Moessbauer studies of 57Fe substitution of Cu ions in superconducting oxides

Since the discovery of high-Tc superconductivity in YBa2Cu3O7 oxides several studies of metal ions substitutions were reported. The observed depression on Tc without a systematic correlation with the charge and magnetic moment of Cu substituents claims for more detailed information about its local properties as can be revealed by 57Fe Moessbauer spectroscopy. The results for different iron concentrations combined with modifications of the superconducting transition are discussed concerning the presence of magnetic moments on the Fe ions and the preferential occupation of Cu(1) sites, recently confirmed by neutron and electron diffraction experiments. The oxygen coordination for the different iron species are proposed on the basis of their dependence on Fe concentration, their behavior at high temperatures as well as the electron diffraction and electron microscopy measurements reported for Fe: YBa2Cu3O7 samples. (author)