Structural chemistry and conductivity of solid solution YBa_1-xSr_xCo₂O_5+δ

Solid solution specimens of YBa1-xSrxCo2O5+delta (0 <= x <= 1) were prepared by solid-solid reactions. The structures were initially investigated using powder X-ray diffraction and further studied using selected area electron diffraction and high-resolution transmission electron microscopy. A 3 x 3 x 1 superstructure derived from oxygen ordering up to 30% Sr doping has been identified. In the Sr-rich compositions, a root 2 x root 2 x 1 superstructure formed due to lattice distortion. The formation mechanisms of these two superstructures and the relation between the unit cell dimensions and the Sr content were discussed. The conductivity of some selected compositions was also measured with a maximum in conductivity in the range 180-320 degrees C, indicating a form of metal to insulator transition at this temperature that is unlikely to be related to a major change in oxygen content.