IDENTIFICATION OF THE HIGH-PRESSURE PHASE IN BI-SR-CA-CU-O SUPERCONDUCTORS BY HRTEM AND XRD

The composition and structure of the main decomposition product of Bi- 2212 annealed under high oxygen pressures was determined by WDS, TEM a nd XRD analyses. The high pressure decomposition reaction for Bi-2212 can be written as: Bi2Sr2Ca1Cu2O8+delta+ O-2-->0.754(Bi2Sr2.48Ca1.19Oy ) + 0.246(Bi2Sr0.53Ca0.39Ox) + 2CuO. The highly oxidised phase Bi2Sr2. 48Ca1.19Oy has the same composition as a less oxidised phase whose sol id solution region was found to be centred on the Bi:(Sr + Ca) 1.2 or (Bi + Cu):(Sr + Ca) 9:(11 + 5) positions. The average atomic positions of the high pressure phase have been determined by high resolution im ages and image simulations along [110] and [011] directions with diffe rent sample thickness. In addition, experimental XRD has been compared with calculated XRD data for possible structures of the phase. The av erage structure was confirmed to be a perovskite-like structure with S r occupying A-sites and Bi and Ca occupying B-sites. The average occup ancy of Bi and Sr positions is 0.75 and the other positions were occup ied by both Ca and Bi ions with an occupancy of 0.5 for Ca and 0.25 fo r Bi.