Structure refinement from ‘digital’ large angle convergent beam electron diffraction patterns

A new electron diffraction approach for structure refinement applied to Ca3Mn2O7

“…The lower sensitivity is reflected in the lower precision of B factors in Table 1. Our previous study (Hubert et al, 2019) found that thermal atomic displacement parameters in GaAs were significantly overestimated in D-LACBED refinements. This also is the case here: Guiblin et al give B Ca = 0.58 Å 2 , B Mn = 0.30 Å 2 and B O ' 0.65 Å 2 at RT.…”

“…Column s gives the difference in picometres between this refinement and the synchrotron X-ray + neutron study of Lobanov et al (2004). Previous work (Hubert et al, 2019) indicates that D-LACBED refinements overestimate atomic displacement parameters B and that the apparent occupancy is influenced by charge transfer between atoms: here all cations appear to have an occupancy slightly greater than one, and oxygen anions less than one. wave simulation program felix and the source code is freely available (Beanland et al, 2019).…”

“…Unlike refinements using spot patterns (e.g. Jansen et al, 1998), in which almost equivalently good solutions can be obtained for different thicknesses, the large number of patterns in D-LACBED data results in a single best fit (Hubert et al, 2019). In order to estimate errors, after a RT best-fit structure was obtained we determined the standard deviation of individual atomic coordinates obtained from a multitude of starting points.…”

“…However, despite its demonstrated sensitivity to many interesting and useful parameters, it is rarely used for structural refinement (Ogata et al, 2004). Recently, we have demonstrated that atomic coordinates can be extracted from 'digital' large-angle convergent-beam electron diffraction (D-LACBED) patterns (Beanland et al, 2013) with subpicometre precision and accuracy using Al 2 O 3 as a model material (Hubert et al, 2019). A similar data-collection approach was also developed by Koch with an additional postspecimen de-scan, known as LARBED (Koch, 2011).…”

Multibeam Electron Diffraction

“…While this technique has been shown to be highly beneficial in many NBED measurements, particularly strain mapping (Mahr et al, 2015), it requires painstaking alignment and often necessitates additional hardware in order to combine precession with STEM. It is also possible to perform diffraction experiments where the precession is applied serially, that is, multiple scans are performed one after the other, where a different initial beam tilt is used for each scan (Beanland et al, 2013;Meng & Zuo, 2016;Hubert et al, 2019). This method removes the need for careful alignment of the different beam tilts since the scans could be aligned during post-processing.…”