Is ferroelectricity in BiMnO₃ induced by superlattice?

The room-temperature structures of the BiMnO₃ ceramic sample synthesized under high pressure have been investigated using transmission-electron microscopy (TEM) and theoretical simulations. The electron diffraction (ED) data revealed that the stoichiometric BiMnO₃ crystallizes in the centrosymmetric C2/c structure, and not in the noncentrosymmetric C2 structure. This was confirmed by further calculations of high-resolution transmission-electron microscopy (HRTEM) images. A second polymorph, characterized as a body-centered pseudocubic superstructure with the lattice constants of 4a_p × 4b_p × 4c_p, was also found in the present sample. Composition analyses and image simulations revealed that this superlattice was formed as the result of an ordered oxygen deficiency. In terms of crystallography, we suggest that the weak ferroelectric polarizations measured on the BiMnO₃ samples originate from the superlattice. This work is very important for further studies of BiMnO₃ and structurally related compounds.