{110} twinning in YBa₂Cu₃O_7-x

Abstract

Large-angle convergent-beam electron diffraction (LACBED) has been used in conjunction with dark-field imaging to determine the nature of {110} twinning in YBa₂Cu₃O_7-x (YBCO). The orthorhombic ratio (b-a)/a has been shown to vary considerably across a number of twin domains in annealed material from 0.006 to 0.02. A number of samples were irradiated at different temperatures (by electrons and ions) and at room temperature this was seen to promote the degradation of the orthorhombic phase to its tetragonal counterpart. This was measured quantitatively using the LACBED technique.

The coexistence of all four twin variants of the orthorhombic structure within the same region of crystal was examined in detail. The nature of the crystal strain associated with this phenomenon was investigated both by dark-field imaging and by LACBED. Kinematic simulations of trial displacement fields based on the misfit between intersecting twins are shown to agree closely with experimental patterns. The strain field allied to this twinning was found to be identical in form with that associated with YBCO following low-temperature irradiation. A variation in oxygen content causing a small deformation of the crystal structure is cited as the origin of these strain fields and the resulting diffraction contrast.