Contribution of electron precession to the study of perovskites displaying small symmetry departures from the ideal cubic ABO₃ perovskite: applications to the LaGaO₃ and LSGM perovskites

Electron microscopy and electron diffraction are well adapted to the study of the fine-grained, faulted pure and doped LaGaO₃ and LSGM perovskites in which the latter is useful for fuel cell components. Because these perovskites display small symmetry departures from an ideal cubic ABO₃ perovskite, many conventional electron diffraction patterns look similar and cannot be indexed without ambiguity. Electron precession can easily overcome this difficulty mainly because the intensity of the diffracted beams on the precession patterns is integrated over a large deviation domain around the exact Bragg condition. This integrated intensity can be trusted and taken into account to identify the 'ideal' symmetry of the precession patterns (the symmetry which takes into account both the position and the intensity of the diffracted beams). In the present case of the LaGaO₃ and LSGM perovskites, the determination of the 'ideal' symmetry of the precession patterns is based on the observation of weak 'superlattice' reflections typical of the symmetry departures. It allows an easy and sure identification of any zone axes as well as the correct attribution of hkl indices to each of the diffracted beams. Examples of applications of this analysis to the characterizations of twins and to the identification of the space groups are given. This contribution of electron precession can be easily extended to any other perovskites or to any crystals displaying small symmetry departures.