Fe ATOM POSITION IN F.C.C. β-La PHASE

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Fe ATOM POSITION IN F.C.C. β-La PHASE

K. Kawano, H. Ino, S. Nishikawa

To cite this version:

K. Kawano, H. Ino, S. Nishikawa. Fe ATOM POSITION IN F.C.C. β-La PHASE. Journal de Physique

Colloques, 1979, 40 (C2), pp.C2-621-C2-623. �10.1051/jphyscol:19792216�. �jpa-00218596�

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JOURNAL DE PHYSIQUE Colloque C2, suppl6ment au n ^O3, Tome 40, mars 1979, page C2-621

Fe ATOM P O S I T I O N

IN

Fa CaC 8 F L a PHASE

K. Kawano, H. Ino and S. Nishikawa

I n s t i t u t e of I n d u s t r i a l Science, University o f Tokyo, 22-1, Roppongi 7 chome, Minato-ku, Tokyo 106, Japan

RCsu&.- Le spectre MEssbauer d'un alliage La-Fe c.f.c., obtenu par trempe B partir de l'Ctat liqui- de, est un doublet paramagnbtique B 300 K ( E = 0,49 m / s , 6 ^a0,06 mmls) et un sextuplet ferromagnb- tique 2 77 K (Hi = 281 kOe). Une analyse d6taillEe montre que le doublet ne contient pas de compo- santes en pic slmple mais se compose deDdeux ou plusieurs doublets. Le paramstre cristallin augmente 1CgGrement jusque 8 at.% de fer (0,005 A1at.Z Fe) mais diminue pour un alliage B 20% Fe. Les atomes de fer n'occupent pas des sites substitutionnels ou interstitiels mais de nouveaux sites B sym6trie non cubique provenant d'arrangement atomique complexe. Plusieurs configurations des atomes de fer dans le reseau c.f.c. du La sont proposbes.

Abstract.- The Gssbauer spectrum of the liquid-quenched f.c.c. La-Fe alloys showed a paramagnetic doublet (E = 0.49 mmls, 6 = -0.06 m / s ) at 300 K and a ferromagnetic sextet (Hi = 281 kOe) at 77 K.

The detailed analysis showed that the doublet had no singlet component but two or more sets of doublet components.

The lattice parameter of the specimen increased slightly (0.005 ;/at.% Fe) with increasing content of Fe up to 8 at.%, but decreased in the La-20 at.% Fe alloy.

The Fe atoms occupy not single substitutional or single octahedral interstitial sites but new sites with non-cubic symmetry due to complex atomic arrangements of Fe atoms. Some possible configurations of Fe atoms in the f.c.c. La lattice are proposed.

I. Introduction.- In recent ten years, there have been many reports on the existence of metal-metal interstitial solid solutions, mainly in diffusion studies

111.

Some metallic elements such as Fe, Co and Au diffuse abnormally fast in rare-earth metals 121. It is, however, difficult to study the details of the atomic arrangement of these fast diffusants only by diffusion technique.

In a previous paper 131 we discussed the lo- cation of Fe atoms in the liquid-quenched La-Fe alloy with atomic size ratio r(Fe)/R(La) = 0.69, which is larger than those of usual metal-metalloid interstitial alloys (r/R

<

0.59). In the present work, a more detailed analysis on the problem will be given.

2. Experimental procedures.- La-Fe alloys containing 1.0, 2.5, 8.5 and 20.0 at.% Fe were levitation- melted and liquid-quenched by a two-piston type de- vice in vacuo. Thin foil specimens, about 20 mm in diameter and 40 % 60 pm in thickness, were prepared.

The lattice parameters were measured accurately by X-ray diffraction step-scanning technique for (420).

(531) and (600) reflections.

3. Experimental results.- The X-ray diffraction peaks of f.c.c. 8-La phase were observed in the liquid-quench( I La-Fe alloys and no peaks for inter- metallic compounds. Only in the case of La-20 at.%

Fe, broad and diffuse diffraction peaks due to amor-

phous or microcrystalline structure were observed, together with the sharp peaks due to the f.c.c.

B-La phase. The lattice parameters of the liquid- quenched 8-La alloys are shown in figure 1. They increase slightly with increasing content of Fe except in the case of La-20 at.% Fe alloy.

5.mm

⁰ ² ⁴ ⁶ ⁸ ¹ ⁰ ₂₀

F e C o n t e n t ( a t o h )

Fig. 1 : The lattice parameters derived from (420).

(531) and (600) reflections in the liquid-quenched f.c.c. La-Fe alloys.

The fractional increment in lattice parameter Aala

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792216

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c2-622 JOURNAL DE PHYSIQUE

is about 0.001(at.Z Fe. The reason why the lattice parameter of La-20 at.% Fe is smaller than that of La-8 at.% Fe alloy is not clear but presumably rela- ted to the formation of amorphous or microcrystalline phase.

The Gssbauer spectra at 300 K and 77 K of the liquid-quenched La-l at.% Fe alloy are shown in figure 2-(a) and 2-(b), respectively.

Fig. 2 : The ~Essbauer spectra of the liquid-quenched La-l at.% Fe alloy.

A paramagnetic doublet with mean values of quadru- pole splitting AE = 0.49 mm/s and isomer shift 6 = -0.06 mm/s is observed in the spectrum at 300 K (Fig. 2-(a)), which transforms to a ferromagnetic one (Fig. 2-(b)). The internal magnetic field is 281 kOe at 77 K. Detail analyses of the central region of figure 2-(a) are shown in figure 2-(c) and 2-(d) . Figure 2-(c) shows the fitting the observed asymmetrical doublet to three peaks using a least- squares minimization program in order to examine the existence of a minor singlet component. As initial three peaks, we chose two peaks A and C with equal intensities and a peak B with a center in the midway between those of A and C. After the operation of the

program, the spectral area of the peak B plus the peakcwas nearly equal (482) to that of the peak A

(52%) and the peak position of B shifted to positive velocity as shown in figure 2-(c). The peak A is a superposition of two peaks ; the one makes a doublet with the peak C and the other makes a doublet with

the peak B. The above result shows that there is no singlet component in the spectrum. The result of another fitting assuming two doublets is shown in figure 2-(d), where the X2-value is 1.8.

4. Discussion.- Considering the size of the Fe atom, a large contraction of the lattice woald be anticipa- ted if Fe atoms substitute for La atoms. The X-ray data, however, showed that the lattice parameter of liquid-quenched La-Fe alloys increases with increasing content of Fe. This fact indicates that at least a part of Fe atoms occupies some interstitial sites.

Since both substitutionals and octahedral interstitial~ in the f.c.c. lattice have cubic symmetrical surroundings, an electric field gradient at the nucleus cannot be expected in these cases. The- refore, the evidence of the doublets in the central region of the Gssbauer spectrum of the liquid- quenched La-Fe alloys indicates that the Fe atoms occupy new sites with non-cubic surroundings due to complex atomic arrangements of Fe atoms.

Possible sites for Fe atoms in the f.c.c. lattice are shown in figure 3.

Fig. 3 : Schematic models of Fe atom position in f .c.c. lattice.

A simple octahedral intersitital site (I) and a simple substitutional one (S) are excluded because of their cubic symmetrical surroundings, when Fe atoms are not their nearest neighbours. The <loo> dumb- bell or split interstitial configuration (I-V-I) is considered to be compatible with the doublet absorp- tion in the Mijssbauer spectrum. In the case of the fitting shown in figure 2-(d), the <110> triplet (S-I-S) may be considered as a probable configuration. Butthe intensity ratio of two doublets shows some deviation from the expected value (2 : 1) in this model, and moreover, even in this fitting, there cannot be obtained a satisfactory result in

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the broadness of the peaks. To determine the loca- References tion of Fe atoms in this complex solid solution,

/I/ Anthony, T.R.,"Vacancies and Interstitials", ed.

further accurate measurements are necessary. Seeger, A. et al. (North-Holland Pub.) 1970, p. 935

-

^958.

Acknowledgements.- The authors express their thanks ^{/ 2 /} Dariel, M.P., Acta Met.

3

(1975) 473, Dariel, M.P. and Erez, G., Phil. Mag.

2

(1969) 1053.

to Dr. Nanao and Dr. Umeyama for their advice in

/3/ Kawano, K., Ino, H. and Nishikawa, S., Scripta experiments and data analysis. Net.

&

(1978) 333.