STACKING-FAULT STRUCTURES IN MELT-PROCESSED YBA2CU3O7-DELTA SUPERCONDUCTORS

An electron microscopy study of melt-textured (1-2-3) superconductors has allowed us to characterize several distinct types of stacking faul ts which form as a direct consequence of the processing method. Extrin sic faulted 1/6[301]-type dislocation loops have a tendency to form in regions where the local stoichiometry of the system has been upset by the formation of 2-1-1 particles. These faults correspond to the inse rtion of extra CuO planes into the 1-2-3 crystal structure parallel to the basal plane. The precise position in the unit cell at which the e xtra material is added critically determines how the faults interact w ith the twin boundaries present in the orthorhombic phase (i.e., wheth er or not the fault displacement vector changes between 1/6[031] and 1 /6[301] in neighbouring twin lamellae).