Specifics of the data processing of precession electron diffraction tomography data and their implementation in the program <i>PETS2.0</i>

TLDR

Electron diffraction tomography resembles X‑ray diffraction but differs by a camel‑shaped rocking curve modeled with a pseudo‑Voigt integral and a high risk of orientation errors from goniometer inaccuracies, crystal deformation, or movement. The study proposes a per‑frame crystal‑orientation refinement method based on least‑squares optimization of simulated diffraction patterns. The authors implemented this refinement and the complete data‑processing workflow in the PETS 2.0 program, incorporating features specific to electron diffraction data. The method achieves typical angular accuracy of frame orientations better than 0.05°.

Abstract

Electron diffraction tomography (EDT) data are in many ways similar to X-ray diffraction data. However, they also present certain specifics. One of the most noteworthy is the specific rocking curve observed for EDT data collected using the precession electron diffraction method. This double-peaked curve (dubbed `the camel') may be described with an approximation based on a circular integral of a pseudo-Voigt function and used for intensity extraction by profile fitting. Another specific aspect of electron diffraction data is the high likelihood of errors in the estimation of the crystal orientation, which may arise from the inaccuracies of the goniometer reading, crystal deformations or crystal movement during the data collection. A method for the refinement of crystal orientation for each frame individually is proposed based on the least-squares optimization of simulated diffraction patterns. This method provides typical angular accuracy of the frame orientations of less than 0.05°. These features were implemented in the computer program PETS 2.0. The implementation of the complete data processing workflow in the program PETS and the incorporation of the features specific for electron diffraction data is also described.