Variation of electronic and atomic structures in ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Fe}}_{\mathrm{x}}$${)}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

We present x-ray-absorption spectroscopy measurements of ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Fe}}_{\mathrm{x}}$ ${)}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$, x=0.015 to 0.15. Both near-edge features and extended x-ray-absorption fine-structure (EXAFS) results demonstrate that Fe preferentially substitutes for Cu(1) atoms at the linear-chain site. The compositional trend of these samples shows that the valence state of Fe (mainly +3, but the possible presence of small amounts of +2 and +4 cannot be ruled out) remains the same and Fe-O bond lengths change minimally in the light of changes in the lattice parameters with composition. Because of its sensitivity to coordination geometry, the reduction of the intensity of the 1s→3d edge feature indicates that the average coordination number of Fe nearest neighbors increases with increasing Fe content. EXAFS data analysis also finds an increase in the number of oxygen neighbors in the linear-chain plane, indicating an introduction of the extra oxygen atom on the vacancies of the a axis. Based on statistical considerations, we have examined the distribution of this extra oxygen atom as well as three possible Fe site coordination geometries (fourfold, fivefold, and sixfold).