THERMOMECHANICAL DETWINNING OF YBA2CU3O7-X SINGLE-CRYSTALS UNDER REDUCED OXYGEN PARTIAL-PRESSURE

The kinetics of the thermomechanical detwinning of YBa2Cu3O7-x crystals Is studied as well as the influence of the starting domain structure, the oxygen content and the temperature on the detwinning process. We observed that after the release of uniaxial pressure at the detwinning temperatures or when detwinned crystals are heated again up to 120-150-degrees-C, nearly the same twin patterns as before the detwinning can reappear spontaneously (memory effect). If appropriate times and temperatures for the detwinning process are used the memory can be cleared. This memory effect is attributed to the presence of impurity and atomic defect clusters near former twin boundaries or to remaining minute twin domains which cannot be detected by optical methods. The time during which the uniaxial pressure has to be applied for clearing the memory follows an Arrhenius relation with an activation energy of 1.09 +/- 0.10 eV. This value is close to the activation energy for oxygen diffusion in YBa2Cu3O7-x. The detwinning was found to be easy and complete when the crystal initially was reduced into the tetragonal phase at 560-600-degrees-C followed by the application of a uniaxial pressure along [100]. After changing the atmosphere to pure oxygen the crystal transformed again into the orthorhombic phase without dividing into domains.