M. L. Marquina

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

Abstract The effects of iron substitutions on the superconducting and structural properties of (Y 0.8 Pr 0.2 )Ba 2 Cu 4− x Fe x O 8 were studied. As iron concentration increases, the rate at which T c diminishes with x is the same as for the YBa 2 Cu 4− x Fe x O 8 system, but faster than in the YBa 2 Cu 3− x Fe x O 7− δ 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 CuO 2 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 CuO 2 planes where superconductivity takes place, and could be responsible of the rate at which T c decreases.

Suppression of Tc in the (Y0.9Ca0.1)Ba2Cu4−xFexO8 system

In this paper, the effects produced by iron substitutions in the (Y0.9Ca0.1)Ba2Cu4−xFexO8 system on the superconducting and structural properties are studied. The Rietveld-fit of the crystal structure and Mossbauer spectroscopy results of (Y0.9Ca0.1)Ba2Cu4−xFexO8 samples indicate that the iron atoms occupy the Cu(1) sites of the (Cu–O)2 double chains in fivefold coordination at low iron concentrations. Besides at high iron concentrations the iron atoms occupy the Cu(1) sites of single Cu–O chains and Cu(2) sites in the CuO2 planes of the (Y0.9Ca0.1)Ba2Cu4O8 phase with structural defects. Simultaneously, as iron concentration increases, a faster decrease of Tc is observed in this material compared with the YBa2Cu3−xFexO7−δ system. According to the charge-transfer model proposed for YBa2Cu4O8 under pressure, the decrease in the Cu(1)–O(4) bond length in parallel to the increase in the Cu(2)–O(4) bond length may affect the charge-transfer mechanism leading to the suppression of Tc.