STUDY OF THE Fe3+/Fe2+ RATIO IN NATURAL CHROMITES (Fex, Mg1-x) (Cr1-y-z, Fey, Alz)O4

HAL Id: jpa-00216688

https://hal.archives-ouvertes.fr/jpa-00216688

Submitted on 1 Jan 1976

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

STUDY OF THE Fe3+/Fe2+ RATIO IN NATURAL

CHROMITES (Fex, Mg1-x) (Cr1-y-z, Fey, Alz)O4

G. Fatseas, J. Dormann, H. Blanchard

To cite this version:

G. Fatseas, J. Dormann, H. Blanchard. STUDY OF THE Fe3+/Fe2+ RATIO IN NATURAL

JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 12, Tome 37, Dkcembre 1976, page C6-787

STUDY OF

TILE Fe3 +/Fez

+

RATIO

IN NATURAL CHROMITES

Mg1-J

(Crl-y-z,

Fey,

AlZ)O4

G. A. FATSEAS and J. L. DORMANN Laboratoire de Magndtisme, C. N. R. S.

1, place Aristide-Briand, 92190 Meudon-Bellevue, France and

H. BLANCHARD

Laboratoire de Mdtallurgie Extractive, Ecole Centrale des Arts et Manufactures 92290 Chgtenay Malabry, France

R6sum6. - Cinq chromites naturels, provenant de Madagascar, U. R. S. S., Iran et de I'Afrique du Sud ont kt6 Btudies par spectroscopie Mossbauer dans la region paramagnetique entre 77 K

(ou 50 K) et 300 K.

Le resultat le plus important est la variation, en fonction de la tempkrature, durapport Fe3+/Fe2+ dii St l'effet de saut klectronique avec une energie d'activation dependant du minerai. Les resultats obtenus nous ont montre que les 5 mineraux peuvent etre classes en trois familles, suivant I'energie d'activation et la gamme de temperatures dans laquelle se manifeste l'effet de saut klectronique. Ces trois familles correspondent aussi St trois formules chimiques differentes proposkes par les resultats combinks d'analyse chimique et d'intensite Mossbauer.

Abstract. - Five spinel chromites minerals from Madagascar, U. S. S. R., Iran and South Africa were studied by Mossbauer spectroscopy in the paramagnetic region between 77 K (or 50 K)

and 300 K. The more important result is the presence of a temperature dependent population Fe3+/FeZf ration due to hopping effect with different activation energy for the different minerals. The whole results allowed us to classify the five samples in three main classes of minerals. These are distinguished by a hopping effect manifested at different temperatures and with a different activation energy and correspond to three different chemical formulae proposed by combined chemical analysis and Mossbauer intensities.

1. Introduction and experimental. - In terrestrial rock-forming minerals, ferrous and ferric states are frequently coexisting [I-31. Since the nuclear hyperfine patterns of FeS7 corresponding to the different valence states and the different crystallographic symmetries are generally well-resolved, problems of valency state, site occupancy, site symmetry and also electronic properties can be studied by Mossbauer spectroscopy. We report here Mossbauer results on five samples of natural spinel chromites of general formula MM'O, (M : Fe, M' : Fe, Cr, Al, Mg). The idealized spinel structure consists of tetrahedral (A) sites and octa- hedral (B) sites in a face-centered cubic oxygen sublattice [4].

The sample used were from Madagascar (two samples denoted MI, M,), USSR (denoted R), Iran (Ir) and South Africa (SA). They contain different amounts of iron (around 15

%

wt) and are mixed with a magnesian silicate mineral (from 3

%

to 5

%).

2. Results and discussion.

-

The obtained Moss- bauer spectra are shown in figures 1, 2 and 3. They all present two broad peaks of unequal intensity and

different line shape and they have been computer analysed in six or seven paramagnetic doublets with equal linewidth for all peaks and all sites of a given spectrum and equal Mossbauer absorption coefficients for all sites.

The figures, from 4 to 9 give the computed results at different temperatures : Intensity (I) of the iron sites, quadrupole splitting E = 114 e2 qQ, isomer

shift relative to the metallic iron and populations Fe3+ /Fe2+ ratio (equal to the Mossbauer absorption ratio). The figure 10 gives calculated curves drown from experimental results of figure 9 and a proposed formula of activation energy for the observed hopping effect.

Before commenting the different figures, it is necessa- ry to explain with what requirements were identified the different iron sites. This, because in the case of spectra composed from several overlapped parama- gnetic doublets it is easy, very frequently, to obtain more than one solution (more than one good repro- duction of the spectra) by different combinations of a given number (or of different numbers) of doublets. The choice among the different solutions is only

C6-788 G. A. FATSEAS, J. L. DORMANN AND H. BLANCHARD counts

I

x103

0

1

counts r l 0 3 5@ K 77 K 1226 _I I 1 300 K 77 K 503 100 K 77 K

1

SA 200°K 77 K 751 250 K 1752 746 300 K - 1 0 1 "'mb

FIG. 3.

-

Spectra obtained at 77 K and 300 K for Ir, SA and Mz chromites.

- 1 0 1 m m/s

FIG. 1.

-

Overlapped experimental and computed Mossbauer spectra of MI chromites for different temperatures.

possible if a solution, among them, is obtained with a certain number of physical requirements depending

counts

l o

@

on the physical properties of the studied compound

and on known previous results. In this case, the

556 - solution obtained may be unique.

For our spectra, the requirements were the follow-

77 K ing :

536 ;

a) a given Mossbauer site (Fe2+ or Fe3+) in a given crystallographic site (A or B) must have the same (reasonably) isomer shift (6) and quadrupole splitting ( 8 ) in all samples at a given temperature,

664 _{and reasonable 6-temperature-variation for a given}

chromite. This was obtained and can be seen in figures 4 and 5.

551 b) As the observed electron transfer Fez+ ++ Fe3+,

is produced between Fe2+ and ~ e of crystallogra- ~ + phic identical sites 15, 61, it is reasonable to put the requirement that the total Mossbauer absorption (I) of the Mossbauer sites within each crystallographic site must be constant with temperature :

- 1 o 1 mm,k (ZIA = constant and XI, = constant).

FIG. 2.

-

Spectra obtained in different temperatures for R

STUDY OF THE Fe3+/Fe2+ RATIO IN NATURAL CHROMITES (Fez, Mgl-*) (Crl-,-B, Fey, Alz)04 C6-789 +--C---l--- -i--,--I +

1

, : 2 h > T K > 0 2 100 200 300

FIG. 4. -Temperature dependence of isomer shift (6) with respect to the metallic iron and of quadrupole splitting E = 14 e2 qQ of the different iron sites for the M I mineral : A, At, A2 and As : tetrahedral sites ; B, B1 and B2 : octahedral

sites. FIG. 6. - Temperature dependence of the Mossbauer intensities of the different tetrahedral (A, At, A2 and As) and octahedral (B, BI and Bz) sites for the M i mineral. Also are indicated the total intensity of the octahedral I = ZZB and of the tetrahedral I = ZZA sites. Broken lines represent non experimental results.

F ~ G . 5.

-

Same caption as figure 4 for the R mineral.

c) The identification in trivalent ~ eand bivalent ~ + Fe2+ tetrahedral and octahedral sites was made from the generally accepted result [7, 81 :

FIG. 7.

-

Same caption as for figure 6 for the R mineral.

This can be seen on the 6 ( T ) curves of the figures 4 and 5. The relation is verified for Fe3' sites but it does not always hold for Fe2+ as it is shown on these curves. All the results of the present work were obtained with the three above requirements.

six sites in R, SA and Ir chromites. MI and M, are sufficiently identical, from 6 and &-values point of view, to be represented both, in figure 4. For the same reason, SA and Ir are represented by the R-curves of the figure 5. The majority of the sites were identified as bivalent Fez+. Only two (in MI and M2) and one (in R, SA and Ir) Fe3+ sites exist in all samples. 2.1. COMMENTS ON THE CURVES.

-

The obtained

C6-790 G . A. FATSEAS, J. L. DORMANN AND H. BLANCmRD

FIG. 8.

-

Same caption as for figure 6 for the SA mineral.

Both groups of atoms exist in octahedral (B) and tetrahedral (A) sites with the exception of the Fe3+ site which is located only in octahedral sites (3' B) in R, SA and Ir minerals (Fig. 5).

Within the group of Fez+ sites, three tetrahedral sites exist, Fe2+(A1), Fe2+(A2) and Fe2+(A3) and two octahedral Fe2' (B,) and Fe2+(B,) in increasing order of quadrupole splitting values (Fig. 4, 5). These sites can be attributed to, crystallographically, non equivalent iron Fez+ positions corresponding to three main &-values ; e 0.25, 0.6 and 0.9 mm/s at 300 K.

These values are frequently observed in minerals [I, 31 but they are lower than the splittings observed in other spinel minerals [I]. The isomer shift is equal, at 300 K, to e 0.20 mm/s for Fe3 '(A), 0.4 for Fe3+(B) and lie between 0.85 and 1.1 mm/s for all Fez+ sites.

2.2 POPULATIONS-RATIO Fe3 +/Fez +

.

-

Using the Mtissbauer absorptions of the iron sites given in figures 6, 7 and 8 the populations-ratio Fe3+/Fe2+ in octahedral and tetrahedral sites was calculated for all samples at different temperatures. The results are shown in figure 9 and they remind the results obtained by Drickamer et al. [9].

As .we reported above, in Ir, SA and R samples, the Fe3 + sites are located only in octahedral (B) positions.

Thus only the ratios ( ~ e ~ + / F e ' + ) , are calculated for these three samples : Ir(,,, SA(,! and

R(,).

In the MI and M2 chromites where all sites exist, both ratios (Fe3 +/Fez +), and (Fe3 +/Fez +), are shown.

The reason for which the ratios (Fe3+/Fe2+), and (Fe3+/Fe2'), were given in figure 9 instead of the total usual ratio Fe3+/Fe2+ is that these ratios are

FIG. 9.

-

Temperature variation of the Fe3+/Fe2+ population- ratio for the different minerals. A and B in parenthesis means, respectively tetrahedral (Fe3+/Fe*+)~ and octahedral

(Fe3+/Fez+)~ ratios.

related to the hopping effect, this effect occurring between atoms of identical crystallographic sites.

These results show that :

- The ratio Fe3+/Fe2 + is different for the different chromites and the different temperatures. At 300 K

for example, it is equal to 2 for Ir(,, and 0.5 for M2(,).

- The temperature-dependence of the electron transfer Fez +

*

Fe3 + is different for the different chromites ; larger temperature-dependence for Ir(,)

and M 1 ( ~ ) .

-

With the exception of MI and M2 (practically of MI only) the hopping effect occurs only in the octahedral sites. This is justified by the fact that the nearest neighbour cations are closer each other in octahedral than in tetrahedral sites [6]. Both, however, hopping between A sites and hopping between B sites are known [lo].

-

The hopping process occurs with a sufficiently low frequency, compared to the Larmor frequency of the nucleus, for Fe2+ and Fe3+ being distinguis- hable ; (unlike the case of Fe304 in which ~ e i + and ~ e ; + atoms give an average site above the Verwey transitions-temperature [l 11).

2.3 ACTIVATION ENERGY ASSOCIATED TO THE HOPPING EFFECT.

-

An even more sensitive measure of the

hopping process is the associated to this process activation energy (E). For a hopping activation energy, we can consider, in analogy with previous results on Fe304 [12], the relation p = exp[- E/kT] where p is the probability that a hopping effect Fez+

*

Fe3+ exists at a given temperature T. This probability is defined here as the ratio

with p = 1 for T = oo K and p = 0 for T = 0 K.

That is : the number of Fe2+ transformed in Fe3' at a temperature T, relative to the whole number of Fez+ which can be transformed in Fe3+ when the temperature is very high

STUDY OF THE Fe3+/Fe2+ RATIO IN NATURAL CHROMITES (Fez, Mgr-Z) (Crt-~-~, Feu, &)O4 C6-791

Inserting the second above equation into the first and taking the logarithm, we have :

This represents a straight line In a = f (117') with a slope equal to Elk and can be drown from the total

FIG. 10.

-

In

+

(l/T)-curves for M1 and R minerals. They were drawn from experimental results of the figure 9 and the proposed formula of activation energy (see text). FeZ+(OK) and FeZ+(TK) are the total intensities of octahedral (B) Fez+ sites at OK and T K respectively, for MI and R minerals, and of

tetrahedral sites, at the same temperature for Mi.

experimental-intensities of tetrahedral and octahedral Fe2+ sites in different temperature T K and from the the extrapolated values at 0 K (Fig. 9). The curves calculated in this way are shown in figure 10 and they allowed us to distinguish the three following groups of results :

- MI(,) and M2(,, have the same rate of hopping effect : high values (relatively) of activation energy ;

E = 0.017 eV (Fig. 10, curve MI, B) corresponding to a low temperature-variation of ( ~ e ~ + / ~ e ' + ) , (Fig. 9, MI,, B). Because of the high activation energy the hopping effect begins approximately at 1 15 K (see Fig. 9, B curve).

- Another group is formed by MI(,,, RcB) and Ir(,,. They have a low activation energy equal to 0.006 eV (MI, A curve, Fig. 10) giving rise to a hopping effect at low temperatures as it is seen by the high variation with temperature of the ( ~ e ~ + / F e ' + ) , ratio on Ir(,, and MI(,, curves (Fig. 9).

-

A third, finally group exist, formed by M,(,, and SA(,) not represented in figure 10. They have Fe3+/Fe2+ ratios varying very little with temperature (Fig. 9), showing the absence (practically) of hopping effect in a11 the investigated temperature-region.

2.4 CHEMICAL FORMULA FROM COMBINED CHEMICAL ANALYSIS AND MOSSBAUER RESULTS.

-

A chemical

formula with detailed cation-distribution can be written from combined typical chemical analysis result and the Mossbauer absorptions of iron sites. The proposed chemical formulae are, at low tempe- ratures, respectively, for the M1, M,, R, Ir and SA minerals, the following (within 0.01 error) :

These formulae give the following results :

a) The Ir and R minerals are very similar, being characterized by large concentration of Mg and low concentration of A1 and Fe atoms. By putting the Mg atoms in the A-sites (which seems reasonable), the vacant positions agree well the Mossbauer absorptions of ~e;' atoms. Some oxygen vacancies exist for charge neutrality reasons.

b) M, and M, are quite similar (and in less extent SA too). They are less rich in Mg and slightly richer in A1 and Fe. Here, the Mossbauer absorptions of ~e:+ and ~ e i + are not sufficient to fill the non occupied (by the Mg) A-sites. Thus, A13+ is located

in these sites. The charge neutrality is realized (no necessity of oxygen vacancies).

SA is not really different from MI and M2 though only Fez+ atoms exist in A-sites. Some oxygen vacancies are, however, necessarily present.

These last results lead to the following conclusions :

-

In Ir and R and also in SA, containing oxygen vacancies, the << hopping effect )> is less strong. It is as

though the vacancies disadvantage the hopping process.

C6-792 G. A. FATSEAS, J. L. DORMANN AND H. BLANCHARD

References [I] MALISHEVA, T. V., YERMACOU, A. N., ALEXANDROV, S. M.,

and KURASH, V. V., Proc. intern. Con$ Mossbauer

effect : Tihani 1969 edit. (Academiai Kiado, Budapest) p. 745.

[2] HERZENBERG, C. L., Mossbauer effect methodology. Edited

b y Irwin, J. Gruverman (Plenum Press) 1970, vol. 5, p. 209.

[3] ANUAR ABRAS and MANSUR, R. A., Proc. Intern. Con$ Mossbauer spectroscopy Poland Cracow, 1975, vol. 1 p. 2, B-16.

141 S m , J., and WIJN, H. P. J., Bibliotk2que Technique Philips @mod Cd.) 1961, p. 146.

151 VERWEY, E. J. W. and DE BOER J. H. Rec. Trav. Chim., Pays-Bas, Belg. 55 (1936) 531.

[6] BANERJEE SUBIR, K., and O'REULY, W., J. Phys. Chem. Sol. 28 (1967) 1323.

[7] FATSEAS, G. A., and KRISHNAN, R., J. Appl. Phys. 39 (1968) 1256.

[8] EVANS, B. J., MiiSSbauer eflect methodology, vol. 4, p. 139 (1968). Editor, see ref. 121.

[9] DRICKAMER, H. G., LEWIS Jr, G. K., and FUNG, S. C. Science 163 (1969) 885.

[lo] VERWEY, E. J. W., HAALJMAN, P. W., and ROMEIJN, F. C., J. Chem. Phys. 15 (1947) 174.

[ll] ITO, A., ONO, K., and ISHIKAWA, Y., J. Pkys. SOC. Japan 18 (1963) 1465.