Nanoscopic unequivocal analysis of twinned crystal structures with tetragonal or orthogonal unit cells by electron microdiffraction

In this study, a particular type of nanoscopic twinned crystal structure which can occur in any tetragonal or orthogonal crystal structure with a ratio of lattice parameters is presented. This type of twinning is characterized by twin components whose c axes are oriented perpendicular to one another, by twin-habit planes parallel to {102} planes, and by a superstructure along the twin boundaries which is described by orthogonal unit cells with lattice parameters 2a, 2a, a. Furthermore, the suitability of electron microdiffraction, a diffraction technique of transmission electron microscopy (TEM) with a moderately convergent beam, for the analysis of such twinned crystal structures is demonstrated. For this demonstration, electron microdiffraction is applied to the chalcogenide compound Cu₂In₃Se₅, which indeed exhibits the proposed twinned crystal structure.