Formation of Grain‐Boundary Carbon‐Containing Phase During Annealing of YBa ₂ Cu ₃ O _6+x

Abstract

In situ annealing studies of YBa 2 Cu 3 O 6+x performed in an optical hot stage revealed that, at temperatures ∧450°, localized melting occurred. On subsequent cooling, a discrete second phase was observed at the YBa 2 Cu 3 O 6+x grain boundaries. Quantitative chemical analysis using X‐ray wavelength dispersive spectroscopy indicated that the second phase was composed of a barium oxycarbonate. The source of the carbon in the second phase was identified to be CO 2 in the atmosphere.