Microstructure and properties of YBa2Cu3O9−δ superconductors with transitions at 90 and near 290 K

We have investigated the microstructure and properties of YBa2Cu3O9−δ that contain resistive transitions at 90 and near 290 K using high-resolution and analytical transmission electron microscopy and Raman scattering techniques. The specimens contain normal orthorhombic (Pmmm space group) phase associated with Tc=90 K, and another phase which we tentatively assign it to be associated with the resistive transitions near 290 K. The new phase has a c axis larger by about 16% and it grows epitaxially on the {100} planes of the orthorhombic (tripled perovskite unit cell) phase having Tc=90 K.