BI AND CU VALENCE CHARACTERIZATION AND CHARGE-TRANSFER IN SINGLE-PHASE BI-2212 CERAMICS

Samples with Bi2.05Sr1.9Ca1.05Cu2O8+delta composition were synthesized by an original process based on the spray drying of a nitrate solutio n of Bi, Sr, Ca and Cu cations complexed with EDTA at pH >6. After cal cination at 800 degrees C and grinding, the samples were sintered in t he range 820-860 degrees C in air and annealed between 600 degrees C a nd 700 degrees C in nitrogen. The relationships between critical tempe rature (T-con and T-coff), Bi and Cu valences (determined by permangan ate and iodometric titration) and cell parameter c were investigated. The relatively weak variation of Bi with respect to that of Cu as a fu nction of the oxygen excess delta in the BiO planes provides evidence for a 'buffer' effect in these planes induced by the charge transfer f rom the CuO2 planes.