MÖSSBAUER SPECTRA OF STOICHIOMETRIC AND NONSTOICHIOMETRIC Fe3O4 MICROCRYSTALS

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MÖSSBAUER SPECTRA OF STOICHIOMETRIC AND NONSTOICHIOMETRIC Fe3O4

MICROCRYSTALS

H. Topsøe, J. Dumesic, M. Boudart

To cite this version:

H. Topsøe, J. Dumesic, M. Boudart. MÖSSBAUER SPECTRA OF STOICHIOMETRIC AND NONSTOICHIOMETRIC Fe3O4 MICROCRYSTALS. Journal de Physique Colloques, 1974, 35 (C6), pp.C6-411-C6-413. �10.1051/jphyscol:1974680�. �jpa-00215837�

JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 12, Tome 35, Dkcembre 1974, page C6-411

MOSSBAUER SPECTRA OF STOICHIOMETRIC AND NONSTOICHIOMETRIC Fe, 0, MICROCRYSTALS

H. TOPSOE ("), J. A. DUMESIC and M. BOUDART Stauffer Laboratories of Chemistry and Chemical Engineering,

Stanford University, Stanford, California 94305, USA

Rbsumb. - On a trouve dans cette etude que des sauts d'electrons ont lieu dans des micro- cristaux superparamagnktiques de Fe304. Ceci est en contradiction avec des etudes preckdentes qui ont probablement BtB faites sur des Bchantillons non stcechiometriques. On montre que meme une exposition B I'air a temperature ambiante suffit pour creer des defauts de stcechiometrie. On peut conclure de la variation des spectres Mossbauer en fonction du temps que la diffusion ⁱⁱ travers une couche d'oxyde du type y-Fez03 est l'etape lente du processus.

Abstract. - In this study electron hopping is found to occur in superparamagnetic Fe304 micro- crystals. This is contrary to previous studies in which the samples were probably nonstoichio- metric. It is shown that even room temperature exposure to air can result in nonstoichiometry.

From the time change of the Mossbauer spectra, it can be implied that the diffusion through a y-Fen03 like oxide layer is the slow step of the process.

Magnetite has the inverse spinel structure in which the cations occupy tetrahedral and octahedral posi- tions in the face-centered cubic close-packed oxygen lattice. The ferric ions are distributed evenly between the tetrahedral and octahedral sites whereas the ferrous ions occupy only the octahedral sites. The Mossbauer spectra of large crystals of magnetite show two six line spectra above the Verwey transition (119 K). In figure 1 the room temperature spectrum is shown. The spectral component A is the contribution from the tetrahedral ferric ions,, and the B spectrum is the contribution from the octahedral ferrous and ferric ions. These ions give rise to only one magnetically split pattern above 119 K where a fast electron exchange occurs.

The previous studies of McNab et al. [I] and Roggwiller and Kiindig [2] of ultrafine Fe,O, par- ticles showed that superparamagnetic behavior is observed by the Mossbauer effect when the particle size is below about 150 A. However, these spectra and the ones by Alief et al. [3] dit not resemble the typical Fe,O, spectrum above the Verwey transition but showed only one broad six line spectrum besides the superparamagnetic component. This was taken as an indication that the structure of small Fe,04 particles resembles that of large crystals of magnetite below the Verwey transition. The possibility of a non-spinel-like structure was also proposed [3]. This paper reports studies [4] to characterize further the structure and superparamagnetic properties of Fe,O, microcrystals in various environments.

, ^{I I}

-10 - ⁵ 0 5 10

VELOCITY (mm i'l

FIG. 1. - Mossbauer spectrum of Fe304 at 298 K.

(*) Present address : HALDOR TOPSBE A/S, Research Laboratories, Vedbaek, Denmark.

1. Experimental. - In order to form stoichiometric microcrystals of Fe,04 the preparation was carried out at 700 K in a C0,-CO gas mixture, and the Moss- bauer measurements were done in situ in this gas mixture. Problems arising due to sintering of the magnetite crystals were minimized by using MgO as a support. Particle size information was obtained from X-ray linebroadening, electron microscopy, magnetic susceptibility, and chemisorption methods (further details on microcrystals of iron and its oxides will appear elsewhere). The Mossbauer spectrometer was operated in the constant flyback mode with a Co5'/Cu source. The spectrometer and the experimental condi- tions have been described previously 14-61. The magnetites used for studying the spectral change with exposure to air were kindly provided to us by Dr. Hans Bohlbro. They were unsupported and contained a few mol % chromium [4].

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1974680

C6-412 H. TOPSDE, J. A. DUMESIC AND M. BOUDART

2. Results and discussion. - The Mossbauer spectra taken at different temperatures of 180 A and 250 A

particles supported on MgO are shown in figures 2 and 3.

I I I I 1 I 1

-1 0 -5 0 5 10

VELOCITY (mm s-I)

FIG. 2. - Mossbauer spectra at different temperatures of 180 A Fe304 particles supported On MgO : a) spectrum at 598 K, b) specttum a t 298 K, and c) spectrum at 195 K.

FIG. 3. - Mossbauer spectra at different temperatures of 250 A Fe304 particles supported on MgO : a) spectrum at 703 K, b) spectrum at 483 K, and c) spectrum at 298 K.

ture. At the same time a decrease in the area of the hyperfine split phase occurs, clearly demonstrating the superparamagnetic behavior. From the temperature at which 50 % of the Fe304 phase is superparamagnetic the anisotropy constant could be calculated (using the procedure given in ref. [2]) to 5 x lo4, 8 x lo4 and 17 x lo4 erg/cm3 for the 250 A, the 180 A, and an 85 A sample, respectively. These values are in close agreement with those reported by McNab et al. [I] and Roggwiller and Kiindig [2].

The present Mossbauer spectra of the Fe30, microcrystals do however contrast to those of the previous investigators since two six line patterns clearly are present in all the spectra studied. Further- more, the intensity ratios of the two six line patterns are close to those in bulk magnetites. The main diffe- rence between the spectra of large crystals and of microcrystals of Fe30, is the fact that as the particle volume is decreasing a broadening of the B spectrum occurs indicating a corresponding slow-down in the relaxation process of the electron hopping. One is therefore led to conclude that the 'inverse spinel structure with electron hopping occurring among the octahedral ions is maintained in the microcrystals studied.

The discrepancy between the present and the pre- vious Mossbauer results can be understood when spectra of magnetites exposed to air at room tempera- ture are considered. Figures 4a and 4b show the spectra of two different magnetites, both about 400A, after exposure to air at room temperature. In both spectra the ratio S of the octahedral to tetrahedral spectral areas has decreased well below 2, the value for stoi- chiometric magnetite. Furthermore, treating the magnetite sample with a low S value in @ure 4b with a CO,/CO mixture regenerates the stoichiometric S ? 2 magnetite (Fig. 4c).

I ^{I I I I 1}

-10 - 5 0 5 10

The central region of the spectra contains a doublet VELOCITY [rnrn s-l)

arising from about half the total iron content which is _{FIG. 4. - a)} and b) are room temperature Mossbauer spectra

present as in ''lid with Mgo. Besides of nonsupported magnetites exposed to air for 80 days, c) sample

this doublet a paramagnetic phase also appears with b) treated at 700 K with a C o 2 / C 0 mixture. Spectrum at room

an intensity that increases with increasing tempera- temperature.

MOSSBAUER SPECTRA OF STOICHIOMETRIC AND NONSTOICHIOMETRIC FesOd MICROCRYSTALS C6-413 The spectra with very low S values and those of

small Fe304 particles oxidized to y-Fe20, resemble quite closely the previous investigators' spectra of small particle magnetites, which indicates that these may not have been stoichiometric. This is also consis- tent with the results for superparamagnetic y-Fe203 where we find much smaller values of the anisotropy constant than that found by Coey and Khalafalla [7].

The values found are rather close to that for Fe30,.

It can be shown [4] that the Mossbauer spectra of a two phase mixture of y-Fez03 and Fe304 (in which the spectrum fol the y-Fe20, phase will coincide with the A spectrum of Fe304) will for a given FeZ+/Fe3+ ratio have the same area ratio S as a single phase solid solution of y-Fe203 and Fe304 in which a localized octahedral Fe2+-Fe3 + pair electron-hopping [8] occurs.

Therefore, it was hoped that a kinetic analysis of the results on the change in stoichiometry with time exposed to air would give information about the origin of nonstoichiometry. Spectra taken up to 3 years after the exposure to air were used for the analysis.

The oxidation of magnetite involves adsorption and ionization of oxygen at the outer surface, the diffusion of cations (and possibly anions), and oxidation of the Fez+ ions. If the adsorption and ionization of the oxygen is rate determining, then a homogeneous nonstoichiometric solid solution would be expected to form ; however, an analysis of the data excludes this mechanism. The best fit of the data was found when the diffusion through an oxidized shell was considered the rate determining step.

According to this mechanism the vacancy concen- tration for small extents of oxidation should be pro- portional to the square root of the time of oxidation since the rate of oxidation will be inversely propor- tional to the thickness of the oxidized layer. From the Mossbauer spectra of the samples exposed to air at different amount of time the vacancy concentrations (equal to (2-S)1(5 S

+

the square root of time is shown in figure 5. The nice linear relationship indicates that the studied non- stoichiometric magnetites consists of a two phase y-Fe203-Fe304 system with the y-Fe203 phase pro- bably having a large range of composition. This was also supported by the X-ray results on samples with low S values where the lattice parameters were always

Refet [I] MCNAB, T. K., Fox, R. A. and BOYLE, A. J. F., J . Appl.

Phvs. 39 (1968) 5703.

[2] ROGG&ILLER,P. and KUNDIG, W., Solid State Commun. 12

(1973) 901.

[3] ALIYEV, L. A., POVITSKI, V. A. and STUKAN, R. A., Pro- ceedings of the Conference on Mossbauer Spectroscopy, Dresden (1971) 452.

141 TOPSBE, H., Ph. D. Dissertation, Stanford University, 1972.

FIG. 5. - The vacancy concentration (=(2 ^- S)/(5 S

+

sample to air at room temperature.

much larger than expected for a homogeneous solid solution. That such changes in stoichiometry can happen at room temperature was novel to us, as stoichiometric magnetite may be found in nature, and must indicate a much higher defect concentration in these synthetic magnetites, also indicated by the calculated diffusion coefficient for the vacancy trans- port which was about thirty orders of magnitude bigger than that for large particle stoichiometric magnetite samples. The mode of preparation and to some extent also the particle size were shown to play an important role in determining the ease of formation of a nonstoichiometric phase ; generally, it is expected that the lower the formation temperature, the greater will be the tendency to forming nonstoichiometric phases. The above mechanism of nonstoichiometry may also be the cause of nonstoichiometry in many of the previously studied magnetites [8-111. Daniels and Rosencwaig [8] concluded that the electron-hopping in nonstoichiometric magnetite is a pair-localized phe- nomenon. This conclusion was based on the magnetites being single phase which from the present study seems doubtful.

In conclusion, the present study shows the need for further in situ Mossbauer studies, especially when small particle stoichiometric compounds are to be studied.

Acknowledgments. - The financial support of NSF Grant G K 17451 X is acknowledged.

[6] TOPSBE, H. and BOUDART, M., J. Catal. 31 (1973) 346.

[7] COEY, J. M. D. and KHALAFALLA, D., Phys. Stat. Sol. (a) 11

(1972) 229.

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Solids 30 (1969) 1561.

[9] EVANS, B. J. and HAFNER, S. S., J. Appl. Phys. 40 (1969) 1411.

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. - ^{, ,}

(1969j2si.

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