Superconducting properties of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Bi}}_{\mathit{x}}$${\mathrm{CuO}}_{4+\mathrm{\delta }}$: Suppression of the phase separation observed for ${\mathrm{La}}_2$${\mathrm{CuO}}_{4+\mathrm{\delta }}$

Superconducting properties of a solid solution, ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Bi}}_{\mathit{x}}$ ${\mathrm{CuO}}_{4+\mathrm{\delta }}$ (0≤x≤0.2), treated at high oxygen pressures were studied by dc magnetic-susceptibility measurements. A remarkably large superconducting volume fraction of up to ∼10% was found for x>0, though ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathrm{\delta }}$ (x=0), which oxygenated under the same conditions, showed a fraction of at most 0.2%. These results have been discussed in relation to the phase separation in ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathrm{\delta }}$ suggested by Jorgensen et al. The Bi-for-La substitution should introduce a random potential for the interstitial oxygen atoms and, thereby, suppress the segregation of the excess oxygen atoms. The large superconducting volume then results from the uniform oxygen distribution.