A new electron diffraction approach for structure refinement applied to Ca₃Mn₂O₇

Abstract

The digital large-angle convergent-beam electron diffraction (D-LACBED) technique is applied to Ca3Mn2O7 for a range of temperatures. Bloch-wave simulations are used to examine the effects that changes in different parameters have on the intensity in D-LACBED patterns, and atomic coordinates, thermal atomic displacement parameters and apparent occupancy are refined to achieve a good fit between simulation and experiment. The sensitivity of the technique to subtle changes in structure is demonstrated. Refined structures are in good agreement with previous determinations of Ca3Mn2O7 and show the decay of anti-phase oxygen octahedral tilts perpendicular to the c axis of the A21 am unit cell with increasing temperature, as well as the robustness of oxygen octahedral tilts about the c axis up to ∼400°C. The technique samples only the zero-order Laue zone and is therefore insensitive to atom displacements along the electron-beam direction. For this reason it is not possible to distinguish between in-phase and anti-phase oxygen octahedral tilting about the c axis using the [110] data collected in this study.

Modelling fine-sliced three dimensional electron diffraction data with dynamical Bloch-wave simulations

“…Nevertheless, dynamical ED intensities are far more sensitive to structure (i.e. atomic coordinates) than temperature factors (Beanland et al, 2021), which is reassuring for structure solution and consistent with the growing number of structures solved by ED. Further improvements may be possible with improved background subtraction, which may be incorrect for high-intensity reflections in this particular example due to a lack of data in the rocking curves.…”