TRANSMISSION ELECTRON-MICROSCOPY STUDY ON THE ANNEALING OF YBA2CU3O7-DELTA

Annealing in oxygen‐rich atmosphere at temperatures between 400° and 600°C is an important step in the manufacture of superconducting YBa2Cu3O7‐δ. The symmetry of the orthorhombic phase requires that if more than one type of twin plane is present within a grain, a distorted region should exist inside the multiple twinned grain. This distorted region hinders the tetragonal‐to‐orthorhombic transformation and may account for some retained tetragonal phase inside an otherwise orthorhombic grain. A physical model is presented describing the formation of such regions and their eventual transformation into low‐angle grain boundaries after long annealing. Extended annealing at intermediate temperatures apparently leads to the formation of planar faults in off‐stoichiometric samples. Transmission electron microscope image contrast and energy dispersive X‐ray analyses of highly defective regions suggest these defects are CuOx (x= 1, 2) extra layers. These extra layers tend to form near grain boundaries or free surfaces, where oxygen is readily available. Copyright © 1990, Wiley Blackwell. All rights reserved