STABILITY OF THE 2223-PHASE IN THE LEAD-SUBSTITUTED BISMUTH CUPRATES

The characteristics of the phase equilibrium in the lead-substituted B i2O3-SrO-CaO-CuO system are specified in the vicinity of the compositi on of the 2223 phase, and for temperatures close to the beginning of m elting. We examine the effect of varying the ratios (Bi,Pb)/Cu as well as the ratio Sr/Ca. The 2223 phase is found to coexist with other pha ses for a large variety of compositions, but its quasi-single-phase fi eld is restricted to compositions departing little from stoichiometry. The kinetics of formation of the 2223 phase is greatly enhanced by an excess of calcium, while an excess of strontium prevents its formatio n. The optimal range of compositions lies close to the stoichiometric composition in contrast with the situation in the lead-free system. In this range, resistivity and magnetic-susceptibility measurements show that T-c is unsignificantly affected by the composition changes. The results are clarified by a qualitative model of the stability of the b ismuth cuprate structure which is based on the idea that the CaCuO2 mu ltilayers present in these structures and the BiO-SrO layers enclosing them are both unstable. The structure is stabilized by the mutual mec hanical coupling of CaCuO2 which its in a ''compressed'' state and of BiO-SrO which is in a ''stretched'' state.