57Fe and57Co Mössbauer studies of highTcY-Ba-Cu oxides

The57Co emission Mössbauer spectra from YBa2Cu3O6.92(1-2-307) and YBa2Cu3O6.00(1-2-306) have been measured and compared with the57Fe absorption spectra from YBa2Cu2.95Fe0.05O7−δin order to clarify decisively the site assignments for the57Fe quadrupole-split doublets in these compounds. Mössbauer spectra obtained from both specimens consist of four components whose hyperfine interaction parameters well agree with each other. It is shown that the Co and Fe atoms mainly substitute at Cu1 chain sites in 1-2-307, but in 1-2-306 the Co atoms occupy randomly the Cu2 plane sites and indicate magnetically-split sextet which converts to a paramagnetic doublet of S-state Fe3+in 1-2-307 by a post-annealing in O2gas.