ELECTRON-MICROSCOPIC STUDIES OF FERROELECTRIC-CRYSTALS - DETECTION OF WEAK SYMMETRY ELEMENTS

Recent HREM studies of ferroelectric are first reviewed. Then it is shown how controlled beam-tilting techniques may be used for the detection of weak symmetry elements, involving small atomic shifts associated with the commensurate superlattice of barium sodium niobate (BNN; Ba//2NaNb//5O//1//5). The principle of the method is that some superlattice reflections are selectively, and asymmetrically, phase-shifted by incident beam tilting. Images are then formed at crystal thicknesses where the subcell beams are relatively weak. The resulting two-dimensional images then show strong contrast within the superlattice unit cell. Thus the presence of symmetry operations of the superlattice may be detected readily.