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MÖSSBAUER EFFECT STUDY OF CATION
DISTRIBUTION IN NATURAL CHROMITES
E. da Silva, A. Abras, A. Sette Camara
To cite this version:
ENVIRONMENTAL STUDIES :
MINERALOGY
-
GEOLOGY
-
MOSSBAUER EFFECT
STUDY OF CATION DISTRIBUTION
IN NATURAL CHROMITES
(*)
E. G. DA SILVA (**) and A. ABRAS
Departamento de Fisica, Universidade Federal de Minas Gerais, Be10 Horizonte, MG-Brazil and
A. 0. R. SETTE CAMARA
Instituto de Pesquisas Radioativas, Belo Horizonte, MG-Brazil
RBsum6. - L'oxydation du Fez* et la distribution des cations de fer dans les sites tktrahC- drique (A) et octahkdrique (B) en fonction de la tempkrature ont Ct6 6tudiCs dans les chromites, par effet Mossbauer, entre 25 "C et 1 200 "C. On a observC l'existence d'un processus d'oxydation du Fez+, superposk k une diffusion de Fe3* des sites B vers les sites A jusqu'k la tempkrature de 900 "C. Au-dessus de 900 "C il y a inversion du sens de la diffusion, et tous les Fe3* retournent dans les sites B. Les knergies d'oxydation et d'activation ont etk calculees en supposant que les concentrations de cations de fer obCissent k une loi d7Arrhenius.
Abstract. - The Mossbauer effect has been used to study the oxidation process of Fez+ and the distribution of iron cations over tetrahedral (A) and octahedral (B) sites in natural chromites. covering a temperature range from 25 "C to 1 200 "C. It was observed an oxidation process for Fez+ (A), supperimposed by a diffusion of Fe3* from B to A sites, up to 900 OC. At higher tempe- ratures the diffusion process is inverted and all Fe3+ (A) ions go back to B sites. The oxidation and activation energies were estimated by assuming an Arrhenius temperature dependence for the iron cations concentrations.
1. Introduction.
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Oxidation of minerals is a process of fundamental interest and great industrial importance. Several aspects of the analysis of minerals involving cations distribution, sample identification and Fe3+/Fe2+ ratios are available to investigate by means +of Mossbauer spectroscopy and a review about its application in mineralogy can be found in the literature [I]. The cation distribution in the chromite spinel system XY204, where X = (Fez+, Mg2+) and Y = (Fe3+, A13+, Cr3+) is very important in the technology of refractory materials due to its physical properties at higher temperatures. Natural chromites were already studied by several tech- niques [2] and the cations distribution in the spinel XY204 has been investigated by many authors [3-61. The Mossbauer spectroscopy was used to investigate ten different samples of natural chromites from deposits throughout Brazil and informations about Fe3.+/Fe2+ ratios and the distribution of Fe3+ and(*) Work supported in part by CNPq, CNEN and FINEP,
Brazil and OAS.
Fe2+ over octahedral (B) and tetrahedral (A) sites were obtained [7] (*). In this paper the oxidation process of Fe2+ and the distribution of cations over A and B sites will be discussed, covering a temperature range from 25 OC to 1 200 OC.
2. Experimental procedure.
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The chemical analysis and the X-ray powder diffraction showed that chromite samples can be represented by the general formula [7] (*) :The powder samples were annealed in open furnace, at different temperatures, for two hours, and then air-quenched. Mossbauer spectra were obtained with a conventional constant acceleration Doppler velo- city spectrometer. All data were taken at room tempe- rature using a 57Co in copper source. The saturation effects due to finite thickness were minimized using absorbers with a whole concentration of about 14 mg/cm2. The data were fitted t o Lorentzian lines with a non-linear, least-square fit computer program.
(**) Present address : Universitat des Saarlandes, Fachbe-
reich Angewandte Physik, 6600 Saarbriicken, W. Germany. (*) Part of this work will be published elsewhere.
C6-784 E. G. D A SILVA, A. ABRAS AND A. 0. R. SETTE CAMARA
3. Results and discussion.
-
A representative set of Mossbauer spectra, as a function of the annealing temperature, is shown in figure 1. The solid lines represent the fitted curves to the experimental data.FIG. 1. - Mossbauer spectra for natural chromites as a function of the annealing temperature.
Each curve consisted of a sum of symmetrical doublets. From the best estimate values of the isomer shift (6) and quadrupole splitting (A) it was possible to identify tetrahedral (A) sites occupied by Fez+ (6 = 1.45 and A = 0.34 in mmls) and Fe3+ (6 = 0.03 and A = 1.06 in mmls) and octahedral (B) sites occupied by Fe3+ (6 = 0.08 and A = 0.60 in mm/s). The values of (6) and (A) above are the averages over the different annealing temperatures.
Below 400 OC the samples, including the unheated one, did not show the presence of the doublet charac- teristic of Fe3+ (A).
The electric field gradient at B site is due to a trigonal field
[a],
and the measured quadrupole splitting A = 0.60 mm/s is in good agreement with the theore- tical value A = 0.56 mm/s reported in the literature [9]. The quadrupole splitting displayed by ~ e ' + at A site can be explained by a deviation from the cubic symmetry caused mainly by the presence of different cations at B sites [5, 91. The quadrupole splitting observed for Fe3+ at A site is about three times larger than that observed for Fez' at the same site, which is not expected from the cubic point symmetry ofthe A site. However, this result may be possible due to thezvarious distributions of cations over B sites as observed in iFeNiAl0, and FeNiCrO, [9].
The X-ray diffraction showed for all samples the presence of only chromite spinel, with an average lattice parameter of 8.308
A.
Therefore, the lattice structure of the spinel was maintained throughout the heat treatment.The concentration of iron cations (K) was deter- mined from the areas under the fitted doublets, assum- ing the same recoilfree fraction for both A and B sites [5, 101. The relative areas, as a function of the annealing temperature, are summarized in table I.
TABLE I
Relative areas ( K ) as a function of the annealing temperature
T ( f 10OC) F ~ ~ + ( B ) Fe3+(A) F ~ ~ + ( A ) - - -
-
400 0.56+
0.08 0.23+
0.08 0.21 f 0.03 500 0.59 f 0.12 0.25+
0.08 0.16 f 0.03 600 0.62+
0.08 0.27+
0.05 0.11 f 0.02 700 0.53 f 0.05 0.40 f 0.05 0.07 5 0.02 800 0.38 & 0.13 0.62 f 0.17-
900 0.19 f 0.09 0.81+
0.18-
950 0.30 _f 0.25 0.70+
0.38-
1 000 1.00 f 0.01-
- 1200 1.00 f 0.01-
-
No significant change in the concentration for ~ e " and Fe3+ ions was observed for temperatures below 400 OC. Therefore, the distribution of the iron cations is shown in the figure 2 only for the temperature interval 400 OC-1 000 OC. From this figure (see also figure 3) one can see three distinct temperature regions :1 ) In the region (400 OC 5 T
5
700 OC) the amount of Fe3+ (-4) ions increases while the amount ofRG. 2.
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Relative concentration of iron cations as a function of the annealing temperature, as given by the relative spectralMOSSBAUER EFFECT STUDY OF CATION DISTFUBUTION IN NATURAL CHROMITES C6-785
-3.0 L/7;o ' ' ' ' '
10,o 1 I T l K-I)
FIG. 3.
-
Arrhenius plot of concentration of Fe3+ B) ( 0 ) andFez+ (A) (e). The solid lines represent a least-square fit to the experimental data points.
Fe2 + (.4) ions decreases, such that the total iron cations remains constant, within the experimental errors. This suggests that an oxidation process of Fe2+ (A) starts at about 400 OC and ends at about 800 OC.
2) In the region (700 OC
5
T
;5 900 OC) only a diffusion process of Fe3+ ions from B to A sites ispresent, and a saturation of tetrahedral sites is observed at about 900 OC.
3) In the region (900 OC
5
T
5
1 000 OC) the diffu-sion process is inverted, and all Fe3+ (A) ions go back to B sites. Above 1 000 OC all iron cations in the chromite spinel are trivalent and distributed over octahedral sites only. By assuming for each region an Arrhenius [ l l ] temperature dependence for the iron cations distribution, we estimated from the plot of In (K) versus 1/T as shown in the figure 3, ihe following activation energies :
1) In the region 1, about 4 kcal/mole is involved in the oxidation process of tetrahedral Fez+ ions.
2) In the region 2, the activation energy for the diffusion process of Fe3+ ions from octahedral to tetrahedral sites is about 17 kcal/mole.
3) In the region 3, the activation energy is about 47 kcal/mole when tetrahedral Fe3+ ions diffuse back to octahedral sites.
One must have a quite different diffusion mechanism for Fe3+ ions in each temperature region, as indicated by the activation energies. It should be mentioned that, an explanation for the difference in the activation energy, in terms of possible diffusion mechanism, is not very easy, specially if we take into account that A13+, Cr3+ and Mg2+ ions and oxygens vacancies are present in our natural chromite samples.
References
[I] See Mossbauer Effect Data Index, J . G. Stevens and V. Ste- vens.
[2] ULMER, G. C., <<High Temperature Oxides-Part I : Magne- sia, Lime and Chrome Refractories>>, Edited by A. M. Al- per (Academic Press) 1970.
[3] CALLEN, H. B., HARRISON, S. E., and KRIESSMAN, C. J.,
Phys. Rev. 103 (1956) 851.
[4] KRIESSMAN, C. J. and HARRISON, S. E., Phys. Rev. 103
(1956) 867.
[5] ROBBINS, M., WERTHEIM, G. K., SHERWOOD, R. C. and BUCHANAN, D. N. E. J. Phys. Chem. Solid 32 (1971) 717.
[6] DE GRAVE, E., DE S I ~ , J. and VANDENBERGHE, R.,
Appl. Phys. 7 (1975) 77.
[7] DA SILVA, E. G., M. Sci. thesis (Universidade Federal de Minas Gerais, Brazil, 1974) unpublished.
[8] E I B S C ~ T Z , M., GANIEL, U. and SHTRIKMAN, S., Phys. Rev. 151 (1966) 245.
[9] MIZOGUCHI, T. and TANAKA, M., J. Phys. Soc. Japan 18
(1963) 1301.
[lo] SAWATZKY, G. A., VAN DER WOUDE, F. and MORRISH, A. H., Phys. Rev. 183 (1969) 383.
