ON THE PERITECTIC TRANSFORMATION IN BI4SR3CA3CU4O16+X AND ITS ROLE IN THE FORMATION OF THE BI2SR2CA2CU3O10 PHASE

We have studied the structural and microstructural changes of Bi4Sr3Ca3Cu4O16+x on annealing close to its melting point and correlated these with the thermal stability and the electrical properties. Thermal analyses suggest that the Bi4Sr3Ca3Cu4O16+x phase undergoes a peritectic or peritectic-like transformation, accompanied by oxygen loss, just before complete melting. This transformation leads to the formation of the Bi2Sr2Ca2Cu3O10 phase as identified by X-ray diffraction (XRD) and confirmed by electrical resistivity measurements. Electron microscopy shows that Bi4Sr3Ca3Cu4O16+x samples maintain their lamellar morphology after transformation and are locally decomposed into a majority Bi2Sr2Ca2Cu3O10 phase bounded by an amorphous phase. A thin layer of Bi4Sr3Ca3Cu4O16+x phase is systematically observed between this amorphous phase and the transformed regions. In the latter, a stacking sequence corresponding to a BiSrCaCuO phase with a structure based on four CuO layers is also observed. Finally, we discuss the consequences of this transformation on electrical properties and on the conditions required for the preparation of a pure Bi2Sr2Ca2Cu3O10 phase. © 1990.