Oxygen stoichiometry-T_c correlations in Bi₂Sr₂CaCu₂O_8+x

The oxygen content of single-phase Bi₂Sr₂CaCu₂O_8+x depends on the annealing temperature and the oxygen partial pressure. The T_c maximum, 87 K, is associated with an excess oxygen content, x, of 0.035+or-0.003, as determined by volumetric analysis and thermogravimetry. This stoichiometry is obtained by, for instance, annealing in air at approximately 820 degrees C or in nitrogen at approximately 400 degrees C. For higher and lower x, T_c decreases. No structural changes associated with this change in oxygen content were observed; variations in T_c appear therefore to be associated with changes in carrier concentration. The rate of change of T_c with x is more than ten times greater than in YBa₂Cu₃O_x.