ON THE MOSSBAUER SITE ASSIGNMENT IN YBA2(CU1-XFEX)3OY

The assignment of the four57Fe Mössbauer sites which are observed in Fe-substituted YBa2Cu3Oy is presented. We present a systematic study of highly-controlled samples with low Fe-concentration over the whole oxygen concentration range. Two sites are found to dominate in the limit of high oxidation (y→7), indentified as Fe4+ on the two crystallographic Cu sites: Cu(1) and Cu(2). These are replaced at lower oxidation (y→6) by two others, which we identify with Fe3+ on these same two sites. Consistency is found with the sign of the electric field gradient (EFG), the asymmetry parameter η and the isomer shift IS for this model. The magnetic structure in the limit y→6 shows changes in the antiferromagnetic (AF) structure dependent on iron concentration. Mössbauer and spin-echo NMR results show a change in the Cu(2) AF layer sequence at low temperatures, without any magnetic moments on the Cu(1) chain sites. © 1990 J.C. Baltzer A.G. Scientific Publishing Company.