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

The Tc of Bi2-Ca-Sr2-Cu2-O8-x can be shifted reversibly over a rangel of at least 15 K by changing the oxygen concentration. Tc shifts downward for increased oxygen content. This is opposite to the shift seen in YBa2Cu3O7- 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 Tc=78.5 K of the 80-K phase is obtained by treating at 600°C in 10-2-bar oxygen. Tc 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 Tc is restored upon retreatment in O2 at low partial pressure. The 110-K phase showed a much smaller shift (Tc4 K) after treatment in O2 at 134 bar.