ELECTROCHEMICAL BEHAVIOR OF CoxMn3-xO4 (1 < x < 2) SOLID SOLUTIONS
NEAR THE POINT OF JAHN-TELER STRUCTURAL TRANSITION
V. B. Fetisov, A. N. Ermakov, G. M. Belysheva, V. M. Kamyshov,
E. A. Pastukhov*
*
Ural State Economic University, Ekaterinburg, Russia
Institute of Metallurgy, Ural Department of RAS, Ekaterinburg, Russia
As is known [1], kinetic parameters of oxidation-reduction processes change abruptly in
ferrites-spinels during a magnetic phase transformation.
An analogous effect may be expected in systems with Jahn-Teller structural transition
caused by orbital-degenerate ions. However, this statement could not be proved by a direct
experimental method for a simple Mn–O system due to a number of reasons. Specifically, the
Mn3-O4  Mn3–O4 transition takes place in this system at temperatures when the
stoichiometry of samples changes insignificantly over a heating-cooling cycle.
Moreover, the Mn3O4 + O2 = Mn2O3 reaction virtually cannot be realized in wide
temperature region [2] because of a large energy barrier preventing the penetration of oxygen
through the gas-solid interface 1.
In this work we endeavored to confirm the aforementioned effect for CoxMn3-xO4
system, where the Jahn-Teller transition occurs with concentration change at x = 1.8 [3].
The voltammetric method employing a carbon-paste electrode (CPE) with an organic binder
was used.
The samples were prepared by a ceramic technology2. The samples were synthesized at
950 C for 180 hours and then were quenched in water. Some characteristics of CoxMn3-xO4
spinels are given in the Table.
1
V. B. Vykhodets, Institute of Metal Physics, Ural Department of Russian Academy of Sciences, Ekaterinburg.
Private communication.
2
The samples were synthesized by Dr. Golikov Yu.V., Institute of Metallurgy, Ural Department of Russian
Academy of Sciences, Ekaterinburg
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Table: Characteristics of CoxMn3-xO4 spinels
Chemical
composition, x
1.00
1.10
1.20
1.30
1.40
1.50
1.60
1.65
1.70
1.75
1.80
1.90
Phase composition, %3
Н
S
100
100
100
100
100
100
80
60
-
20
40
100
100
100
100
а, Ǻ
5.778 (4)
5.783 (1)
5.786 (1)
5.796 (1)
5.805 (1)
5.817 (1)
5.835 (1)
5.855 (2)
Unit cell parameters***
Н
S
с, Ǻ
а, Ǻ
8.954 (1)
8.900 (1)
8.842 (1)
8.789 (1)
8.721 (1)
8.634 (1)
8.555 (2)
8.734 (1)
8.442 (6)
8.353 (2)
8.329 (1)
8.324 (1)
8.315 (1)
8.295 (1)
The method of electrochemical examination has been described in detail in Ref. [4] of this
collection of papers. Figure 1 presents a cathodic polarization curve for Co1.5Mn1.5O4, which
was recorded in a 1M aqueous solution of HCl at different potential scan rates.
Fig. 1. Cathodic polarization curves for Co1.5Mn1.5O4, which were recorded in
1-M HCl at different scan rates (υs). The numerals at the curves denote υs (mV/s)
These voltammetric curves are typical of all compositions in the system CoxMn3-xO4. A
comparison of Fig. 1 with analogous literature data obtained for other oxide systems
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immediately reveals an unusual shape of our polarization curves (cf., for example, Figs. 1 to 3
in [4]). Let us point out basic distinctions:
– The anodic branch of the peak does not follow the Butler-Volmer relation, which
shows an exponential dependence of the current on the applied potential. Instead of this, the
process rate increases directly with E in the initial section.
– The slope of the initial section of the potentiodynamic curve is independent of the
potential scan rate at the given acidity of the electrolyte.
– No overvoltage relative to the stationary potential arises during cathodic and anodic
polarizations of the electrode.
Probably, electrochemical reduction of CoxMn3-xO4 solid solutions is realized in two
stages: one stage corresponds to the rectilinear section in the i–E dependence, while the
second stage shows up as a classical polarization curve with a peaked current response. This
supposition is confirmed by the behavior of the potentiostatic curves (chronoamperometric
studies), which were recorded in the corresponding intervals on the potential scale (Fig. 2).
Fig. 2. Chronoamperometric curves for Co1.7Mn1.3O4, which were recorded in 1-M HCl
at E = 0.6 V (1) and E = 0.2 V (2)
It is worth noting that cycling near the stationary potential (Est  E) gives an
anhysteretic cyclogram passing through the "oxidation-reduction" transition point (E = Est, i =
0) without any singularities. One may think therefore that an equilibrium potential, which
depends on the degree of the material nonstoichiometry, is established at the "electrolyte-
3
The X-ray phase and structural analyses were made by Dr. R.G. Zakharov and Dr. S. A. Petrova, Institute of
Metallurgy, Ural Department of Russian Academy of Sciences, Ekaterinburg
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nonstoichiometric oxide" interface in the system at hand when the circuit is open [5]. In this
case, initial stages of cathodic and anodic polarizations of the electrode represent
electrochemical reduction and oxidation respectively within the homogeneity interval of the
nonstoichiometric oxide.
The maximum reduction current Im and the initial slope of the i-E curve served as the
indicator characteristic of the "composition-properties" dependence. The dependence of Im on
the composition of CoxMn3-xO4 solid solutions is shown in Fig. 3. This curve exhibits a
complicated -shaped behavior with a sharp extremum at x = 1.680.02 (the interface between
the one-phase spinel and two-phase (H + S) regions).
Fig. 3. Variation of Im with the CoxMn3-xO4 composition (supporting electrolyte: 1-M HCl,
υs = 5 mV/s)
In accordance with the phase diagram of the Co-Mn-O system, thermally inequilibrium
states, which are produced by quenching in water from 950 C [2], have three regions with
different phase states in the interval 1  x  2 (see Table). Two boundary regions (1 < x < 1.65
and 1.7 < x < 2) represent one-phase solid solutions having the structure of hausmannite
(spinel lattice has tetragonal distortions caused by the cooperative Jahn-Teller effect) and
spinel respectively. A two-phase region (H + S) is located between these boundary regions.
The corresponding fragment of the state is shown at the bottom in Fig. 3.
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A comparison of the Im = f(x) relation with the aforementioned phase diagram leads to
the following conclusions:
– The phase composition and the electrochemical properties clearly correlate in the
CoxMn3-xO4 system.
– Kinetic parameters of electrochemical oxidation-reduction processes change abruptly
near the point of Jahn-Teller structural transition.
Details of the electrochemical processes taking place in the system were beyond the
scope of this study. The essence of the problem does not depend on the mechanism of the
electrochemical transformation.
Acknowledgement
The authors are grateful to Prof. Kh.Z. Brainina and Prof. A.Ya.Fishman for useful
discussions.
The work was supported by the Russian Foundation for Basic Research, grant 00-03-32362.
REFERENCES
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1996, v. 350, No. 6, pp. 789-791.
2. Balakirev V.F., Barkhatov V.P., Golikov Yu.V., Maizel S.G. Manganites:
Equilibrium and Unstable States, Ekaterinburg, Izd-vo UrO RAN, 2000, 398 p.
3. Wickham D. G., Groft W., – J. Phys. Chem. Sol., 1958, v. 7, №2, p. 351–360
4. Fetisov V. B., Ermakov A. N., Belysheva G.M., Kamyshov V.M., Voltammetric
study of magnetite using a carbon – paste electrode on, – ibit
5. Fetter K. Electrochemical Kinetics, Moscow, Khimiya, 1967, 856 p.
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