XANES STUDY OF RARE-EARTH VALENCY AND 4F HYBRIDIZATION IN PB(2)SR(2)LN(1-X)CA(X)CU(3)O(8) FOR LN=CE, PR AND TB

Pb2Sr2Ce0.5Ca0.5Cu3O8 and Pb(2)Sr(2)Ln(1-x)Ca(x)Cu(3)O(8) with Ln = Pr , To and x = 0.0 and 0.5 have been studied by rare-earth L(II,III) edg e XANES spectroscopy. The spectra of Pb2Sr2Ce0.5Ca0.5Cu3O8 show that C a doping of Pb2Sr2CeCu3O8 causes a partial oxidation of Ce. Analysis w ithin the framework of the Anderson single-impurity model yields an f electron occupation of 0.5 in Pb2Sr2Ce0.5Ca0.5Cu3O8. The partial oxida tion of Ce in Pb2Sr2Ce0.5Ca0.5Cu3O8 prevents the formation of electron holes in the CuO2 layers and thereby explains the lack of superconduc tivity in this compound. The XANES spectra of the Pr and Tb containing compounds show that the rare-earth ions in these compounds are trival ent and that no oxidation of the rare-earth ions is induced by Ca dopi ng.