Synthesis conditions and superconductivity of Pb_0.7Cu_0.3Sr₂Y_0.5Ca _0.5Cu₂O_y

Pb_0.7Cu_0.3Sr₂Y_0.5Ca _0.5Cu₂O_y was synthesized as a mother specimen in air and then annealed under two different experimental conditions, in an oxidizing and in a reducing condition. The mother specimen was a nonsuperconductor, but both annealed specimens, one quenched from high temperature and the other treated in high-pressure O₂, were superconductors with T_c's of about 50 K and about 60 K, respectively. The low Cu-valence of the quenched specimen which showed superconductivity suggests a decrease in the Pb-valence, while the high T_c of the specimen treated in high-pressure O₂ would be explained by high Cu-valence and Pb⁴⁺.