Abstract
Structural features mainly associated with the ordering of oxygen in YBa 2Cu 3O x are studied by electron microscopy. The geometry and the mechanism of twin formation has been studied in detail. Models are proposed for twinning dislocations and it is shown that the ordering stresses resulting from the tetragonal-orthorhombic transition induce a synchroshear glide motion of these dislocations along the twin interfaces. The model also leads to an atomic mechanism for the detwinning of crystals under a uniaxial compressive stress. In oxygen deficient samples evidence is found for vacancy ordered long period superstructures of the nao type with n = 2,3,4,… The presence of these phases is related to T c. The 2√2 × 2√2 structure, previously considered as being a vacancy ordered structure, is suggested to be a deformation modulated structure. The distortions are of the Jahn-Teller type and are located in the CuO 2 layers.
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