Perovskite based materials can be tailored to exhibit a host of physical properties, ranging from ferroelectricity and ferromagnetism, to superconductivity and ion conductivity. Any success in synthesizing new perovskite-based materials always opens up opportunities for attempting to improve the properties available in the already known perovskites. During this Ph.D., different perovskite compounds Ln2−xMxMn2-yFeyO6-δ have been made, specifically in a search for new multiferroic materials. The materials studied in this project are: La1-xAxSrMn2O5+δ (with A =Ag and Li), LaBaMnFeO6-δ, LaBaMnFe0.5Zn0.5O6-δ, LaBaMnFe0.5Ti0.5O6-δ, LaBa0.5Ag0.5MnFeO6-δ and LaBa0.5Na0.5MnFeO6-δ, LaBaFe2O6-δ, LaBaFeTiO6-δ. However, after all properties have been measured, the compounds show either semiconductivity or conductivity, depending on the sample, next to a ferromagnetic transition, but no multiferroicity. To provide valuable knowledge for future searches for multiferroics, the Ph.D. study needs to be completed with the explanation why certain properties or present or absent. For this, we need to know the structures of the compounds, since the structure dictates the properties. However, the different techniques used so far all point to different structures, Mössbauer shows oxygen-vacancy order, X-ray diffraction shows disordered but undistorted perovskite and electron diffraction shows there has to be either some form of order or some kind of distortion. Therefore, the last step of this Ph.D. study is to explain these apparent
contradictions (size effects ? defect structure ? ... ?) and solve the structure of the compounds, to be able to explain the properties. This last stage will be completed using the transmission electron microscopy facilities and crystallographic expertise present at the University of Antwerp.