Layered Perovskite-Like Pb2Fe2O5 Structure as a Parent Matrix for the Nucleation and Growth of Crystallographic Shear Planes

The Pb2Fe2O5 compound with a layered intergrowth structure has been prepared by a solid-state reaction at 700 degrees C. The incommensurate compound crystallizes in a tetragonal system with alpha = 3.9037(2) angstrom, c = 3.9996(4) angstrom, and q = 0.1186(4)c star, or when treated as a commensurate approximant, alpha = 3.9047(2) angstrom, c = 36.000(3) angstrom, space group I4/mmm. The crystal structure of Pb2Fe2O5 was resolved from transmission electron microscopy data. Atomic coordinates and occupancies of the cation positions were estimated from high-angle annular dark-field scanning transmission electron microscopy data. Direct visualization of the positions of the oxygen atoms was possible using annular bright-field scanning transmission electron microscopy. The structure can be represented as an intergrowth of perovskite blocks and partially disordered blocks with a structure similar to that of the Bi2O2 blocks in Aurivillius-type phases. The A-cation positions at the border of the perovskite block and the cation positions in the Aurivillius-type blocks are jointly occupied by Pb2+ and Fe3+ cations, resulting in a layer sequence along the c axis: -PbO-FeO2-PbO-FeO2-Phys(7/8)Fe(1/8)-O1-x-Fe-5/8/Pb-3/8-O-2-Fe5/8Pb3/8-. Upon heating, the layered Pb2Fe2O5 structure transforms into an anion-deficient perovskite modulated by periodically spaced crystallographic shear (CS) planes. Considering the layered Pb2Fe2O5 structure as a parent matrix for the nucleation and growth of CS planes allows an explanation of the specific microstructure observed for the CS structures in the Pb-Fe-O system.