Conversion of Cr(III) and Co(III) during the synthesis of Co/Cr codoped bismuth niobate pyrochlore according to NEXAFS data

Cubic pyrochlore of the composition Bi$_{2}$Co$_{0.5}$Cr$_{0.5}$Nb$_{2}$O$_{9+\Delta}$ (sp. gr. Fd-3m, a = 10.4838(8) Å) was synthesized in several stages using a solid-phase reaction from oxide precursors at a final temperature of 1050 $^\circ$С. Using NEXAFS spectroscopy data, the electronic state of cobalt and chromium ions during the synthesis process was studied. It has been established that before the formation of phase-pure pyrochlore, Cr(III) ions are converted to Cr(VI), and then again to Cr(III); Cobalt ions Co(III) are reduced to Co(II). NEXAFS Cr2p spectra of ceramics synthesized at 650 $^\circ$С, according to the main characteristics of the spectrum, coincide with the spectrum of K$_2$Cr$_2$O$_7$ and indicate the chromium content in the oxide ceramics in the form of tetrahedral CrO$_4^{2-}$ ions, and according to the nature of the Co2p spectrum, cobalt ions are in the Co(II) state and Co(III). In the composition of pyrochlore Bi$_{2}$Co$_{0.5}$Cr$_{0.5}$Nb$_{2}$O$_{9+\Delta}$, synthesized at 1050 $^\circ$С, cobalt and chromium appear predominantly in the form of Co(II) and Cr(III) ions. Analysis of phase transformations showed that changes in the oxidation state of transition element ions and the color of ceramics are associated with the formation of intermediate synthesis products.