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 faults which form as a direct consequence of the processing method. Extrinsic 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 insertion 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 extra material is added critically determines how the faults interact with the twin boundaries present in the orthorhombic phase (i.e., whether or not the fault displacement vector changes between 1/6[031] and 1/6[301] in neighbouring twin lamellae).