It was noted that discrepancies existed in the literature concerning the formation and atomic structure of intercalated CuO layers in this material. Two different types of stacking fault, with 1/6[301] or 1/6[031] displacement vectors, were associated with the intercalation of a CuO layer into the matrix. One of these stacking faults, which was associated with 1/6[031], was thought to have a lower energy. An analysis of the formation of these extended defects, and their interaction with a mirror twin boundary, led to the conclusion that a dislocation with a 1/2<110> Burgers vector was nucleated at mirror twin boundaries when such an extended defect propagated in a twinned orthorhombic structure. The atomic structure of the faults was the same in the 2 variants, but a change in the diffraction contrast was found upon crossing the mirror twin boundary. When the stacking fault existed before twin nucleation, the displacement vectors across the mirror twin boundaries were the same, using the tetragonal notation, but the atomic structures of the faults were different in the 2 variants, although the same diffraction contrast was found all over the stacking-fault area. This corresponded to the propagation of a twin within the faulted region. This faulted region was suggested to have formed within an non-twinned orthorhombic structure or, more probably in the tetragonal structure. It was concluded that the presence of screw dislocations with a 1/2<110> Burgers vector at mirror twin boundaries provided a signature for a stress-relief mechanism which was caused by CuO layer insertion into the orthorhombic phase. The other configuration corresponded to a stress-relief mechanism that was caused by CuO layer insertion into the tetragonal phase.

J.Rabier: Philosophical Magazine A, 1996, 73[3], 753-66