High-pressure synthesis of CuBa2Can−1CunOx (n=3, 4) superconductors without external oxidizing agents

We have successfully synthesized non-stoichiometric BaCuO2+δ as a precursor for the high-pressure synthesis of CuBa2Can−1CunOx [Cu-12(n−1)n] superconductors. The oxygen content of BaCuO2+δ was determined by a coulometric titration method. Using this precursor but no external oxidizing agents, samples which were superconducting with Tc=116 K have been obtained by sintering at 1100°C under 5 GPa. The Tc=116 K phase was identified to be the CuBa2Ca3Cu4Ox (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 Tc at 80 K was also obtained at a temperature as low as 950°C. The 80 K phase was identified to be the CuBa2Ca2Cu3Ox (Cu-1223) phase. Thus, the Cu-1234 and Cu-1223 phases were successfully synthesized without the presence of any external oxidizing agents.