Local Oxygen-Vacancy Ordering and Twinned Octahedral Tilting Pattern in the Bi_0.81Pb_0.19FeO_2.905 Cubic Perovskite

The structure of Bi 0.81 Pb 0.19 FeO 2.905 was investigated on different length scales using a combination of electron diffraction, high-resolution scanning transmission electron microscopy, synchrotron X-ray powder diffraction, and Mössbauer spectroscopy. In the 80–300 K temperature range, the average crystal structure of Bi 0.81 Pb 0.19 FeO 2.905 is a cubic Pm 3̅ m perovskite with a = 3.95368(3) Å at T = 300 K. The (Pb 2+, Bi 3+ ) cations and O 2– anions are randomly displaced along the ⟨110⟩ cubic directions, indicating the steric activity of the lone pair on the Pb 2+ and Bi 3+ cations and a tilting distortion of the perovskite framework. The charge imbalance induced by the heterovalent Bi 3+ → Pb 2+ 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 (FeO 1.25 ) layers that are approximately 6 perovskite unit cells apart and transform every sixth layer of the FeO 6 octahedra into a layer with a 1:1 mixture of corner-sharing FeO 4 tetrahedra and FeO 5 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 Bi 0.81 Pb 0.19 FeO 2.905 at low temperatures.