Quantitative three dimensional atomic resolution characterisation of non-stoichiometric nanostructures in doped bismuth ferrite

Over the last decade, the lead-free, environmentally-friendly multiferroic material, BiFeO3 (BFO), has once again received tremendous attention from researchers, not only for its fundamental properties, but also for its potential applications such as novel devices that can be written by an electric field and read by a magnetic field. However, one of the most important limitations for applications is the high leakage current in pure materials. Doping has proved to be an effective way to reduce the leakage current caused by the electron hopping between Fe2+ and Fe3+. In this work, a series of Nd3+ and Ti4+ co-doped BFO compositions have been studied using a combination of atomic resolution imaging and electron energy loss spectroscopy in STEM, especially concentrating on nanostructures within the Bi0.85Nd0.15Fe0.9Ti0.1O3 composition, as nanostructures can play an important role in the properties of a crystal. Two types of novel defects – Nd-rich nanorod precipitates and Ti-cored anti-phase boundaries (APBs) are revealed for the first time. The 3D structures of these defects were fully reconstructed and verified by multislice frozen phonon image simulations. The very formation of these defects was shown to be caused by the excess doping of Ti into the material and their impact upon the matrix is discussed. The nanorods consist of 8 atom columns with two Nd columns in the very center forming the Nd oxide. Density functional theory calculation reveals that the structures of the nanorod and its surrounding perovskites are rather unusual. The Nd in the core is seven coordinated by oxygen while the coordination of B site Fe3+ at its surroundings are just five-coordinated by oxygen due to the strain between the nanorod and the surrounding perovskite. The APB is nonstoichiometric and can be treated as being constructed from two main structural units - terraces and steps. Within the terraces, Ti4+ occupy the centre of the terrace with Ti/Fe alternately occupying either side of the terrace. As for the step, this is constructed from iron oxide alone with a structure similar to -Fe2O3, and Ti is completely absent. Quantitative analysis of the structure shows the APB is negatively charged and this results in electric fields around the APBs that induce a local phase transformation from an antiferroelectric phase to a locally polarised phase in the perovskite matrix. Based on this thorough investigation of these defects, a new ionic compensation mechanism was proposed for reducing the conductivity of BiFeO3 without the complications of introducing non-stoichiometric nanoscale defects