DETWINNING MECHANISM, TWINNING DISLOCATIONS AND PLANAR DEFECTS IN YBA2CU3O7-DELTA

The detailed geometry of twin boundaries in YBa2Cu3O7-δ is discussed on the basis of high-resolution electron microscopy observations. "Sharp" as well as "diffuse" twin boundaries are observed, depending on the composition and thermal history of the sample. It is shown by means of Monte Carlo simulations based on a two-dimensional anisotropic Ising model that the concentration of oxygen ions in the CuO1-δ layers exhibits a minimum in the region adjacent to the twin interface. The vacancy enriched region is more pronounced at low temperature and disappears at high temperature. A model is proposed for twinning dislocations in YBa2Cu3O7-δ and it is shown that the ordering stresses created by the tetragonal-orthorhombic phase transition induce the "synchro-shear" motion of such twinning dislocations, leading to the lateral displacement of twin interfaces. It is shown that this atomic model for twinning dislocations provides an atomic mechanism for the "detwinning" of crystals under uniaxial compression. Crystallographic shear planes accommodating an excess of CuO are observed; Frank dislocations bordering such shear planes have been analysed as well. © 1990.