Evidence for Fe/sup 4+/ in YBa/sub 2/ (Cu/sub 1-//sub x/M/sub x/)/sub 3/O/sub 7-//sub y/ (M = /sup 57/Fe, /sup 57/Co) by absorption and emission Moessbauer spectroscopy

/sup 57/Fe absorption and /sup 57/Co emission Moessbauer spectroscopy was applied to iron- and cobalt-doped YBa/sub 2/(Cu/sub 1-//sub x/M/sub x/0/sub 3/O/sub 7-//sub y/ high-temperature superconductors (M = /sup 57/Fe, /sup 57/Co; 3.5 x 10/sup -5/less than or equal toxless than or equal to0.1, 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/sub Fe/greater than or equal to0.06. 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 Fe/sup 4+/; C is high spin Fe/sup 3+/. We provide evidence thatmore » Co is mostly in the four-valent state, too. Four-valent Fe and Co substitution in YBa/sub 2/Cu/sub 3/O/sub 7-//sub y/ is in reasonable agreement with the dopant concentration dependence of T/sub c/. Possible crystallographic assignments of all four iron (cobalt) sites are discussed.« less