A new electron diffraction approach for structure refinement applied to Ca₃Mn₂O₇

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 A2₁am 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.