Oxygen concentration effect on T_c of the Bi-Ca-Sr-Cu-O superconductor

The T_c of Bi₂-Ca-Sr₂-Cu₂-O_8-x can be shifted reversibly over a rangel of at least 15 K by changing the oxygen concentration. T_c shifts downward for increased oxygen content. This is opposite to the shift seen in YBa₂Cu₃O_7-δ and may be caused by reduction in the Fermi level reducing conductivity in the Bi-O layers. Oxygen concentration is varied by treatment in O₂ at appropriate pressures. The highest T_c=78.5 K of the ‘‘80-K’’ phase is obtained by treating at 600 °C in 10^-2-bar oxygen. T_c shifts down to 65.4 K after treatment at 137 bars. The transitions remain sharp, with no evidence of significant structural change by x-ray powder diffraction. The original T_c is restored upon retreatment in O₂ at low partial pressure. The 110-K phase showed a much smaller shift (ΔT_c≊4 K) after treatment in O₂ at 134 bar.