High-temperature structure of Co3O4 : Understanding spinel inversion using in situ and ex situ measurements

Abstract

In this paper we investigate the high-temperature structure of ${\mathrm{Co}}_{3}{\mathrm{O}}_{4}$, 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 ${\mathrm{Co}}_{B}{}^{3+}$ ions with high spin state. However, we do show an evolution in the magnetic moment from magnetic structure refinement from ($2.4\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{\text{B}}$) to ($2.7\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{\text{B}}$) 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.