Chemical bonding and valence state of 3d-metal ions in Ni1−xCoxCr2O4 spinels from X-ray diffraction and X-ray photoelectron spectroscopy data

Ni1−xCoxCr2O4 solid solution samples with х = 0, 0.005, 0.01, 0.02, 0.3, 0.5, 0.8 and 1 are studied by the methods of X-ray diffraction and X-ray photoelectron spectroscopy (XPS). Ni2p3/2,1/2, Co2p3/2,1/2, Cr2p3/2,1/2, and O1s spectra are measured. The spectra are calculated with allowance for multiplet splitting, crystal field, and charge-transfer effect. Ni2+ ions are found to be present in tetrahedral and octahedral positions. Relative numbers of Ni2+ ions in tetrahedral and octahedral sites are determined via fitting the experimental spectra with superposition of the spectra of Ni2+ ions calculated in tetrahedral and octahedral crystal field. Cobalt ions are bivalent and occupy mostly tetrahedral sites. Chromium ions are trivalent and occupy predominantly octahedral positions. However, due to a defective lattice, the samples may contain Cr ions in non-equivalent states, either chemically or structurally.