Synthesis of YBa 2Cu 3O 7−x by sol-gel route formation of YBaCuO oxycarbonate intermediate

Abstract

High- T c superconducting powders of the Y-Ba-Cu-O system are prepared by a solution-polyacrylamide gel using citric acid as a complexing agent. This method provides an easy way to prepare reactive YBaCuO powders. However, sol-gel synthesis of this oxide involves the formation of intermediate phases which impedes the obtaining of the pure phase at low temperature. An intermediate oxycarbonate phase forms between 800 and 850°C in flowing oxygen. From powder X-ray diffraction, thermal analysis and IR spectroscopy, it is concluded that the intermediate oxycarbonate has an average tetragonal structure — SG P4/ mmm — similar to the parent oxide with a stoichiometry close to YBa 2Cu 2.95(CO 3) 0.35O 6.6. The carbonate group is located in the center of the basal CuO square. This compound has superconducting properties. A pure 123 phase is obtained when the sol-gel precursor is annealed at 925°C in O 2.