Site occupancy and Tc degradation in iron substituted YBa2(Cu1−xFex)4O8+δ

Abstract A series of YBa 2 (Cu 1− x Fe x ) 4 O 8+ δ samples with x =0.00625, 0.0125, 0.01875, 0.025 and 0.05, were synthesized and characterized by resistance vs. temperature measurements, X-ray diffraction and by Mossbauer spectroscopy. In order to make the site assignment for the Fe atoms, a Rietveld refinement of the X-ray spectra was performed and a point charge calculation of the quadrupole splitting for all the possible oxygen environments around the two Cu sites was carried out, taking into account the two possible ionic states of the iron atoms that occupy those sites. We found that, for low level iron concentrations, the Fe atoms occupy only the Cu(1) sites of the structure in a fivefold pyramidal coordination, with the apical oxygen atom placed along the a -axis between the double chains of the unit cell. The presence of these extra oxygen atoms in the crystal structure could be related to the rapid T c reduction as iron concentration increases.