Oxidation state of Ce in Pb2Sr2Ce1-xCaxCu3O8

The structural and electronic properties of Pb2Sr2Ce1-xCaxCu3O8 have been studied using neutron and x-ray diffraction, magnetization, and x-ray absorption near edge structure (XANES) experimental techniques. Our experiments show that although Pb2Sr2Ce1-xCaxCu3O8 has the same crystal structure as any of the other Pb2Sr2R1-xCaxCu3O8 (R=Y,Pr-Lu) compounds, superconductivity is not observed in the cerium-based materials. Neutron diffraction data also reveal an orthorhombic to monoclinic structural transition in Pb2Sr2YCu3O8 above room temperature, and this transition is expected to occur in other rare-earth Pb2Sr2R1-xCaxCu3O8 materials also. Pr L3-edge XANES measurements show that Pr is trivalent in Pb2Sr2Pr1-xCaxCu3O8. In contrast, XANES measured on both the Ce L3 and Ce K edges indicate that cerium is tetravalent in Pb2Sr2R1-xCaxCu3O8. The valence of R is not affected by the Ca doping either for cerium or for any of the other rare-earth ions studied. Pb L3 and Cu K edge XANES indicate that some of the extra charge introduced by cerium is transferred to the Cu-O layers responsible for the super- conductivity changing Cu2+ to Cu1+. This charge transfer may explain the absence of superconductivity in Pb2Sr2Ce1-xCaxCu3O8. copyright 1996 The American Physical Society