${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ absorption and ${}_{}{}^{57}{}_{}{}^{}\mathrm{Co}$ emission Mössbauer spectroscopy was applied to iron- and cobalt-doped ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{M}}_{\mathrm{x}}$$0_3$${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$ high-temperature superconductors (M${=}^{57}$Fe, ${}_{}{}^{57}{}_{}{}^{}\mathrm{Co}$; 3.5×${10}^{\mathrm{-}5}$≤x≤0.1, y=0 and y=0.8) between 4.2 and 295 K with and without external magnetic field. four iron species A, B, C, and D, are observed with different dominance at different x and y. Aftereffects of the electron capture are ruled out. At 4.2 K spontaneous antiferromagnetic order is shown by contributions A, B, and C in spectra of samples with $x_{\mathrm{Fe}\ge 0.06}$. A model-independent separation of the magnetically split subspectra shows an asymmetry parameter of the electric field gradient above 0.8 for A and B. Temperature and external magnetic field dependence of the magnetic splittings is indicative of superparamagnetic relaxation of small clusters. The species A, B, and C are magnetically interrelated. We suggest a preferential Cu(1) substitution by Fe and Co. Three of the four iron species, viz. A, B, and D have been assigned to high-spin ${\mathrm{Fe}}^{4+}$; C is high spin ${\mathrm{Fe}}^{3+}$. We provide evidence that Co is mostly in the four-valent state, too. Four-valent Fe and Co substitution in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{y}}$ is in reasonable agreement with the dopant concentration dependence of $T_c$. Possible crystallographic assignments of all four iron (cobalt) sites are discussed.

• Received 15 July 1988

DOI:https://doi.org/10.1103/PhysRevB.38.11373