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 Bi2Co0.5Cr0.5Nb2O9+∆ (sp. gr. Fd-3m, a = 10.4838(8)
\n˚A) was synthesized in several stages using a solid-phase reaction from oxide precursors at a final temper-
\nature of 1050 ◦С. Using NEXAFS spectroscopy data, the electronic state of cobalt and chromium ions
\nduring the synthesis process was studied. It has been established that before the formation of phase-
\npure pyrochlore, Cr(III) ions are converted to Cr(VI), and then again to Cr(III); Cobalt ions Co(III)
\nare reduced to Co(II). NEXAFS Cr2p spectra of ceramics synthesized at 650 ◦С, according to the main
\ncharacteristics of the spectrum, coincide with the spectrum of K2Cr2O7 and indicate the chromium
\ncontent in the oxide ceramics in the form of tetrahedral CrO2
\n4 ions, and according to the nature of
\nthe Co2p spectrum, cobalt ions are in the Co(II) state and Co(III). In the composition of pyrochlore
\nBi2Co0.5Cr0.5Nb2O9+∆, synthesized at 1050 ◦С, cobalt and chromium appear predominantly in the form
\nof Co(II) and Cr(III) ions. Analysis of phase transformations showed that changes in the oxidation state
\nof transition element ions and the color of ceramics are associated with the formation of intermediate
\nsynthesis products