Absence of Twinning in the 60K Superconducting Phase of YBa sub 2 Cu sub 3 O sub 7-- delta

The sequential observation of a number of areas of a large single crystal of YBa sub 2 Cu sub 3 O sub 7-- delta using both optical and TEM microscopy has shown that in those regions in which there are not observable optical domains, there is also very little evidence for oxygen chain-related twinning. In these same areas, electron diffraction (SAD) patterns clearly show the presence of the (1/2 0 0) superlattice associated with an O concentration close to O sub 6.5 (i.e. delta approx 0.5). This has in turn been linked to the appearance of the 60K transition, and suggests that the lattice energetics of this alternate Cu--O chain occupany do not favour twinning. Graphs, Photomicrographs, Diffraction patterns. 11 ref.--AA