Oxidation-Reduction of Pb₂Sr₂Ln_l-xM_xCu_3-yAg_YO_8+δ

The effects of oxygen partial pressure (po2)and temperature on the oxygen content, structure, and phase stability of Pb₂Sr₂YCu₃O_8+δ, Pb₂Sr₂GdCu₃O_8+δ, Pb₂Sr₂Y_0.60Ca_0.40Cu₃O_8+δand Pb2Sr₂YCu_{2 50}Ag_0.50O_8+δ are determined. In the range 300–600 °C the compounds with δ = 0 are reversibly oxidized to δ as high as ≈1.6. The extent of this reaction depends on the heating rate as well as the temperature and P_O2. The structure changes from orthorhombic to tetragonal, and the unit cell expands on oxidation. The oxygen content in the phase passes through a maximum around 500 °C. Above ~630 °C an irreversible oxidative decomposition occurs depending on the Po2 and nature of the substitutions. The initial decomposition product is a pseudocubic perovskite material, but at still higher oxygen contents a SrPbO₃-like perovskite forms. At lower P_O2where this decomposition does not occur, the original structure undergoes a higher order transformation from orthorhombic to tetragonal symmetry near 750 °C. At temperatures ≳850 °C the oxidized products lose 0₂ and revert to the original phase and oxygen content. Quenching from above 850 °C or slowly cooling in decreased Po2 can be used to synthesize the orthorhombic structure with δ = 0. The nature and extent of the particular substitutions determine the exact conditions required for various oxygen contents, structures, and phase stability.