Title: Suppression of Tc in the (Y0.9Ca0.1)Ba2Cu4-xFexO8 system

Abstract: In this paper, the effects produced by the 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 chain in fivefold coordination at low iron concentrations. Besides at high iron concentrations the iron atoms occupy the Cu(1) sites of single Cu-O chainss and Cu(2) sites in the CuO2 planes of the (Y0.9Ca0.1)Ba2Cu4-xFexO8 phase with structural defects. Simultaneouly, as iron concentration increases, a faster decrease of Tc is observed in this material comapred with the YBa2Cu3-xFexO7-y 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.