Valence state of transition metal ions in Co1−xFexCr2O4 (x = 0.1, 0.2, 0.5) ceramics from X-ray photoelectron and Mössbauer spectroscopy data

• Ceramics Co1−xFexCr2O4 (x = 0.1, 0.2, 0.5) are synthesized.
• Valence state of 3d metal ions is studied with XPS and Mössbauer spectroscopy.
• Ionic states of Co and Cr are 2+ and 3+; Fe ions are Fe2+ and Fe3+, Fe3+ prevailing.
• Relative Fe3+/Fe2+ content grows with x; it is the same in the bulk and near surface.
• Fe atoms substitute both Co and Cr atoms forming partly inverse spinel structure.

Ceramics with nominal composition Co1−xFexCr2O4 (x = 0.1, 0.2, 0.5) are synthesized via a solid state reaction method. Crystal structure of samples is studied with X-ray diffraction. Actual elemental composition is determined using X-ray microanalysis and X-ray photoelectron spectroscopy (XPS). Samples’ morphology is studied with scanning electron microscopy. Valence state of the Co, Fe and Cr ions is determined from 2p X-ray photoelectron spectra and Mössbauer spectra. XPS are assigned based on calculations with allowance for the multiplet splitting, crystal field, and the charge-transfer effects. Cobalt ions are found to be bivalent in tetrahedral coordination; chromium ions are trivalent in octahedral coordination. Fe ions are mostly in trivalent states, although Fe2+ ions are also present in significant amounts. Fe2+ ions are in tetrahedral coordination while Fe3+ ions occupy both tetrahedral and octahedral sites. Bulk and near-surface elemental compositions of the ceramic grains are noticeably different. Relative Fe3+/Fe2+ contents are the same in the volume and at the surface of the samples; relative number of Fe3+ ions increases upon the increase of x. A model of partly inverse spinel structure is suggested where the atoms of iron substitute both Co and Cr atoms.