OXYGEN CONTENT AND TWEED/TWIN TRANSITION OF YBa₂Cu_3-xAl_xO_6+y SINGLE CRYSTALS Schmahl¹, W. W., Wruck¹, B., Brecht², E., Wolf², Th, Andersen³, N. H., Rodewald⁴, M., Miehe⁴, G., Fueß,⁴ H. ¹Institut fuer Mineralogie, Universtitaet Tuebingen, Germany, ²INFP, FZ Karlsruhe, Germany, ³Riso Natl. Laboratory, Denmark, ⁴FB Materialwissenschaft, TH Darmstadt, Germany.

Our single-crystal neutron and x-ray diffraction data show that Al³⁺ substitution on the Cu(1)-'chain'-site of the YBCO-structure significantly decreases the oxygen-content y in the basal layer compared to pure material prepared under identical conditions in O2-atmosphere (e.g. YBa₂Cu₃O_6.98 <-> YBa₂Cu_1.8Al_0.2O_6.71). In reducing conditions, however, the Al³⁺ ions pin excess oxygen in the basal layer (e.g. YBa₂Cu_1.8Al_0.2O_6.25 compared to YBa₂Cu₃O_6.05 for the pure material). Both facts are a consequence of tetrahedral coordination of Al3+, which persists even in extremely reducing conditions and limits the correlation-length of the ortho-I chains in the fully oxidized structure. The oxidized as-grown crystals show a macroscopically tetragonal tweed-structure. If these crystals are annealed in reducing atmospheres above 950 K, they develop ortho-I long-range order and lamellar {110} -type twinning during reoxidation at 673 K. The transition between tweed- and lamellar twin-structure is reversible: the tweed-structure is restored in the long-range ordered, twinned crystal by annealing in oxygen atmosphere above 950 K. The orthorhombicity of the reoxidized material decreases with decreasing temperature of the reduction treatment prior to reoxidation. The effect can be attributed to clustering of the AlO_n defects in reducing conditions and declustering in oxidizing conditions with a kinetic cutoff-temperature of 950 K for these diffusion processes. Computer simulations show that clustering allows ortho-I long-range-order because (i) the mean defect-free area is increased, and (ii) most of the relevant clusters are neutral with respect to the ortho-I domain orientation.

The work was supported by the Deutsche Forschungsgemeinschaft DFG and the EU large installations program LIP.