Ordering of Mg and Nb in the octahedral positions of the “cubic” perovskite structure of Pb3MgNb2O9

Abstract

Selected-area electron-diffraction patterns and x-ray diffraction powder patterns were taken of crushed single crystals of Pb₃MgNb₂O₉. Although the x-ray patterns did not indicate any deviation from the cubic perovskite structure, the single-crystal electron-diffraction patterns showed additional spots which were attributed to a superlattice. Those superlattices which are compatible with the experimental data were derived. The diffuse phase transition of Pb₃MgNb₂O₉ and the direction in which the spontaneous polarization occurs may be explained by the proposed B-site ordering.