Study of the Oxidation States of Mn, Cr, and Co in Spinel Type
Materials
A. Hagen and L. Mikkelsen
Materials Research Department, Risø National Laboratory, Frederiksborgvej 399, DK-4000 Roskilde, Denmark
Oxides with the spinel structure are interesting candidates for protective coatings on interconnects
in solid oxide fuel cell stack systems [1]. Spinels are versatile compounds, which can comprise
several transition metals in different oxidation states. The properties depend on the nature of the
constituting atoms, their coordination/positioning in the structure and oxidation state. X-ray
absorption spectroscopy is a unique method that allows these kind of studies for each specific atom
even in multicomponent structures, at elevated temperatures and in the presence of different gas
atmospheres. Two compositions: MnCr2O4 and MnCo2O4 spinels were addressed in the present
contribution.
The spinel powders were prepared by a glycine-nitrate combustion synthesis [2] and the presence
of single phase cubic spinel patterns (>95%) was confirmed by XRD. XANES measurements were
performed at DESY, beam line E4. The obtained raw XANES spectra were energy calibrated with
the respective metal foil spectra (edge energy: first inflection point), background corrected by two
linear polynomials in the pre-edge and after the main edge region, and normalized to an edge jump
of one above the edge.
In the present report, the results with respect to the positions of the energy features (edge and edge
width - difference between energy of main absorption and edge) are qualitatively evaluated. In Fig.
1, the room temperature XANES spectra of the samples and some oxidic references compounds are
shown. The energy positions in the Mn-XANES spectrum of MnCr2O4 (Fig. 1a) were at significantly
lower values than those of the reference compound Mn3O4. This result indicates the presence of Mn ions
in a low oxidation state in the MnCr-spinel sample. Furthermore, the extremely large edge width
(difference between absorption maximum and edge) is a strong indication for a mixed valence among
the Mn ions in the spinel sample. The Cr-edge energy in the Cr-XANES spectrum of the MnCr2O4
sample (Fig. 1b) was in the same region as for Cr2O3, characteristic for an oxidation state of three.
2.0
Mn-XANES
Cr-XANES
2.5
(c)
MnCo2O4
MnCr2O4
Mn3O4
0.5
Normalized intensity in a.u.
2.0
1.0
0.0
6505
(b)
Normalized intensity in a.u.
Normalized intensity in a.u.
(a
1.5
1.5
1.0
0.5
6705
Energy in eV
6805
1.5
1.0
0.5
MnCr2O4
Cr2O3
MnCo2O4
Co3O4
0.0
0.0
6605
Co-XANES
2.0
6.0
6.1
6.2
Energy in keV
6.3
7.7
7.8
7.9
8.0
Energy in keV
Figure 1: XANES spectra of Mn (a), Cr (b), and Co (c) recorded at room temperature
In the Mn-XANES spectrum of MnCo2O4 (Fig. 1a), the edge energy was at a similar value as in the
Mn3O4 reference sample, comprising Mn(II) and Mn(III) ions. Evaluating the Co-XANES spectrum
of the MnCo2O4 sample, the Co-edge position and shape were also very similar to the respective
features of the Co3O4 reference compound with a mixed valence of Co(II) and Co(III), though there
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was a slight shift towards lower energies (Fig. 1c). The distribution of oxidation states in the
MnCo2O4 sample should thus be similar as in Co3O4, probably with a higher portion of Co(II) ions.
Summarizing the results, it was shown that the oxidation states and distribution over the sites in the
spinel structure were different for the two samples, depending on the chemical composition. A
mixed valence with strong prevailance of a lower oxidation state was concluded for Mn in
MnCr2O4. When Cr was substituted for Co in the spinel structure, the valence distribution of Mn
was similar as in Mn3O4, with a ratio of Mn(II) to Mn(III) of 1/2.
In MnCr2O4, the Cr-ions were mainly in an oxidation state of three, although the presence of small
portions of Cr(II) species cannot be excluded. On the other hand, there was a valence distribution
between Co(II) and Co(III) ions with a ration of close to 1/2 in MnCo2O4.
The XANES method proved to be a valuable tool to assign the valence states of Mn, Cr, and Co in
the investigated structures and to study the effect of chemical composition on the distribution of the
ionic species.
The authors are grateful to HASYLAB for providing beam time and thank K. Klementiev for the
valuable technical assistance and the Danish Natural Science Research Council for the financial
support through DANSYNC.
References
[1] S. Taniguchi, M. Kadowaki, H. Kawamura, T. Yasuo, Y. Akiyama, Y. Miyake, and T. Saitoh, J.
Power Sources 55 (1) 73 (1995).
[2] L.A. Chick, L.R. Pederson, G.D. Maupin, J.L. Bates, L.E. Thomas, and G.J. Exarhos, Mater. Lett. 10
(1-2) 6 (1990).
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