Superconducting and microstructural properties of Fe-doped Tl-1223 cuprates

The Fe-doping effects on superconductivity, transport and microstructure ar e investigated by means of Hall coefficient measurements, thermogravimetric analysis, Mossbauer spectroscopy and the calculation of the point-charge-e ffective-valence model (PCEV model) in Fe-doped Tl0.5Pb0.5(Sr0.8Ba0.2)(2)Ca -2(Cu1-xFex)(3)O-y (Tl-1223) superconductors. In the doping level of Fe (x = 0-0.05), both zero-resistance temperature T-co and carrier concentration n(H) decrease linearly with Fe dopants increasing. The striking linear beha vior of T-co and n(H) suggests the potential effects of charge localization on the transport properties and also shows that Fe dopants occupy differen t Cu sites in a stable manner with Fe content increasing. The thermogravime tric analysis shows the Fe3+ ions substituting for Cu2+ ions can bring exce ss oxygen atoms into lattice and form some Fe-O defect clusters. Mossbauer measurements are performed to investigate the microstructure of Fe-O defect s. In addition, the atomic position of Fe dopant and the electric field gra dient (EFG) on Fe sites are investigated under the PCEV model. We found tha t the signs of the EFG on Fe sites are positive. According to quadrupole sp littings deduced from Mossbauer spectroscopies and results of model calcula tion, it is further confirmed that there is a small shift of Fe ion relativ e to the original center of the CuO2 network, and in some different kinds o f Fe-O defect clusters, Fe dopants can lead to excess oxygen atoms entering lattice. These results provide a further support for the interpretation ab out the displacement of Fe dopant and the assignment of Fe site. Our discus sion suggests that the microstructural distortion and the excess oxygen def ect induced by Fe doping encourage in the localization of cruising carrier on CuO2 planes, which is one of the reasons to explain the decrease of T-c and carrier concentration. (C) 1999 Published by Elsevier Science B.V. All rights reserved.