Structure ofLa2NiO4.18

The published structure of ${\mathrm{La}}_2$ ${\mathrm{NiO}}_{4+\mathit{y}}$ has unusually short (1.94 Å) Ni-O bond lengths. We reexamined the structure of ${\mathrm{La}}_2$ ${\mathrm{NiO}}_{4.18}$ using monochromatic synchrotron x-ray radiation. Rietveld refinement of the structure in space group Bbcm yielded Fourier synthesis (${\mathit{F}}_{\mathrm{obs}}$) maps that suggest a modified structural model for ${\mathrm{La}}_2$ ${\mathrm{NiO}}_{4.18}$. The refined structure contains chemically more reasonable interatomic distances (e.g., shortest Ni-O distance of 2.00 Å). A comparison of this structure with that of the superconductor ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathit{y}}$ reveals disparities in the conduction planes of the two materials that may explain differences in electrical behavior.