Formation of high-T _c phases of the system Bi-Pb-Sr-Ca-Cu-O in a fluxed KCl melt

Abstract

The influence of the conditions of synthesis and annealing on the ratio of the volumes and the temperature T ⁿ_c at which transition to the superconducting state begins is investigated on the basis of diamagnetic susceptibility measurements and x-ray phase analysis for bismuth 2212 and 2223 phases obtained from solution in a KCl melt. It is found that the value of T ⁿ_c for the 2212 phase decreases as the temperature and the holding time are increased in synthesis. The 2212–2223 transition in the fluxed KCl melt takes place in the presence of an oxygen deficiency, and the width of the transition interval to the superconducting state of the 2223 phase depends on the cooling rate of the fluxed melt after isothermal holding and also on subsequent annealing in air. Temperatures T ⁿ_c =107 K, 90 K, and 20 K are observed for unannealed crystals of the 2223 phase with average dimensions 50×50 µm. Air annealing leads to oxygen saturation of their lattice, and T ⁿ_c =107 K throughout the entire structure of the crystal.