The interface of orthogonally oriented twins in YBa2Cu3O7-δ

Twinning on the (110) and the equivalent (110) planes is frequently encountered within one crystal grain in YBa2Cu3O7-δ. Using high-resolution electron microscopy and electron diffraction analysis, we studied the interface and its vicinity where the (110) and (110) twins meet. We found that the crystal lattice of the impinging set of twins rotates around the c axis when they confront the orthogonally oriented twins. Every second one of the impinging twins is pointed, but does not taper off to zero width, yielding a small-angle grain boundary; the other twin forms a coherent boundary with a twin belonging to the orthogonal set. Thus, the interface of the orthogonally oriented twins comprises of an alternation of twin boundaries and grain boundaries. Furthermore, we found that there is a limiting wedge-angle for the tapered twins, and if this angle is exceeded, the tip of the twin splits. The structure of the interface and its vicinity is determined by the equilibrium of elastic strain energy due to the tetragonal-orthorhombic phase transformation and the interfacial energy of the twin boundary and the grain boundary. The shape of the tapered twin was analysed based on the observed twinning dislocation and the associated twinning steps in YBa2Cu3O7-δ.