Local crystal symmetry and structure at CaTiO₃twin boundaries

The local crystal symmetry and crystal structure at CaTiO3 twin boundaries were examined using convergent-beam electron diffraction (CBED). CBED patterns obtained right at the twin boundaries exhibited one mirror symmetry not observed in the bulk form. Two types of crystal structures were identified at the boundaries on different boundary widths and symmetry breaking. Quantitative analysis using CBED patterns revealed atomic shifts of Ti and Ca. A distortion of oxygen octahedra was also produced, which was smaller than the corresponding distortion obtained in the bulk form. The refined crystal structure conforms to the polar structure expected only at CaTiO3 twin boundaries.