Oxidation at Ba sub 2 YCu sub 3 O sub x at High P sub O sub 2

Dense large, grain ceramic samples of Ba sub 2 YCu sub 2 O sub x have been oxidized at 400 to 500C in 100 to 300 atm oxygen for 1 to 10 days. The high pressure treatment has increased the equilibrium content x slightly above 7 but there is no concomitant increase in T sub c . At x =7.03 the superconducting transition, measured by a.c susceptibility, has an onset temperature of ~ 91K and DELTA T ~ 4 and no change in lattice parameter is observed. High oxygen pressure increases the rate of oxidation.