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

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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.