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

The kinetics of the thermomechanical detwinning of YBa2Cu3O7-x crystal s 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 detwinnin g temperatures or when detwinned crystals are heated again up to 120-1 50-degrees-C, nearly the same twin patterns as before the detwinning c an reappear spontaneously (memory effect). If appropriate times and te mperatures for the detwinning process are used the memory can be clear ed. This memory effect is attributed to the presence of impurity and a tomic defect clusters near former twin boundaries or to remaining minu te 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 comp lete when the crystal initially was reduced into the tetragonal phase at 560-600-degrees-C followed by the application of a uniaxial pressur e along [100]. After changing the atmosphere to pure oxygen the crysta l transformed again into the orthorhombic phase without dividing into domains.