Valence state of transition metal ions in Co1−Fe Cr2O4 (x= 0.1, 0.2, 0.5) ceramics from X-ray photoelectron and Mössbauer spectroscopy data

Ceramics with nominal composition Co 1− x Fe x Cr 2 O 4 ( 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 Mossbauer 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 Fe 2+ ions are also present in significant amounts. Fe 2+ ions are in tetrahedral coordination while Fe 3+ ions occupy both tetrahedral and octahedral sites. Bulk and near-surface elemental compositions of the ceramic grains are noticeably different. Relative Fe 3+ /Fe 2+ contents are the same in the volume and at the surface of the samples; relative number of Fe 3+ 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.