HREM Study of Compounds in the Bi-Rich Part of the Ba–Bi–O System

A series of perovskite-like compounds with general composition BanBin+mOy(m, n––integer) has been studied by means of electron diffraction and high-resolution electron microscopy. For a number of simple, discrete values of the cation composition ratio Ba:Bi the structures were found to be Ruddlesden–Popper–like derivatives of the perovskite structures of BaBiO3, with composition-dependent long periods. For more complicated values of the cation composition ratio the perovskite blocks are further fragmented by planar interfaces into a two-dimensional periodic island structure. This fragmentation gives rise to complicated local structures formed by a mosaic of square or (and) rectangular islands, which often produce incommensurate diffraction patterns. Such diffraction patterns are interpreted in the framework of the "fractional shift" method for interface modulated structure. Structure models and compositions can be made compatible by assuming that substitution of barium ions by bismuth ions must take place, predominantly along the island boundaries.