SYNTHESIS OF YBA2CU3O7-X BY SOL-GEL ROUTE - FORMATION OF YBACUO OXYCARBONATE INTERMEDIATE

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 ag ent. This method provides an easy way to prepare reactive YBaCuO powde rs. 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-degrees-C in flowing oxygen. From powder X-ray diffraction, th ermal analysis and IR spectroscopy, it is concluded that the intermedi ate oxycarbonate has an average tetragonal structure - SG P4/mmm - sim ilar to the parent oxide with a stoichiometry close to YBa2Cu2.95(CO3) 0.35O6.6. The carbonate group is located in the center of the basal Cu O square. This compound has superconducting properties. A pure 123 pha se is obtained when the sol-gel precursor is annealed at 925-degrees-C in O2.