COMPARATIVE MOSSBAUER-SPECTROSCOPY AND NEUTRON-DIFFRACTION ANALYSIS IN YBA2(CU1-XFEX)(3)O-Y .1. STRUCTURAL TRANSITIONS

The structural and electronic transitions induced by oxygen reduction in YBa2(Cu1-xFex)3O(y) compounds have been studied by Mossbauer spectr oscopy, and the results are compared with neutron diffraction data, ob tained on the same Fe-57 isotope enriched samples. It was found that a fter reduction the value of the quadrupole splitting for the Fe ions i n Cu2 sites (the D3 doublet in the Mossbauer spectral is essentially s maller than that in the oxidized state. This is explained by a shift o f the Fe ion along the c-axis towards the apical O1 oxygen, which make s the Fe position inside the oxygen pyramid (k = 5) more symmetric. Up on reduction, the Cu1 --> Cu2 iron migration really takes place, and t he particular Cu1 site, from which Fe migrates to Cu2 sites, is the py ramidal D2 local site. After the reduction, the charge and spin states of all ions in Cu1 layers change remarkably, and high spin Fe3+ state s appear in Cu1 sites (in the D1 and D2' local sites). A part of the F e ions in the pyramidal Cu1 sites (the D2 '' local sites) is shifted a long the a-axis towards the apical O5 oxygen. Thus, not only the oxyge n coordination but also the local crystal structure and the electronic state of the Fe ions change upon doping.