Local Oxygen-Vacancy Ordering and Twinned Octahedral Tilting Pattern in the Bi0.81Pb0.19FeO2.905 Cubic Perovskite

The structure of Bi0.81Pb0.19FeO2.906 was investigated on different length scales using a combination of electron diffraction, high-resolution scanning transmission electron microscopy, synchrotron X-ray powder diffraction, and Mossbauer spectroscopy. In the 80-300 K temperature range, the average crystal structure of Bi0.81Pb0.19FeO2.905 is a cubic Pm (3) over barm perovskite with a = 3.95368(3) angstrom at T = 300 K. The (Ph2+, Bi3+) cations and O2- anions are randomly displaced along the < 110 > cubic directions, indicating the steric activity of the lone pair on the Pb2+ and Bi3+ cations and a tilting distortion of the perovslcite framework. The charge imbalance induced by the heterovalent Bi3+ -> Pb2+ substitution is compensated by the formation of oxygen vacancies preserving the trivalent state of the Fe cations. On a short scale, oxygen vacancies are located in anion-deficient (FeO1.25) layers that are approximately 6 perovskite unit cells apart and transform every sixth layer of the FeO6 octahedra into a layer with a 1:1 mixture of corner-sharing FeO4 tetrahedra and FeO5 tetragonal pyramids. The anion-deficient layers act as twin planes for the octahedral tilting pattern of adjacent perovskite blocks. They effectively randomize the octahedral tilting and prevent the cooperative distortion of the perovskite framework. The disorder in the anion sublattice impedes cooperative interactions of the local dipoles induced by the off-center displacements of the Pb and Bi cations. Magnetic susceptibility measurements evidence the antiferromagnetic ordering in Bi0.81Pb0.19FeO2.906 at low temperatures.