Electron microscopy study of twin behavior in GdBa2Cu3Ox and YBa2Cu3Ox

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Abstract
Twins are ubiquitous in the perovskite superconductors. They form during cooling from high temperatures through the tetragonal‐to‐orthorhombic phase transformation. Their behavior has been studied by transmission electron microscopy using a liquid‐nitrogen‐cooled specimen holder. It is observed that extra twins sometimes form in coarse‐twinned regions to give a “refined” twin structure; this is thought to be induced by thermal stresses from the electron beam. Prolonged electron irradiation (with little beam heating) results in transformation to tetragonal with a loss of the twin structure; this is probably due to disordering of the oxygen sublattice by knock‐on displacement. The same transformations can be induced by deliberate electron beam heating. In both cases the twinned orthorhombic structure can be restored by re‐ordering of the oxygen ions, in the first case by removing the beam and in the second case by allowing the specimen to cool.