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

Abstract

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 Bi ₂O ₃, SrCO ₃, and CuO) processed at 700 °C in air the Bi ₂Sr ₂CuO ₆ composition (221) crystallizes to a mixture of CuO, SrCO ₃, and the rhombohedral Bi ₂O ₃ · _x SrO solid solution. At 800–830 °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 Å and a = 3.804 Å, a “collapsed” 221 structure with c = 23.6 Å, 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 Å 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” 221 structure is inhibited; (2) for the ideal 221 phase the c-cell dimension decreases significantly (0.2 Å) and the a-cell dimension increases slightly (0.02 Å); (3) the low temperature resistivity behavior changes from superconducting with T _c 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. Rapid quenching (<5 s) from temperatures near the melting point (900 °C) can raise the superconducting T _c onset to 9 K. Independent of the cell variation with Sr content, quenching causes the c-cell dimension to expand by 0.03 Å on average while the a-cell dimension remains invariant. A small number of oxygen vacancies are quenched in from high temperature, and presumably originate in the Bi ₂O ₂ layer. As grown from the melt, crystals of the ideal 221 phase exhibit semiconducting behavior at low temperature; but with an additional high-temperature anneal in oxygen, metallic resistivity is restored with a superconducting onset near 5 K. Ca doping does not increase T _c in the ideal 221 phase. La and Y substitution occurs for Sr in the ideal 221 phase and ruins superconductivity.