Effect of Fe substitution in the structure and superconducting properties of the (Y0.8Pr0.2)Ba2Cu4-xFexO8 system

The effects of iron substitutions on the superconducting and structural properties of (Y0.8Pr0.2)Ba(2)Cu(4-x)FexO(8) were studied. As iron concentration increases, the rate,at which T. diminishes with x is the same as for the YBa2Cu4-xFexO8 system, but faster than in the YBa2Cu3-xFexO7-delta system. The Mossbauer spectra of the different samples, together with Rietveld refinements of their corresponding X-ray spectra, indicate that the iron atoms occupy preferably the Cu(1) sites of the double (CuO)(2) chains in fivefold coordination; that is, due to their tri-valency, the iron atoms attract extra oxygen atoms, named O(5), that place themselves along the a-axis in between two adjacent planes of the double (CuO)(2) chains. As x is increased, the Cu(1)-O(4) bond length decreases, whereas the Cu(2)-O(4) one increases. Also, a reduction of the occupation factors of the oxygen atoms in the CuO2 planes is observed. Some of these changes may be due to the presence of O(5) atoms, which in turn affect the charge-transfer mechanism between the (CuO)(2) double chains and the CuO2 planes where superconductivity takes place, and could be responsible of the rate at which T-c decreases. (C) 2002 Elsevier Science B.V. All rights reserved.