The superconducting properties of YBa2Cu3O7-δ (YBCO) were strongly dependent on atomic structure and composition. While the average structures of YBCO were well documented, the local O concentration and ordering have received much less attention owing to the difficulty of experiments and analysis. Several samples of YBCO were examined by quantitative high-resolution transmission electron microscopy, and geometric phase analysis was used to determine their local atomic structure and symmetry. The results revealed that adjacent domains separated by the (110) interfaces, commonly considered to be twin domains, could have differing ao/bo ratios. The domains therefore did not retain the ideal twinned configuration commonly expected for this system. Assuming that the ao/bo ratio was proportional to the O concentration, it was suggested that these domains resulted from an inhomogeneous O distribution. Multiple measurements in both bulk and thin-film samples indicated that YBCO could consist of different phases, which could yield locally variable superconducting properties.

Structural Inhomogeneity and Twinning in YBa2Cu3O7-δ Superconductors - High-Resolution Transmission Electron Microscopy Measurements. C.L.Johnson, J.K.Bording, Y.Zhu: Physical Review B, 2008, 78[1], 014517 (7pp)