Ab-initio HRTEM simulations of ionic crystals: a case study of sapphire

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Solving the atomistic structure of crystal defects by HRTEM often requires comparison of experimental images with simulated images of trial structures. Although many materials of interest exhibit a strong component of ionic bonding, HRTEM image simulations conventionally rely on the scattering factors of neutral atoms. In the present case study of sapphire we thus examine the influence of ionicity on dynamical electron scattering in the specimen and on the HRTEM image. For this purpose, we compare simulated images obtained with ion electron scattering factors with conventional simulated images, obtained with the scattering factors of neutral atoms. We introduce several approaches to include ionicity in image simulations. The most reliable results were obtained by means of self-consistent ab-initio electron bandstructure calculations. After converting the charge density distribution obtained by these calculations into electron scattering factors of the corresponding ions we have implemented the data in our HRTEM image simulation software and used it for simulating HRTEM images. It turned out that these'realistic' simulated images do not differ significantly from simulated images of neutral atoms. At least for sapphire, this result justifies the usage of'conventional' image simulations.

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