High-pressure synthesis of CuBa 2Ca n−1 Cu nO x ( n=3, 4) superconductors without external oxidizing agents

Abstract

We have successfully synthesized non-stoichiometric BaCuO 2+ δ as a precursor for the high-pressure synthesis of CuBa 2Ca n−1 Cu n O x [Cu-12( n−1) n] superconductors. The oxygen content of BaCuO 2+ δ was determined by a coulometric titration method. Using this precursor but no external oxidizing agents, samples which were superconducting with T c=116 K have been obtained by sintering at 1100°C under 5 GPa. The T c=116 K phase was identified to be the CuBa 2Ca 3Cu 4O x (Cu-1234) phase. The Meissner volume fraction of the sample was estimated at ca. 30% at 5 K. A sample which contained a superconducting phase with T c at 80 K was also obtained at a temperature as low as 950°C. The 80 K phase was identified to be the CuBa 2Ca 2Cu 3O x (Cu-1223) phase. Thus, the Cu-1234 and Cu-1223 phases were successfully synthesized without the presence of any external oxidizing agents.