Optimization of the synthesis and tuning the oxygen content of the CuBa 2Ca 2Cu 3O 8+ δ (Cu-1223:P) superconductor

Abstract

The high-pressure (5 GPa) synthesis of the superconducting CuBa 2Ca 2Cu 3O 8+ δ (Cu-1223:P) phase was optimized in terms of temperature, the starting metal composition and the amount of AgO used as an external oxygen source. The most phase-pure Cu-1223:P samples were obtained from the 2BaCuO 2+ x –Ca 2CuO 3–CuO starting stoichiometry when performing the synthesis at 1050–1100°C and using 200 mol% of AgO. The as-synthesized Cu-1223:P samples with T c=67 K were found to be strongly overdoped by a thermoelectric power measurement. With post-annealings carried out in a thermobalance in a flowing Ar gas at temperatures ranging from 300 to 420°C the hole-doping state in the as-synthesized Cu-1223:P material could be tuned in the same continuous manner observed for the YBa 2Cu 3O 6+ δ (Cu-1212:P) compound, even though all the obtained Cu-1223:P samples were, in contrast to so far obtained YBa 2Cu 3O 6+ δ samples, in an overdoped state. The decrease in the hole concentration in the Cu-1223:P phase with increasing annealing temperature was accompanied with increases in T c and in both lattice parameters of the tetragonal unit cell. The highest obtained T c was 118 K for the sample annealed at 400°C. Above 400°C in Ar the Cu-1223:P phase was found to decompose. From the observed overall weight loss upon the Ar-annealing up to decomposition temperature the amount of removable oxygen in the Cu-1223:P phase could be established at Δ δ≈0.4.