OXYGEN DEFECTS IN FE-SUBSTITUTED TL-SYSTEM SUPERCONDUCTORS

For Fe-doped Tl-1223 phase, the excess oxygen defects induced by Fe do pants are studied by means of Hall coefficient, thermogravimetric meas urements, Mossbauer spectroscopy, and the model calculation of the eff ective bond valence. The extra oxygen defects have effects on carrier density and microstructure of the superconductors. In the light doping level of Fe (x=0-0.05), the superconducting transition and carrier de nsity have significant corresponding relation- the zero resistance tem perature T-co and carrier densities decrease linearly with Fe dopants increasing. The thermogravimetric measurements show that the Fe3+ ions ' substituting for Cu2+ ions can bring the extra oxygen into the latti ce to form extra oxygen defects. The calculation of the effective bond valence shows that the decrease of carrier density originates the str ongly localized binding of the extra oxygen defects. The distortion of Cu-O layer induced by the extra oxygen defects decreases the supercon ductive transition temperature. The microstructures of the extra oxyge n defects including the oxygen occupying and the shifts of Fe and O ar e discussed.