Oxygen-rich samples of YBa2Cu3-xFexOy with different concentrations
have been prepared by solid state reaction in both superconducting (SC) and
antiferromagnetic (AFM) phases. Electrical resistivity measurements indicated
the preference of Fe to substitute Cu in Cu (1) sites for low iron concentrations
in the Sc phase. Mössbauer measurements have been carried out at room
temperature, 77 K and 4.2 K in order to elucidate Fe distribution just below the
SC-AFM phase transition of x=0.45 and up to x=1. From the analysis of the
obtained MS spectra, it is concluded that in the SC phase with x=0.4, Fe
substitutes Cu in both square planar and induced pyramidal Cu (1) sites. The
latter is formed by oxygen filling the unoccupied O (5) site. With increasing
iron content, Fe accommodated in Cu (2) sites prevail, together with the Cu (1)
sites of pyramidal configuration. The obtained small QS values for Fe in Cu(2)
sites suggest the formation of quasi-octahedral Fe site by additional oxygen
atom insertion in the mid plane of the Cu (2)-O bilayer. At 4.2 K, a suppressed
hyperfine ordering with Bhf value of 472 kOe is assigned to Fe in Cu (1) sites
for the superconducting sample with x=0.4. On the other hand, a hyperfine field
of 502 kOe, slightly increasing with x, is suggested to associate Fe in Cu (2)
sites with subsequent increase in iron content.