A STUDY OF THIXOTROPIC β-FeOOH BY MÖSSBAUER EFFECT

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A STUDY OF THIXOTROPIC β-FeOOH BY

MÖSSBAUER EFFECT

D. Hanžel, F. Sev Ek

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C6, suppl6ment au no 12, Tome 37, B h m b r e 1976, page C6-277

A STUDY

OF THIXOTROPIC P-FeOOH BY MOSSBAUER EFFECT

D.

HANZEL

and F.

SEVSEK

J. Stefan Institute, University of Ljubljana, Yugoslavia

Rhum6.

-

Le comportement thyxotropique des particules collofdales de 8-FeOOH a ete 6tudiB par l'effet Mossbauer. Ces particules en forme de batonnet montrent un comportement superparamagnetique en phase sol et gel. Une densite energetique anisotropique a et6 constatee. Les constantes hyperfmes de B-FeOOH en phase gel et sol gel& ont 6tB comparkes avec celles des Bchantillons secs de 11-Fe00H. La variation de l'aire relative en fonction de la temperature au-dessus de 240 K a Bt6 expliquee par la temptrature de liquefaction des particules disperskes.

La temperature de Debye des Bchantillons secs de BFeOOH a et6 d6terminBe.

Abstract.

-

Using colloidal 8-FeOOH particles, thixotropic behaviour has been studied by the Mossbauer effect. These rod like particles show superparamagnetic behaviour in the sol and gel phase and the anisotropy energy density was estimated.

The hyperfine parameters of BFeOOH in the frozen sol and the gel phase were compared with those obtained for the dried sample of 8-FeOOH. The difference in the temperature dependence of the relative area above 240 K was related to the melting temperature of the dispersed medium. For the dried BFeOOH samples the Debye temperature was determined.

l . Introduction. - Some colIoidaI systems under certain conditions exhibit the property which is called thixotropy [l, 2, 31. This means that the colloidal sys- tem undergoes an isothermal gel-sol-gel transforma- tion upon mechanical agitation and subsequent resting. In the gel phase the particles form a loose network structure which immobilises the dispersed medium in such a manner that a quasi-rigid structure appears.

Therefore some authors [4] consider the process of gelation as a kind of crystallization in which a lattice of crystal threads is formed which entrains the liquid. In order to obtain additional information about the microscopic properties of thixotropic systems, the present paper discusses a Mossbauer investigation of P-FeOOH in the frozen sol and gel phase. The contri- bution of the dispersed medium to the hyperfine parameters of 57Fe was estimated from measurements of 57Fe spectra using dried samples of P-FeOOH. The colloidal particles in the thixotropic system used for this study exhibit superparamagnetic behaviour [5] ;

that is a relaxation of the macroscopic spin of the whole particle with the period :

z = zo exp KV/kT (1)

where z0 is usually 10-9-10-12 S, K the anisotropy

constant, V the particle volume, and

kT

the thermal energy. For kT E KV, small particles behave as

magnetically disordered. Due to the size distribution of the particles, superparamagnetic behaviour is seen over a large temperature range. Iron hydroxide P-FeOOH in bulk, as well as in the form of small particles has been studied by Mossbauer spectroscopy in conside- rable detail by various authors [6,7, 81. However the spectra are not in agreement with one another, proba-

bly due to different modes of preparation which might cause nonstoichiometric and defect structure. We hoped that the hyperfine spectra parameters of particles in colloidal suspension, where surface effects cannot be neglected, would give some additional information about the physical properties of the luidiq where the measured particles were to be found.

2. Experimental. - The thixotropic liquid used for this study was prepared from commercial BDH dialised hydrosol by addition of an appropriate amount of NaOH. The amount of iron oxide Fe,03 was found by chemical analysis to be 5.8 wt

%.

With a composi- tion of 53.4 g Fe203/l and 1.5 g NaOH/l the sol changes into a gel after a period of half an hour. Thixotropic behaviour, which refers to the variation of viscosity with time after mechanical agitation, can be repeated as often as desired with the described samples. The gel obtained has the same colour as the original hydrosol. The small particles of P-FeOOH were identified by X-ray analysis, Mossbauer spectra and electron microscopy. Electron micrography showed discrete rod-like particles with an axial ratio of 8 : I. An average volume in the range of 1.3 - 1.6 X 10-l* cm3

was obtained (Fig. 1).

Mossbauer spectra were measured at various temperatures from 77-300 K with samples in the sol and gel phase cooled quickly and then mounted in the cryostat. Dried samples of P-FeOOH were obtained by evapo- ration of the liquid from the sample.

The Doppler velocity scale was calibrated with metallic iron, which was also used as the reference point for the centre shift parameter.

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C6-278 D. HANZEL AND F. S E V ~ E K

FIG. 1. - Electron micrograph of the thixotropic sol phase.

3. Results and discussion. - The Mossbauer spectra

of P-FeOOH in frozen sol and gel phase are similar. Some characteristic spectra of P-FeOOH in the sol phase are given in figure 2. They consist of a super- position of a quadrupole doublet and a magnetically split 6 line pattern with broadened resonance lines. The intensity ratio of both is temperature dependent, which is typical for superparamagnetic behaviour of small particles.

FIG. 2.

-

Mossbauer spectra of 57Fe in frozen 8-FeOOH at

various temperatures taken with a source of 57C0 in Pd.

Using the procedure of Kiindig and Bommel [5] we find nearly the same value for the anisotropy energy ; KV = 3.7 X 10-l4 erg in both phases on the

assumption of z0 = 5 X 10-9 s and the blocking

temperature of 160 K. As the average volume of the particles is about 1.5 X 10-l8 cm3, we obtained an

anisotropy constant K = 2.5 X 104 erg/cm3, in good

agreement with the values 1 X 104 to 5 X 104 reported

in the literature for iron oxydes [5, 9, 101.

The spectra for the colloidal P-FeOOH particles can also be least-squares fitted using the relaxational model given by Van der Woude and Dekker [ll]. Assuming that the atomic spin flip period Q-' is equivalent to relaxation time of the magnetization vector of the particle, we obtained that

for temperature range 80 K < T

<

200 K. Inserting

these values into equation (l), we obtain again for the anisotropy energy KV = 4.2 X lO-I4 erg and a time

factor zo = 5 X 10-9 S.

The temperature at which the magnetic hyperfine pattern begins to appear is not dependent on the reset time which characterises the changing of the sol into the gel phase.

The hyperfine parameters of 57Fe in the P-FeOOH sol and the gel phase obtained from analysing the spectra at 77 K are as follows :

Sol Gel

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Heff = 477 f 5 kG Heff = 475 f 5 kG C.S=0.54+0.01 mm/s C.S=0.55+0.01 mmls.

The quadrupole splitting parameter is quite tem- perature independent and above 240 K it is : AEQ = 0.68 f 0.02 mm/s for both phases. At a tempe-

rature of 240 K the obtained value for the center shift is

0.40 f 0.01 mm/& This parameter is again the same

for both phases in the range of experimental accuracy. Temperature dependences of the relative area under the resonance lines are given in figure 3. The interval in which the resonance effect decreases is nearly the same in sol and gel phase. The fact that different internal structure of both phases give a similar temperature dependence of resonance effect could be understood if we take account of the surface of the particles. Ions

FIG. 3. -Temperature dependence of relative area of the Mossbauer spectra of dried B-FeOOH sample a) and frozen

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A STUDY OF THIXOTROPIC 8-FeOOH BY MOSSBAUER EFFECT C6-279

oscillate there with a larger mean square amplitude than inside the particle. In accordance with the result of Lisichenko et al. [12], this is possible because the melt-

ing of the liquid starts at lower temperature near the surface of the particle. Spectra of 57Fe in the sol and the gel phase give evidence that the melting might start there already at about 240 K, while the macroscopic melting appeared near 275 K for both phases. Since the ordered structure of the gel phase is changed into the sol phase after melting, similar temperature dependence of the resonance effect is expected. The reason for the lowering of the melting point at the surfaces of particles can be the adsorption and electric double layer effects at the interfaces between particle and liquid, which also play an important role in the physical behaviour of whole system [13].

There is evidence that in the temperature range above 270 K Brownian motion of particles influences the broadening of the spsctra. Further measurements in this temperature rangc are in progress to decide on the appropriate mechanism due to which this broadening of the lines appears.

It should be mentioned that the way of cooling and melting plays an important role in determining the interval and behaviour of the temperature dependence of the resonance effect above 240 K. Commonly the lower the initial temperature at which the sol or gel was cooled, the larger the resonance effect and the smaller the interval in which the resonance effect decreases. Ordered structure of the gel phase was always destroy- ed after thawing and the time necessary for the rear- rangement became much longer.

The spectra of the dried sample obtained by evapo- ration of the liquid from the sol phase in vacuum, resemble quite closely the data of P-FeOOH in the sol or the gel phase, indicating that we are dealing with the same compound.

Dried samples of P-FeOOH decompose to Fe2O3 at about 650 K in accordancc with the results obtained from references [6, 141 though this temperature

depends upon the concentration of C1 ions and the excess crystal water in the sample [15]. The center shift for the dried sample is shown as a function of the temperature in figure 4. The second order shift data

FIG. 4.

-

Temperature dependence of center shift of 57Fe in

dried 8-FeOOH sample relative to metallic iron.

were fitted according to a simple Debye model which gives a Debye temperature 8, = 200 +_ 50 K. Nearly the same result was determined from the temperature dependence of the recoil-free fraction. This indicates that Debye model might be appropriate for description of the lattice of dried 8-FeOOH sample.

The nondestructive study of thixotropy is relevant not only for the classical example of a weak iron hydroxide gel, but also for other physico-chemical questions, liquid crystals, organic and biological systems which exibit similar rheological behaviour. However it is believed that in concentrated sols thixotropy will influence the Mossbauer spectra in a more pronounced way than in the here studied dilute iron hydroxide colloidal system, which gives sufficient resonance effect only in frozen state.

Acknowledgements.

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The authors would like to thank Dr. M. Schara for helpful discussions, Dr. V. KraSevec for kind assistence in the study of the samples by electron microscopy, and Mrs. I. Levstik for the preparation of the samples.

References

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