HIGH TEMPERATURE PHONONS IN IRON

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HIGH TEMPERATURE PHONONS IN IRON

A. Vallêra

To cite this version:

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JOURNAL DE PHYSIQUE

CoZZoque C6, supple'ment au n o 12, Tome 42, de'cembre 1981 page C6-398

H I G H TEMPERATURE PHONONS I N IRON A.M. Vallcra

Fac. de CiCncias, Lisboa, and LNETI, Sacavem, Portugal.

Abstract.

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The phonon frequencies of Fe + 5 at % Si were extensively measured at room temperature, 933K, 1147K, and 1423K using neutron inelastic scattering. The most interesting features are the frequency shifts of obvious magnetic origin, with large gradients at the Curie temperature. Their interpretation in terms of magnetic contributions is discussed, and preliminary magnetic force constants are given.

We have been studying the variation with temperature of the phonon frequencies of bcc iron by neutron inelastic scattering. The data presented here refers only to Fe + 5 at % Si single crystals, although Fe samples have been also used for compa- rison at temperatures below the ct -t y transition point.

Most of the data were obtained with the twin-chopper spectrometer at reactor PLUTO, Harwell. A few modes were later studied in detail with the three-axis spe

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ctrometer IN3, at the ILL, Grenoble (1). All the chopper data were obtained at four temperatures only (296K, 933K, 1147K, and 1423K) in the 1i0 plane. The phonon frequencies at each temperature are well described by 5-neighbour Born-von-1:arman mo

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dels (2), and are consistent with the ultrasonic elastic constants (3). As an exam- ple, in Fig. 1 we show the phonon frequencies along the symmetry directions measured at 933K in Fe(Si).

In Fig. 2 we present the frequency shifts relative to room temperature. One should remark the large relative shifts occuring in low lying modes. That these

0

I P

r

I I

r

I P

Fig. 1. Phonon frequencies of Fe(Si) at 933IC : experiment ( 0 - chopper meas.;

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3-axis meas.) and fitted 5-neighbour model.

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Fig. 2 Relative frequency shifts of Fe (Si) phonons

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9333

---

1147K

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1423K

Fig.3. Variation with temperature of the frequencies of (XXO)T1 modes

&fts are of magnetic origin is clearly suggested by Fig. 3. But these magnetic effects are not restricted to such modes; all of them show measurable kinks at the Curie temperature, although smaller.

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C6-400 JOURNAL DE PHYSIQUE

Pig. 4. The separated magnetic contribution to the phonon frequencies of Fe(Si): experiment and fitted 3-neighbour model.

+,---

higher frequency branch;

+

,--

lo- wer frequency branch. The negative point at 3/5(1 1 0) was left out of the model fit. Square dots refer to 3-axis data, also not included in the fit.

butions to the shear elastic constants Ci = 2.2, C44,m = 0.5, and to the bulk mo- dulus Bm = 2.7 (units of 1''0 ~m-')

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estimated from (3). The fitted force constants are given in the table below. Although a few of the important features of the ex

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perimental data have been confirmed (1) by more careful investigation of a few modes (the squares in Fig. 4), these force constants should be regarded as prelimi

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nary only, firstly because of the large errors and total absence of data in some branches, and secondly because we now feel that the value of B used in the fit is

m

rather uncertain, and should be left out until low q (XO0)L and (MO)L modes are

studied in detail.

Table I

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Magnetic contributi'on to the force constants (in Nm-')

1st neighbour (111) 2d neighbour (200) 3d neighbour (220)

References :

1. A.M. vall&a, C.G. Windsor, being submitted to J. Phys. F. 2. A.M. vall&a, Ph.D. thesis, Cambridge University, 1977