Fe57Mössbauer study of the superconductorYBa2(FexCu1−x)3Oy

${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ Mössbauer measurements were made on the ${\mathrm{YBa}}_2$(${\mathrm{Fe}}_{\mathit{x}}$ ${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$ ${)}_3$ ${\mathrm{O}}_{\mathit{y}}$ superconducting compounds, for the Fe concentrations of x=0.001, 0.005, 0.01, 0.03, and 0.1. The Mössbauer spectra, obtained from 4.2 K to room temperature, are dependent on concentration and show several quadrupole doublets that can be related to different Cu sites, to the number of oxygen vacancies, and to the presence of a high-temperature tetragonal phase. Two doublets in the dilute samples (up to x=0.01) are associated to ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ in the Cu1 site of the compound with different oxygen configurations. X-ray diffraction and Mössbauer spectroscopy indicate that the YBaCuO structure is strongly perturbed for higher iron concentration. dg magnetization measurements show a correlated decrease of the transition temperature $T_c$. The 4.2-K spectra of the more concentrated samples (x=0.03 and 0.1) show complex magnetic hyperfine patterns indicating the presence of magnetic moments on the Fe.