Synthetic Conditions and Structural Properties of the High-T_c Phase in the Superconducting Bi-Sr-Ca-Cu-(Pb)-O System

Abstract

Synthetic conditions, structural and thermal properties of the high-T_c phase (T_c > 100 K) in the Bi-Sr-Ca-Cu-O system have been investigated. Three methods have been found to be effective in increasing the volume fraction of the high-T_c phase in the Bi-Sr-Ca-Cu-0 system: (1)starting from a nominal composition with more Ca and Cu than Bi₂Sr₂Ca₂Cu₃O (2) the addition of Pb to Bi-Sr-Ca-Cu-0 system (3) annealing at 870°C under higher oxygen partial pressures than 0.2 atm. Sample with the nominal composition of Bi₂Sr₂Ca₆Cu₈O_x fired at 87O°C for 120 hours was found that it consisted of mainly the high-T_c phase. Unit cell dimensions of the high-Tc phase determined by X-ray powder diffraction and transmission electron diffraction measurement are a = 5.40 Å, b = 27.0 Å and c = 36.8 Ä, indicating a pseudotetragonal symmetry. Transmission electron microscopy showed that there were stacking faults in the high-T_c phase. The high-T_c phase and the low-T_c phase (T ~ 80 K) decompose into Bi₂Sr₂CuO_x by annealing under oxygen partial pressure of 0.02 and 0.1 atm at 87O°C. Under higher oxygen partial pressures than 0.2 atm at 870°C, the high-T_c phase was remarkably formed from the possible disproportionation reaction of the low-T_c phase.