Effect of Co partial substitution on the valence state of Ru in the Gd2-x Co x Ru2O7 pyrochlore

Polycrystalline samples of Gd2-x Co (x) Ru2O7 with x = 0.0, 0.1 and 0.4 were synthesized by the molten salt method. The samples were studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electrical resistivity measurements. Rietveld refinements of the XRD patterns and XPS measurements showed that the Co2+ ion replaces Gd3+ sites. As a result, the lattice parameter a and the Ru-O bond length decrease; then, the Ru-O-Ru bond angle increases. Those changes induce a charge compensation which was detected by XPS measurements. The analysis of these results shows that the Ru 3d (5/2) core level could be fitted assuming the contribution of two different chemical states of the Ru. The Ru 3d (5/2) core level is localized at 280.7 and 281.6 eV, which corresponds to Ru4+ and Ru5+. The valence band XPS spectra show an increase in Co 3d states at the Fermi level as the Co content increases, which contribute to the decrease in the electrical resistivity.