High-temperature structure of Co 3 O 4 : Understanding spinel inversion using in situ and ex situ measurements
Résumé
In this paper we investigate the high-temperature structure of Co3O4, a compound that has been studied extensively over the last 60 years due to its unresolved high-temperature structure. In situ thermal analysis and x-ray diffraction confirm previously reported high-temperature structural changes and show that these changes are unrelated to the high-temperature decomposition to CoO. Raman-active peaks are also extinguished over the same temperature range. By considering the changing lattice parameter, A-O, and B-O bond lengths as well as cation size we are able to calculate the degree of inversion which reaches a maximum of 0.6. To further study the structure in this experimentally inaccessible range we quench samples and perform ex situ measurements including redox titration, x-ray photoelectron spectroscopy, and neutron diffraction. We do not observe any evidence of large oxygen vacancy concentrations or octahedral CoB3+ ions with high spin state. However, we do show an evolution in the magnetic moment from magnetic structure refinement from (2.4μB) to (2.7μB) that coincides exactly with the high-temperature anomaly and suggests partial inversion (0.46) of the spinel structure in fairly good agreement with the inversion calculated from bond lengths.
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