Using aberration corrected STEM to study the atomic structure of incommensurate antiferroelectrics

Some atomic structures are just not amenable to traditional diffraction-based solution using X-rays or neutrons. One example of this is irregular or disordered layered structures, where the disorder on the nanoscale makes any interpretation of diffraction data extremely difficult. This is exactly the situation in lanthanum-doped zirconium-rich lead zirconate titanate where the unit cells often appear to have a period in one direction which is not a simple multiple of the number of building blocks of simple cells that they are made of (i.e. the full cell is incommensurate with the primitive cell). It appears that this happens because different layered structures can be formed and they can be stacked together irregularly. In this situation, the only way to determine the local atomic structure is via an imaging based technique with atomic resolution.This project will use high resolution scanning transmission electron microscopy at the world-leading EPSRC-supported SuperSTEM facility, which can image features down to below 1 +. In particular, the samples will be imaged using the High Angle Annular Dark Field technique, which is especially sensitive to heavy elements and which will consequently be invaluable for locating Pb and Zr ions; this will provide a powerful complement to HRTEM investigations being performed separately (also funded by the EPSRC under Grant Reference EP/H028218/1) which is more sensitive to oxygen ion locations. Images from both techniques will then be analysed quantitatively to extract atom positions and using images from more than one direction, we will then determine the three dimensional atom positions in each stacking sequence. This will then enable us to understand how the polarisation is ordered within the different stacking in order to produce antiferroelectric structures. This will provide a powerful demonstration of the capabilities of aberration corrected STEM in conjunction with quantitative data analysis to analyse nanoscale structures with atomic resolution, and will also lay the groundwork for future work on electric field induced phase transformations and possible applications of antiferroelectric lead zirconate titanate compositions.

The primary impact of this work will be it's benefits for two academic communities. Firstly, it will be of interest to researchers concerned with ferroelectric materials in that it explicitly studies in atomic level detail the little explored borderlands between ferroelectric and antiferroelectric orderings and will start to explain how this changeover takes place. Secondly, it will be of interest to the electron microscopy community in its innovative use of the possibilities provided by the sub-+ngstrm resolution available in the new generation of aberration-corrected microscopes to solve the atomic structures of objects that are very spatially limited in periodicity. These impacts are described in more detail under Academic Beneficiaries . As stated there, to ensure these impacts accrue to the communities of interest, the latter part of the project will mainly be concerned with the dissemination via both journal publication and conference presentation. At present, there is little immediate impact for this work in a more technological or commercial setting, although since this work will make possible further studies on the mechanisms by which field-induced antiferroelectric-ferroelectric phase transformations occur, then this could inform the development of actuators based on switching between the antiferroelectric and ferroelectric states, and thus this work could represent a very early stage in the development of technologically useful devices. An additional benefit of this work will be the training of the PI in both experimental methodologies and in the use of different software tools for data analysis. This will be of further benefit to researchers and PhD students in Glasgow since they will be applied in the PIs research group for studies in both related materials systems, as well as completely different areas, as appropriate. Thus, this investment in a key player in the UK electron microscopy scene will provide further benefits in coming years through the training of early career researchers in cutting-edge techniques for sub-+ngstrm science.