MÖSSBAUER EFFECT STUDY OF LATTICE VIBRATION OF FINE PARTICLES OF IRON

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MÖSSBAUER EFFECT STUDY OF LATTICE VIBRATION OF FINE PARTICLES OF IRON

M. Hayashi, I. Tamura, Y. Fukano, S. Kanemaki, Y. Fujio

To cite this version:

M. Hayashi, I. Tamura, Y. Fukano, S. Kanemaki, Y. Fujio. MÖSSBAUER EFFECT STUDY OF

LATTICE VIBRATION OF FINE PARTICLES OF IRON. Journal de Physique Colloques, 1979, 40

(C2), pp.C2-661-C2-662. �10.1051/jphyscol:19792229�. �jpa-00218610�

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JOURNAL DE PHYSIQUE Colloque C2, suppl6ment au n O 3, Tome 40, mars 1979, page C2-661

MOSSBAUER EFFECT STUDY OF L A T T I C E V I B R A T I O N OF F I N E P A R T I C L E S OF IRON

M. ~ a ~ a s h i ~ , I. ~amura*, Y. Fukanoe, S. ~anemaki** and Y. ~ujio***

ff ' ~ o ~ a m a Medical and Phamnaceutical University, Physics Department, Toyama, Japan Nagoya University, College o f General Education, Nagoya, Japan

szx Shirnadzu Seisakusho Ltd., Kyoto, Japan

Rdsum6.- On a ddtermind la probabilitd d'absorption sans recul de photons y par des petites particules de fer en fonction de la tempgrature. Les particules sont enrobdes dans des matrices diver- ses. Pour des particules de taille supdrieure ou Qgale 1 100

A,

la tempdrature de Debye n'est pas vraiment diffdrente de celle du matdriau massif, dans la limite de quelques %.

Abstract.- The probability of recoilless absorption of y-quanta by small particles of iron metal embedded in various matrices was determined as function of temperature. The analysis of the result revealed that, for the particles with diameter

t

100

11,

the Debye temperature does not differ from the bulk value by more than several percent.

Introduction.- Small crystallites, 100 ;i or less in size, are expected to exhibit properties different from bulk crystals. Recently, a number of authors have reported of softening of the lattice or decrease in the Debye temperature

OD

of fine particles of metals as compared to the bulk. Quantitatively, however, there are considerable discrepancies among

those results. The X-ray diffraction experiment on Ag particles by Kashiwase et al. /I/ and the measurement of the electrical conductivity of films of Ag and A1 soot by Fujita et al. /2/ indicate a decrease in OD by several tens percent for the particles with average diameter of about 100

1.

On the other hand. Matsuo et al. conclude, from the super- conductivity experiment, that the decrease in OD is by several percent for the particles of Al, In and Pb of the same size / 3 / . Viegers and Trooster suggest a value of

OD

practically equal to the bulk value for Au particles of sizes 30

-

¹⁷⁰

1,

^on

the basis of the Gssbauer effect measurement / 4 / . Harada et al. suggest, analyzing their X-ray experimental result on Au particles, that considerable part of the Debye-Waller factors are due to some kind of static distortion in the particles / 5 / .

The present paper reports on the ~Gssbauer effect study of Fe fine particles. In the ~Gssbauer effect, the probability of y-absorption is not affected by the static distortion of lattice.

Various ways of fixing particles were adopted to examine the effect of the motion of the particles as a wole, the importance of which was emphasized by Viegers and Troo;ter /!I/.

Experiment.- Iron particles of two sizes, 100 and 150 A 0 in average diameter, were prepared by the gas

evaporation technique. Particle size was determined from the electron micrographs, which revealed that the samples consisted of entangled chains of the particles.

The probability of recoilless absorption fa was determined as function of temperature in the temperature range 8 0

-

^{300 K}from the spectra obtained by the usual transmission technique. The MGssbauer spectra always consisted of the typical six lines- pattern, superparamagnetism not occurring because of the strong dipole interaction among the particles.

At the lowest temperatures investigated, slight traces of oxides, mostly magnetite, were recognized in the spectra. It was inferred that the surface of the particles was oxidized. The oxide layer was es- timated to be 1

-

2 layers thick from the absorption area for iron.

The areal density of iron was less than 10mg/cm2 for all samples of fine particles, thus the saturation effect was neglected. The natural iron foil, which was examined as the bulk for comparison, had an areal density of about 20 mg/cm2 and the observed recoilless fraction was corrected for the saturation effect.

Result.- The observed temperature dependences of fa relative to the value at 80 K are shown in figure

I for three samples with average particle diameter of 100 and for the bulk. In one of the samples the particles were compressed under a pressure of about 1.5 kbar without matrix, while in the other two samples the particles were embedded in paraffin and cyanoacrylic resin, respectively. It is found that the slope of the curves coincide with each other and with the curve calculated on the Debye

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792229

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c2-662 JOURNAL DE PHYSIQUE model using OD = 470 K, the value found for bulk

iron, within the experimental error which is about 30 K in OD. Similar result was obtained with the

150 A specimens.

TEMPERATURE (K)

Fig. I : Observed temperature dependence of the relative recoilless fraction for iron particles with average diameter z 100

A.

In three different samples, the particles are compressed under a pressure of

=

1.5 kbar without matrix (0) and embedded in paraffin (@) and cyanoacrylic resin (A), respectively. Data gor the bulk sample is also given

(A).

The dashed curves represent the theoretical curve calculated on the Debye model using OD = 470 K, the value of bulk iron.

Discussions.- In experimentally determining the recoilless fraction of fine particles, it is important to take into account the effect of the motion of the particles as a whole 141. Since there is no coupling between the motion of the particles and the lattice vibration within the particles, the mean-square vi- bratima1 amplitude of the y-absorbing nucleus can be written as

<52> =

<x2>

+ a2>

part latt'

where <x2> is the mean-squared vibrational am- part

plitude of the particle as a whole. Thus the observed recoiless fraction equals

part latt

fa

⁼

fa fa .

part Numerical calculation shows that fa ⁵1 in the temperature range 0

-

300 K for the particles embedded in matrices, if the force constant of the particle vibration is assumed to be of the same order of magnitude as that of the intermolecu- lar vibration in typical molecular substances.

Then, the observed fa can be regarded as solely due to the lattice vibration in the particles. It is worthwhile to note that faPart has a great influence on fa in the case of the Massbauer effect for

"'Au, for which the y-energy is much higher /4/.

In the compressed specimens, each particle is

supposed to contact with several neighbouring particles and the force constant of vibration will be as large as in the case of the embedded specimens.

It was found that, if the sample was more locse- ly packed without matrix, the absorption intensity decreased and the slope of the curve fa vs. T became steeper. For very loosely packed samples, the absorp-

tion was observed only at the lowest temperatures investigated, even for particles as large as 800

K

in diameter. These facts indicate that the vibrational amplitude of the particles is very large in such specimens and this may be due to the transverse vibration of the chains for which the force constant will be very small.

In order to minimize the vibrational amplitude of the particles, the specimens have to be embedded in matrix or compressed. Under such conditions, however, the particles will contact with the matrix molecules or the neighbouring particles, and this may affect the nature of the free surface of the particles, the existence of which is considered to be the origin of the lattice softening. This is a problem in the Gssbauer effect study of fine particles. In addition, the surface of the present particles is oxidized, which provides another problem, although its influence is supposed to be unimportant.

For the investigation of the temperature dependence of f of iron, an experiment in a temperature range higher than the room temperature would be more effective because f varies with temperature more rapidly there, but unfortunately, suitable ma-

trix substances usable at higher temperatures were not available.

Conclusion.- From all these results we conclude that, for iron particles with diameter 2100 b;, the Debye temperature does not differ from the bulk value by more than several percent.

References

/I/ Kashiwase, Y., Nishida, I., Kainuma, Y. and Kimoto, K., J. de Physique Colloq.

2

S. ('1977) C2-157.

/ 2 / Fujita, T., Ohshima, K. and Kuroishi, T., J.

Phys. Soc. Japan

40

(1976) 90.

131 Matsuo, S., Sugiura, H. and Noguchi, S., J.Low Temp. Phys.

15

(1974) 481.

/4/ Viegers, M.P.A. and Trooster, J.M., Phys. Rev.

Bl5 (1977) 72.

-

/5/ Harada, J.. Yao, S. and Ichimiya, A . , Acta Cryst. Part S3 (1975) 204.