THE COMPLEX DISTRIBUTION OF IRON IN THE (Y, CA)BA2(CU, FE)3O6+Y CUPRATE - A MOSSBAUER STUDY

Mossbauer studies with Fe-57 in Y1-zCazBa2Cu3-xFexO6+y for x=0.09 and 0.24, z=0.09 and 0.24, and y between 0 and 1, allowed, for fixed z and x values, different distributions of iron on the copper sites to be observed according to thermal treatment under different gaseous atmospheres (Ar, air, oxygen). Three types of coordination - a tetrahedral and two pyramidal - are proposed for Fe on Cu(1) sites; one of the pyramidal Cu(1) sites is compatible with the existence of small iron clusters (Fe2O9, Fe3O13, etc.). The accomodation of tetrahedral coordination by a fraction of iron atoms induces the transition from orthorhombic to tetragonal symmetry with increasing x. The migration of iron from these sites into the Cu(2) layer restores the orthorhombic symmetry. An increase in the occupancy of Fe on the Cu(2) site leads to a sharp detrimental effect on the critical temperature T(c); this confirms the predominant role of the Cu(2) sites in superconductivity for the ''123''-structure.