Structural Variations of BiMnO_3+<i>x</i>Revealed by Electron Diffraction

Electron diffraction studies have been performed on a number of samples within the series BiMnO3+x (for the range –0.11 < x < 0.16). Samples with a large value of x (either positive or negative) crystallize as a monoclinic perovskite supercell structure not dissimilar from the stoichiometric ideal. However, the variation of oxygen composition manifests itself as disorder in either the cation or anion sub-lattices, leading to a reduction in symmetry (to C2, P21 or P2). In addition, three new structures have been identified at compositions closer to the perovskite ideal. One structure is similar to a mixed cation phase reported by Hughes et al, while another is an n = 2 Ruddlesden-Popper phase; importantly both of these phases are acentric and so could support multiferroic properties. The third is as yet not fully characterised but seems to crystallise into a different monoclinic structure than the stoichiometric ideal phase.