Structure of the interstitial oxygen defect in ${\mathrm{La}}_2$${\mathrm{NiO}}_{4+\mathrm{\delta }}$

Using Rietveld refinement of neutron powder diffraction data, we show that the excess oxygen in ${\mathrm{La}}_2$${\mathrm{NiO}}_{4+\mathrm{\delta }}$ (0.13<δ<0.18) is incorporated as an interstitial oxygen defect. The defect is located near the ((1/4, 1/B) / 4 ,(1/4) site in the orthorhombic Fmmm structure, which provides a favorable coordination to four La atoms but requires four nearby oxygen atoms to be displaced from their normal positions. The defect concentration determined from structural refinement agrees well with the overall oxygen stoichiometry determined by hydrogen reduction. For intermediate oxygen contents (δ≊0.07) the system separates into two phases with different defect concentrations. Structural data suggest that the nearly stoichiometric phase, 0<δ<0.02, incorporates excess oxygen by forming a different defect. Except for large differences in the solubility of the oxygen defect, ${\mathrm{La}}_2$${\mathrm{NiO}}_{4+\mathrm{\delta }}$ exhibits behavior remarkably similar to superconducting ${\mathrm{La}}_2$${\mathrm{CuO}}_{4+\mathrm{\delta }}$, suggesting that the oxygen defect structures are the same in both systems.