Disordering of Pb and Cu arrangements in the block layers of Pb₂Sr₂YCu₃O_8+δ and PbBaSrYCu₃O_7+δ by oxygen introduction

The crystal structures and phase stabilities of oxygen-annealed Pb₂Sr₂YCu₃O_8+δ (Pb3212) and PbBaSrYCu₃O_7+δ (Pb2212) were investigated by X-ray powder diffraction, high-resolution transmission electron microscopy and thermogravitmetry. The Pb and Cu arrangements in block layers of these compounds depend on the oxidizing temperatures. Metal arrangements in the block layers of Pb3212 annealed at 440°C were very similar to those of quenched Pb3212. That is, the block layers are constructed by stacks of PbO, CuO_δ and PbO layers, which is consistent with the results reported by Marezio et al. However, we found that the disordering of Pb and Cu in the block layers occured by annealing at 520°C, though the oxygen contents of the two phases annealed at 440°C and 520°C were nearly equal to each other. A similar structural change was also observed in Pb2212. The block layers in Pb2212 were completely ordered to form PbO and Cu layers by annealing at 290°C, but they became disordered by annealing at 460°C. The metal disordering at relatively low temperatures around 500°C reflects the fact that the oxygen introduction enhances atomic diffusion. The annealing temperature region where the metal-disordered phase of Pb3212 is formed is quite narrow, because the CuO_δ layer sandwiched by two PbO layers is very stable, while the corresponding temperature region for Pb2212 is much wider.