A complementary study by electron microscopy of modulated phases in Ba2NaNb5O15

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Abstract
A complementary study by electron microscopy and electron diffraction of the phase transitions in barium sodium niobate was performed whereby particular attention was paid to the commensurate incommensurate transition at 543 K. The evolution of the discommensuration configurations was followed by in situ thermal treatment and the nucleation stage of the incommensurate phase could be shown to be associated with the appearance of fourfold closed loops of discommensurations. By electron diffraction and satellite dark field imaging the low temperature phase (<105 K) could be characterized as an orthorhombic modulated structure consisting of periodic arrays of discommensurations. Preliminary results are reported of a high resolution electron microscopic study of remarkable defects of the subnanometer size which could serve as pinning centres for discommensurations.