We have discovered that the apical oxygen with a 2.35-Å La-O bond length is removed when La is substituted by Sr in polycrystalline ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ under normal preparation conditions. This apical oxygen can be partially filled by oxygen annealing. It is reasoned that a defect oxygen is trapped at an interstitial site near the Sr atom. We present evidence that this defect oxygen is intrinsic to Sr doping, independent of processing conditions. We propose that this defect oxygen serves as a mechanism for hole doping similar to that in the superconducting oxygen-rich ${\mathrm{La}}_2$${\mathrm{CuO}}_{4+\mathit{y}}$.

• Received 29 November 1989

DOI:https://doi.org/10.1103/PhysRevLett.64.2715