Oxygen concentration effect onTcof the Bi-Ca-Sr-Cu-O superconductor

The $T_c$ of ${\mathrm{Bi}}_2$-Ca-${\mathrm{Sr}}_2$-${\mathrm{Cu}}_2$-${\mathrm{O}}_{8\mathrm{-}\mathrm{x}}$ 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 ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ and may be caused by reduction in the Fermi level reducing conductivity in the Bi-O layers. Oxygen concentration is varied by treatment in ${\mathrm{O}}_2$ at appropriate pressures. The highest $T_c$=78.5 K of the ‘‘80-K’’ phase is obtained by treating at 600 °C in ${10}^{\mathrm{-}2}$-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 ${\mathrm{O}}_2$ at low partial pressure. The 110-K phase showed a much smaller shift (Δ$T_c$≊4 K) after treatment in ${\mathrm{O}}_2$ at 134 bar.