Characterization and superconducting properties of phases in the Bi-Sr-Cu-O system

Phase formation in the system Bi-Sr-Cu-O has been examined as a function of composition, temperature, ambient atmosphere, cooling history, and annealing time. Ceramic processing and melt crystallization techniques were used. For the ceramic materials (using Bi2O3, SrCO3, and CuO) processed at 700 degree C in air the Bi2Sr2O6 composition (221) crystallizes to a mixture of CuO, SrCO3, and the rhombohedral Bi2O3. xSrO solid solution. At 800--830 degree C in air for short durations (5 min to 2 h) the reacted products consist principally of the ideal 221 phase with minor amounts of CuO. For longer reaction times (2--400 h) the reacted products consist of the ideal 221-type structure with c=24.64 A and a=3.804 A, a ''collapsed'' 221 structure with c=23.6 A, and CuO. With increasing reaction time the ''collapsed'' 221 phase grows gradually at the expense of the ideal 221 phase. The ''collapsed'' 221 phase is not an oxycarbonate and appears to be a distinct ternary compound near the 221 composition, with a layered structure having a 1 A smaller stacking repeat. The ideal 221 phase is a solid solution with variable Sr content. With decreasing Sr in the starting mixture [2 to 1.25 atoms per formula unit (afu)] we observe the following: (1) the formation of the ''collapsed'' 21 structure is inhibited; (2) for the ideal 221 phase of c-cell dimension decrease significantly (0.2 A) and the a-cell dimension increases slightly (0.02 A); (3) the low temperature resistivity behavior changes from superconducting with Tc onset of 6 K for Sr>1.5 afu to semiconducting for Sr≤1.5 afu; (4) the positions of the superlattice peaks around the (001) reflections become more incommensurate with respect to the parent structure