OXYGEN ANNEALING AND THE RELATED THERMODYNAMICS IN HGBA2CUO4+DELTA

Several well-characterized HgBa2CuO4+delta samples were annealed under various conditions. Above 300-degrees-C and at various oxygen partial pressures, both reversible oxygen intake/release and irreversible Hg-loss take place with different time constants. By choosing the proper annealing temperature T(a), oxygen pressure (P(O2) and annealing time t, the Hg-loss effects were negligible and the bulk superconducting transition temperature T(c) was reversibly changed with delta from 0 K on the underdoped side (an insulator) through a peak of 97 K, then down to approximately 20 K on the overdoped side. The measured 3D variation of T(c) vs. (1/T(a), ln P(O2) appears as a parabolic surface with iso-T(c) segments projected on a 2D 1/T(a)-ln P(O2) plane as straight lines. The slope [partial derivative ln P(O2)/partial derivative(1/T(a))]T(c) of the projections corresponds to the oxidation enthalpy DELTAH, and the derivative (partial derivative ln P(O2)/partial derivativeT(c))T(a), which is a measure of the T(c) variation with the oxygen partial pressure used in the annealing, is related to the defect-interactions and the T(c)-delta correlation. The data for YBa2Cu3O7-delta, Bi2Sr2CaCu2O8+delta, and HgBa2CuO4+delta are compared.