Ordered Oxygen-Deficient Superlattice From Electron Irradiation Of Biferroic Bimno3

The structural evolution of biferroic BiMnO3 caused by electron-beam irradiation has been systemically studied by means of electron diffraction (ED), high-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS), and energy dispersive analysis of x-ray (EDX). The ED and HRTEM observations demonstrate clearly that the well-known C2 monoclinic form of BiMnO3 is easily and irreversibly transformed to another modulated form by electron-beam irradiation. This irradiation-induced form shows an apparently higher symmetry and can be described by a body-centered pseudocubic superstructure with a approximate to b approximate to c approximate to 15.8 A and alpha approximate to beta approximate to gamma approximate to 90 degrees. The EELS and EDX measurements reveal a small decrease of O/Mn ratio, but no change of Bi/Mn ratio in the irradiation-induced form as compared with the C2 monoclinic form. From the decrease of the O/Mn ratio, it can be concluded that the irradiation-induced form is produced from ordered oxygen deficiencies. A possible structure model is proposed.