ION COORDINATION AND STRUCTURAL TRANSITION OF YBa2Cu3-xFexOy

Abstract

Critical temperature, phase structure, oxygen content and coordination of oxygen atoms were studied for Fe doped superconducting oxides YBa2Cu3-xFexOy(0.01≤x≤ 0.2) after an-nealing at different temperatures by use of M?ssbauer spectroscopy and X-ray diffraction. The result shows that Fe ion occupies mainly Cu(l) position. Both the increase of Fe content and vacuum annealing at high temperature lead to the structural transition from orthorhombic, to tetragonal phase, and the critical temperature Tc goes to zero. However, in the former case, during the increase of Fe content the transition occurs when appreciable amount of Fe3+ ion with sixfold coordinate oxygen atoms exists, and is thus accompanied by the increase of oxygen content. But for vacuum annealing at high temperature, with the increase of annealing temperature the number of Fe ion with double oxygen coordination increases and oxygen content decreases, and the transition occurs when this double oxygen coordination dominates. Both these processes destroy the planar Cu-O configuration and lead to the decrease of Tc .Besides, the valence states of Fe atoms at different lattice sites are also determined and discussed.