The Madelung energy of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ca}}_{1+\mathit{x}}$${\mathrm{Cu}}_2$${\mathrm{O}}_{6\pm \mathrm{\delta }}$ system, whose properties change from semiconductor to superconductor under high-oxygen-pressure annealing, is calculated. We find the increase in Δ${\mathit{V}}_{\mathit{A}}$, the difference in Madelung site potential between the apex and the ${\mathrm{CuO}}_2$ planes’ oxygen, promotes the superconducting transition in this system. Slight intercalation of oxygen between two ${\mathrm{CuO}}_2$ planes is found to be of primary importance in determining Δ${\mathit{V}}_{\mathit{A}}$.

• Received 10 January 1994

DOI:https://doi.org/10.1103/PhysRevB.49.12315