Richard Beanland (condensed matter) and Rudo Roemer (theory).

Until now, electron diffraction has not determined atomic positions in a crystal to the same accuracy (10^-13 m) as X-rays. We describe a new method which does this and more. We use computer-controlled data acquisition and processing to produce digital large angle convergent beam electron diffraction (D-LACBED) patterns. We demonstrate refinements of atomic coordinates and isotropic Debye-Waller factors (DWFs) for well-known materials using simulations produced with a neutral, spherical independent atom model. We find that atomic coordinate refinements in Al2O3 have sub-pm precision and accuracy. Isotropic DWFs are accurate for Cu, a simple fcc metal, but do not agree with X-ray measurements of GaAs or Al2O3. This lack of agreement is probably caused by bonding and change transfer between atoms. While it has long been appreciated that CBED is sensitive to bonding, examination of D-LACBED data shows that some regions exhibit large changes in intensity from small changes in the periodic crystal potential. Models of bonding will be essential to fully interpret D-LACBED data.

• Caption to figure: Some D-LACBED patterns from Al2O3
• Publication: Ultramicroscopy 198, 1-9 (2019)
• DOI: https://doi.org/10.1016/j.ultramic.2018.12.007%20