B. Gillot

Particle size effects on the oxidation-reduction behavior of Mn3O4 hausmannite

The oxidation of hausmannite, Mn3O4, of different particle sizes has been studied by thermogravimetry with a large range of linear heating programs under an oxygen pressure of 10 5 Pa. For high heating rate (600C/h) and whatever the particle size, Mn3O4 was converted to a-Mn2O3. For small particle size (32 nm) and low heating rate (3C/h) the oxidation shows the formation of two metastable manganese oxides, a cation deficient spinel of the formula Mn 2O2:92 with Mn 2C ions oxidation and Mn5O8 associated to Mn 3C ions oxidation. The difference in the kinetic behavior of Mn 2C and Mn 3C ions is interpreted with regard to a structural change from the cation deficient spinel to Mn5O8 initiated by the oxidation temperature.

Infrared studies on the behavior in oxygen of cobalt-substituted magnetites: Comparison with zinc-substituted magnetites

Infrared spectroscopic measurements were carried out on the cobalt-substituted magnetites (Fe3+)A(Co2+xFe2+1−xFe3+)BO2−4, pretreated in oxygen, to investigate as a function of temperature the defect phases γ and their transformation to hematite. It has been found that the defect spinels for which x < 0.30 show a partial vacancy ordering on octahedral sites. Referring to the disappearance of the 720-cm−1 absorption band of the defect phases γ or the appearance of the 470-cm−1 absorption band of αFe2O3, we show that the transition temperature γ → α increases with cobalt substitution. By comparison with zinc-substituted magnetites, the divalent cation distribution is shown to be important to vacancy ordering and to setting the temperature of hematite precipitation.

Etude de la cinetique d'oxydation de magnetites finement divisees. I - Influence de la substitution par l'aluminium

Abstract The kinetics of finely divided, (Fe 2+ Fe 3+ 2−x Al 3+ x )O 4 type (0 ⩽ x ⩽ 2), substitution magnetites oxidation into metastable phase γ (Fe 3+ 1−y Al 3+ y ) 2 O 3 (x = 3y) has been accounted for by means of the law of diffusion, under variable working conditions, of the cationic vacancies generated at solid-gas interface with a diffusion coefficient that decreases when the stoichiometric difference is high. The crystallites size and the trivalent substituant percentage increases, in both cases, slow down the reaction rate.

Availability of Fe2+ ions in Cr- or Al-substituted magnetites with relevance to the process of oxidation in defect phase γ

Abstract During oxidation of the aluminum- or chromium-substituted magnetites, (Fe 2+ Fe 3+ 2− x M 3 f x )O 2− 4 , with 0.4 x 2+ ions in the tetrahedral sites (A sites) of the spinel structure is much less than that in octahedral sites (B sites) and in both cases depends on the extent of aluminum or chromium substitution. The influence of cation distribution in A and B sites on the oxidation temperature is shown directly by differential thermogravimetric analysis and by electrical conductivity.

Infrared investigation of aluminum- and chromium-substituted magnetites and of the lacunar spinels resulting from their oxidation

When Fe3+ ions are substituted by aluminum or chromium on magnetite octahedral sites, the ir spectrum shows the conversion of an inverse spinel to a normal spinel. Both broad bands of magnetite are gradually replaced by the four characteristic bands of normal spinels II–III. They are also observed for solid solutions, FeCr2O4FeAl2O4, with, however, a further band at 780 cm−1 which may be assigned to Al3+ ions on tetrahedral sites. Low-temperature (<400°C) oxidation of these compounds whose sizes are less than 2000 A results in lacunar spinels III–III. The ir spectrum of these solids is characterized by two absorption bands (as for inverse spinels II–III) except for compounds close to pure γFe2O3 in which an order of vacancies could be put in evidence.

Ionic configuration and cation distribution in cubic nickel manganite spinels NixMn3−xO4 (0.57<x<1) in relation with thermal histories

Cation distribution has been determined in cubic manganite spinels NixMn3−xO4 (0.57<x<1) for samples slowly cooled (5 K/min) by combination of powder neutron diffraction and thermogravimetric oxidation results. In the composition range 0.80<x<1 a not negligible (⋍0.10) fraction of Mn3+ ions is found at A sites of the spinel lattice and an estimation of the Mn3+- O distance is carried out. Therefore, a fine analysis of overall data indicates that the Mn3+ concentration at A site cancels for x<0.65. The changes in electrical conductivity and in infrared spectrum are also accounted for by a cationic migration dependent on the thermal history of the specimens.

Cation valencies and distribution in the spinels CoxCuyMnzFeuO4+δ (δ≥0) thin films studied by X-ray photoelectron spectroscopy

Abstract X-ray photoelectron spectroscopy (XPS) has been used to elucidate the valencies and cation distribution of the copper and manganese in the spinels Co x Cu y Mn z Fe u O 4+ δ ( δ ≥0) as thin films prepared by r.f. sputtering on glass substrates. The results obtained show that identification of the two Cu (Cu + and Cu 2+ ) and three Mn (Mn 2+ , Mn 3+ , Mn 4+ ) species in the stoichiometric ( δ =0) thin film is possible using XPS in the Cu2 p 3/2 region and in the Mn2 p 3/2 , Mn3 p and Mn3 s regions. By a fitting process yielding the amounts of the Cu and Mn ions in the different oxidation states it is possible to determine the cation distribution among the two sublattices of the spinel structure. Significant changes were noted when the thin film was oxidized in cation deficient spinel. These changes included the total oxidation of Cu + and Mn 2+ ions with a variation in the relative amounts of Mn 3+ and Mn 4+ ions in the oxide film. A good agreement was obtained with the cation distribution established by thermogravimetry of the oxidation of thick films.

Cation distribution and mechanism of electrical conduction in nickel-copper manganite spinels

Abstract With the view of understanding the electrical conduction mechanism in the nickel-copper manganite spinels, the cation distribution and the ionic configurations of these compounds have been established by associating powder neutron diffraction, thermogravimetric measurements and XPS spectroscopy analysis. Neutron diffraction measurements allow us to reach the scattering lengths in both tetrahedral and octahedral sites. From this information we deduce lower and upper limits for the different cation fractions in both sites. The XPS spectroscopy analysis shows a predominately divalent copper, localized in tetrahedral sites. The monovalent copper is also present, but at a much smaller content. The additional information supplied by Mn 2+ tetrahedral thermogravimetric measurements resolves the cation distribution in these compounds. Finally, the discussion of the electrical properties using these structural formulas has shown that the presence of copper probably induces a mean-range hopping, the average distance of jumps being estimated to ≈ 9 A.

Oxidation mechanism of manganese-substituted magnetite

During oxidation in air of finely grained manganese-substituted magnetites structure, the availability for oxidation of Mn2+ ions in tetrahedral sites (A sites) is much less than that of Fe2+ and Mn3+ ions in octahedral sites (B sites). The oxidation kinetic of Mn2+ ions is well interpreted by diffusion under variable working conditions with an activation energy of about 140 kJ mol−1. The defect phases γ obtained at 400°C undergo stoichiometry changes due to manganese ions in the temperature range 450–550°C as a function of oxygen pressure. Above 600°C, Mn2+ ions that are not oxidized at lower temperatures are transformed into Mn3+ with a phase change from a spinel to a corundum structure. The kinetic curves for this transformation are sigmoidal with an activation energy depending on the amount of Mn3+ ions.

Oxydation menagee du chromite de fer stoechiometrique a basse température, réduction du composé obtenu

Abstract The influence of oxygen on stoichiometric, finely crystallized iron chromite gives a compound with formula FeCr2O4,5. The overall rate of the transformation is controlled by the diffusion of iron cations in the spinel lattice and the simultaneous formation of iron vacancies. Oxidation is followed by means of thermogravimetric, radiocrystallographic, E.P.R. and electrical conductivity analysis. The system can be brought back to the initial state by reduction of the oxidized product with hydrogen. This reverse reaction occurs with the same experimental energy of activation and the same rate pressure law. The data obtained are explained by a vacancy Fe3+ ion association mechanism.