Electron Diffraction and Electron Microscopy of the Superconducting Compound Ba2YCu3O7–δ

The superconducting transition temperature in the compound Ba2YCu3O7–δ exhibits a positive correlation with the magnitude of the volume fraction of the orthorhombic phase in two-phase material, showing that this phase is responsible for the high transition temperature (>90 K). It cannot be excluded that the low temperature tetragonal phase is also superconducting with a lower transition temperature. The orthorhombic phase is fragmented in fine coherent twin lammellae parallel with (110) and (110) planes; they result from the decrease in symmetry 4/mmm → mmm on cooling through the high temperature tetragonal (TH) → orthorhombic (O). The orthorhombic phase can be transformed back into the high temperature tetragonal phase by “in-situ” heating in the electron microscope. This reversible phase transformation takes place at a temperature of the order of 750 °C, where the vacancies on the oxygen sublattice become disordered. In well annealed specimens weak superstructure spots are sometimes observed at 1/2 0 0 and 0 1/2 0 positions; they are attributed to a 2a0 × b0 superstructure resulting from vacancy ordering. Weak streaking along [001]* is attributed to disordering in the direction of vacancy ordering in successive CuO planes. The high temperature tetragonal phase can be retained on fast cooling. The high resolution images of the orthorhombic phase along the [100], [001] and [010] zones are consistent with the crystal structure of Ba2YCu3O7–δ proposed recently. Evidence is presented for the occurrence of disorder in the cation sublattice. In rapidly cooled specimens the cation disorder causes lattice deformation.