Oxidation-Reduction of Pb(2)Sr(2)Ln(1-x)M(x)Cu(3-y)Ag(y)O(8+delta)

The effects of oxygen partial pressure (P-O2) and temperature on the oxygen content, structure, and phase stability of Pb2Sr2YCu3O8+delta, Pb2Sr2GdCu3O3+delta, Pb2Sr2Y0.60Ca0.40Cu3O8+delta, and Pb2Sr2YCu2.50Ag0.50O8+delta are determined. In the range 300-600 degrees C the compounds with delta = 0 are reversibly oxidized to 5 as high as approximate to 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 degrees C. Above similar to 630 degrees C an irreversible oxidative decomposition occurs depending on the P-O2 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-O2, where this decomposition does not occur, the original structure undergoes a higher order transformation from orthorhombic to tetragonal symmetry near 750 degrees C. At temperatures greater than or similar to 850 degrees C the oxidized products lose O-2 and revert to the original phase and oxygen content. Quenching from above 850 degrees C or slowly cooling in decreased P-O2 can be used to synthesize the orthorhombic structure with delta = 0. The nature and extent of the particular substitutions determine the exact conditions required for various oxygen contents, structures, and phase stability.