SITE OCCUPANCY AND T-C DEGRADATION IN IRON SUBSTITUTED YBA2(CU1-XFEX)(4)O8+DELTA

A series of YBa2(Cu1-xFex)(4)O8+delta a samples with x = 0.00625, 0.01 25, 0.01875, 0.025 and 0.05, were synthesized and characterized by res istance vs, temperature measurements, X-ray diffraction and by Mossbau er spectroscopy. In order to make the site assignment for the Fe atoms , a Rietveld refinement of the X-ray spectra was performed and a point charge calculation of the quadrupole splitting for all the possible o xygen environments around the two Cu sites was carried out, taking int o account the two possible ionic states of the iron atoms that occupy those sites. We found that, for low level iron concentrations, the Fe atoms occupy only the Cu(1) sites of the structure in a fivefold pyram idal coordination, with the apical oxygen atom placed along the a-axis between the double chains of the unit cell. The presence of these ext ra oxygen atoms in the crystal structure could be related to the rapid T-c reduction as iron concentration increases. (C) 1998 Published by Elsevier Science B.V. All rights reserved.