Some aspects of the thermodynamic behavior of the lead-doped Bi-2223 system

A thermodynamic assessment of lead-doped Bi-2223 with emphasis on compositions and oxygen partial pressures within the homogeneity region prior to solid-state decomposition is presented. Equations for the variation of oxygen partial pressure with composition and temperature have been derived from our EMF measurements. Long-term metastability was indicated during cycling over a temperature range of similar to 700-815 degrees C of a lead-doped Bi-2223 sample having an oxygen-deficient stoichiometry of 9.64 prior to solid-state decomposition corresponding to the diphasic CuO-Cu-2,O system. A trend of increasing negative values of the partial molar enthalpy Delta (H) over bar (O-2) and entropy a Delta (S) over bar(O-2) with increasing oxygen deficiency of the condensed phase indicated an increase in ordering of the cuprate structure prior to solid-state decomposition.