5O15 and its homologues Sr2KNb5O15 and Pb2KNb5O15. The displacement vector of the discommensuration walls is one fourth of a lattice vector of the orthorhombic commensurate superstructure, explaining the typical coalescence pattern of the discommensuration dislocations. The magnitude of the incommensurability is consistent with the density of discommensuration walls. Similarities and differences between the homologues are pointed out.
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Tendeloo et al. (1985) studied this question.
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