XPS and NEXAFS study of Cu/Ni codoped bismuth tantalate pyrochlore

The chemical state of transition element cations in oxide pyrochlore Bi$_2$Cu$_{1/2}$Ni$_{1/2}$Ta$_2$O$_{9\pm\delta}$ (sp. gr. Fd-3m, a=10.5378(6)Å) synthesized by the solid-phase method was studied using X-ray absorption spectroscopy (NEXAFS, XPS). The parameters of the XPS spectra of Bi4f, Ta4f, Ta5p, Ni2p, Cu2p for multielement pyrochlore are compared with the parameters of transition element oxides. A characteristic shift of the Ta4f spectrum to lower energies by 0.7 eV is observed for pyrochlore, due to which the effective charge of tantalum cations is +(5-$\delta$). It is shown that the NEXAFS Cu2p spectra of oxide ceramics, according to the main characteristics of the spectrum, represent a superposition of the spectra from the Cu(I) and Cu(II) cations. Based on the analysis of the relative intensity of peaks in the XPS spectrum of Cu2p for pyrochlore, the content of cations Cu(I)/Cu(II)=2/3 was estimated. According to the nature of the NEXAFS and XPS Ni2p spectra, nickel ions are in the state of Ni(II,III).