Dependence of T_c on the excess oxygen content δ in Bi₂Sr₂CaCu₂O_8+δ annealed under high pressures of oxygen

The dependence of the superconducting transition temperature T_c on the excess oxygen content δ has been investigated for Bi₂>Sr₂>CaCu₂>O_8+δ annealed under high pressures of oxygen. With increasing δ, T_c decreases for δ≲0.3 and then becomes constant for δ≳0.3. It has been found by chemical titration that this strange δ dependence of T_c is related to the δ dependence of the average valency of Cu ions. That is, the average valency increases with increasing δ for δ≲0.3 and then is saturated for δ≳0.3. For δ≳0.3, the Curie-Weiss component in the DC magnetic susceptability has been found to increase with increasing δ. Therefore, the saturation is ascribed to the fact that most of the excess oxygen atoms exceeding δ≈0.3 are introduced in the form of O₂ instead of O^2-.