Structural investigations of AgNbO3 phases using high-energy X-ray diffraction

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The perovskite ferroelectric system AgNbO3 exhibits a successive phase transition. Recent synchrotron X-ray and neutron powder diffraction measurements have revealed a monoclinic phase remains down to low temperature. In the present work we describe a Rietveld analysis of the detailed structure in the wide temperature range. The structure analysis indicated a possibility that a rhombohedral structure exists in a temperature region lower than monoclinic structure. The monoclinic structure can be viewed as a condensation along one of the ＜110＞ direction of the local displacements present in the tetragonal phase. It equally well corresponds to a freezing-out of the local displacements along the ＜100＞ directions recently reported by Noheda et al. for PZT. The monoclinic structure therefore provides a microscopic picture in which one of the locally monoclinic phases in the average rhombohedral or tetragonal structures freezes out, and thus represents a bridge between two phases.
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