VALENCE CHANGE OF EUROPIUM ATOMS DUE TO COLD-WORKING OF MgEu ALLOYS

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VALENCE CHANGE OF EUROPIUM ATOMS DUE

TO COLD-WORKING OF MgEu ALLOYS

G. Shenoy, B. Dunlap, R.C. Pirich, C. Burr

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C6, suppliment au no 12, Tome 37, Ddcembre 1976, page C6-927

VALENCE CHANGE OF EUROPIUM ATOMS DUE

TO COLD-WORKING OF MgEu ALLOYS

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G. K. SHENOY, B. D. DUNLAP, J. D. PHILLIPS Argonne National Laboratory, Argonne, Illinois 60439 U. S. A.

R. C. PIRICH, C. R. BURR

State University of New York, Binghamton, New York, U. S. A.

Rbum6.

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On a r6alisk une etude d'alliages de Mg contenant 50 % 6 300 ppm d'Eu. Les donnkes de spectroscopie Mossbauer de 15lEu, de susceptibilit6 magnktique et de rnicroscopie Bectronique

B balayage montrent que ce syst&me ne consiste pas en une solution solide mais que Eu prkipite dans deux phases eutectiques. Tandis qu'en l'absence de contrainte les ions Eu sont dans 1'6tat divalent k toute tempkrature, un laminage a froid induit une conversion k 1'6tat trivalent d'une fraction Blevke des ions. Des mesures de recuit isochrone montrent que l'etat divalent est rktabli pour des temp6ratures de recuit supbieures a environ 450 "C. Les rksultats sont discut6s en invo- quant la formation et le pi6geage de defauts en ligne ainsi que leur recuit ult6rieur.

Abstract.

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An investigation has been made of alloys of 50 to 6 300 ppm Eu in Mg. Data obtained by the use of the Mossbauer spectroscopy of IslEu, magnetic susceptibility and scanning electron microscopy show that solid solutions do not form in this system, but that the Eu precipitates into two eutectic intermetallics. While the Eu ions are in a divalent state at all temperatures for strain-free samples, introduction of cold-work causes a large fraction of the ions to convert to the trivalent state. Isochronal annealing measurements show that the divalent state is recovered at annealing temperatures above

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450 C. Those results are discussed in terms of the formation and trapping of line defects and their subsequent annealing.

1. Introduction.

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It is a common truism that rare-earth ions are present in solid materials in the trivalent state. The usual exceptions are Ce4+ and Yb2' (at the ends of the lanthanide series) and Sm2+ and EuZC (near the middle of the series). Recently there has been interest in these ions due to the presence of c( interconfigurational fluctuations D arising from the fact that the binding energies in various valence states are not very different [l-31. A related problem concerns a simpler investigation into the rules of stabi- lity for the various configurations, and the experimental conditions under which they are obtained. In this paper we present a study of dilute alloys of Eu in Mg, utilizing the lSIEu Mossbauer spectroscopy, magnetic susceptikility measurements and scanning electron microscope investigations. Discussion is given concerning the metallurgy of the system, the electronic configuration of the Eu ions and the relation of that configuration to mechanically introduced cold-work in the materials.

2. Experimental.

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Alloys have been prepared ranging in concentration from 50 to 6 300 ppm Eu

in Mg. Constituent materials were melted in a graphite (*) Work supported in part by the U. S . Energy Research and Development Administration.

crucible under a helium atmosphere, and the polycrys- talline samples grown by lowering the crucible through a temperature gradient in a Bridgeman-type drop furnace. Strain-free samples were cut from the resulting material with either an acid string saw or a spark cutter.

Due to the low density and low atomic number of Mg, samples as thick as

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1.5 cm can be used without inordinately attenuating the 22 keV 'Eu gammaray in the Mossbauer investigations. Therefore samples can be prepared with a sufficient thickness of Eu to allow Mossbauer spectra to be obtained in a reasonable length of time.

Mossbauer spectra were obtained at temperatures ranging from 1.6 K to 300 K, using a 151Sm20, source. Magnetic susceptibility measurements were performed in the same temperature range using the Faraday technique. Results have been obtained by both techniques on samples in the strain-free state, on samples which have been cold-worked by rolling, and on those same samples after subsequent annealing.

3. Results and discussion.

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Mossbauer spectra obtained for the 6 300 ppm Eu in Mg sample taken at room temperature are shown in figure 1, as a function of the degree of coldwork. This was introduced by

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C6-928 G . K. SHENOY, B. D. DUNLAP, J. D. PHILLIPS, R. C . PIRICH, C. R. BURR

I I I l l I I r l

-20 -10 0 10 20

VELOCITY, mm/s

FIG. 1. - Mossbauer spectra of lslEu in an alloy of 6 300 ppm

Eu in Mg, as a function of the degree of cold-work.

working on a strain-free sample, in a rolling mill. The percentage of cold-work (i. e. percentage reduction in thickness) is given in figure 1.

We consider first the strain-free sample, marked 0

%

in the figure. At room temperature, essentially all Eu atoms are seen to be in a divalent state, with an isomer shift of

--

11.8 $. 0.1 mm/s vs Sm203. This is verified by the susceptibility, which follows a Curie-Weiss law down to 30 K. Below 10 K, the susceptibility becomes temperature independent. Mossbauer spectra show this to be due to magnetic transitions in the material. Spectra taken at 4.2 K show the presence of two distinct sites in roughly equal population, with appro- ximately the same isomer shift but with hyperfine fields of 100 kOe and 280 kOe. Clear magnetic transitions are seen for these two sites at 7 K and 16 K,

respectively. The presence of rather large, well-defined transition temperatures indicates that the Eu is not randomly distributed in the Mg matrix, but exists as intermetallic compounds. This has been verified by a scanning electron micrograph analysis and by energy dispersive X-ray techniques to show that the sample consists of large areas of a pure Mg matrix interspersed with channels containing two different types of concen- trated Eu-Mg alloys.

In the strain-free spectrum of figure 1, it is seen that there is a small number of trivalent ions present. One

also sees that introduction of lattice defects by cold- worki~g causes a conversion of a large proportion of the E U ~ + ions into the Eu3+ state ; this proceeding monotonically with increasing cold-work until the sample is so severely work-hardened that it does not retain its mechanical integrity. Subsequent annealing of the sample returns the Eu3 + ions to the Eu2+ configuration. Thus it seems clear that this is a phenomenon related to lattice defects rather than, for example, an increased preference for oxidation as the surface area of the sample increases. These results are verified by magnetic susceptibility measurements.

Further information is gained by the following susceptibility measurements : A sample containing 4 500 ppm Eu in Mg was work-hardened to the point where 20

%

of Eu ions were divalent at room temperature. The sample was than isochronally annealed for two hours at various temperatures and measured at room temperature. It was found that the concentration of Eu2+ ions first increased as a function of annealing temperature, reaching a maximum of 35

%

at

--

450 OC then rapidly dropped to its pre-work-hardened state where all ions were divalent.

These results can all be explained in a general way. Upon cold-working the material, lattice defects are introduced. With the extent of damage present here, isolated point defects are not likely. Rather there will be extensive dislocation lines, edge dislocations, shear planes, etc., throughout the material. In the absence of detailed information, we refer to these in the following merely as line defects. If an Eu ion finds itself near a line defect, it will experience an internal pressure due to the volume loss necessary to accommodate the defect. As a result it will prefer to be in the trivalent state, where the atomic volume is smaller. As more line defects are introduced, the probability of finding a trivalent ion becomes larger. Due to the large size difference between the Eu and Mg ions, and the small concentration of Eu even in the intermetallic clusters, the Eu ions act as impurities which provide pinning centers so far as the line defects are concerned [4]. The primary effect of annealing is to increase the line defect mobility. Thus the probability of a line defect moving until it finds a Eu ion is increased and the Eu3+ concentration grows. Above 450 OC, the Eu-dislocation trapping is overcome, all defects anneal out through migration to grain boundaries, free surfaces, etc., and all Eu ions again assume the divalent configuration. It may be noted that isochronal annealing measurements of the resistivity of cold- worked pure magnesium [5] shows a recovery attri- buted to dislocation movement producing recrystalli- zation in the temperature range of 160-180 OC, some- what lower than that observed in the present case with the dislocations pinned by impurities.

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VALENCE CHANGE OF EUROPIUM ATOMS DUE TO COLD-WORKING OF MgEu ALLOYS C6-929

py, magnetic susceptibility and scanning electron microscopy measurements show that Eu does not go into solid solution, but rather precipitates into two different eutectic alloys.

Studies of Mossbauer spectra and magnetic susceptibility show that the introduction of cold-work into the alloys causes a conversion of Eu2+ ions into the Eu3+ state, believed to be due to dislocation pinning by the Eu ions. Isochronal annealing shows depinning

effects occurring at W 450 OC, returning the sample

to its undamaged state. Thus constitutes the first measurement of a valence change due to mechanical deformation and offers the possibility of a different technique for the study of recovery processes in workhardened materials.

We wish to thank S . Das for taking the scanning electron microscope photographs and for useful discussions.

References

[l] MAPLE, M. B. and WOHLLEBEN, D., in Magnetism and Magnetic Materials-1973, AIP Conference Proceedings No. 18, edited by C. D. Graham, Jr., and J. J. Rhyne (AIP, N. Y.) 1974, p. 447.

[Z] H m s ~ , L. L., in Magnetism and Magnetic Materials-1974,

AIP Conference Proceedings No. 24, edited by C. D. Graham, Jr., G. H. Lander, and J. J. Rhyne (AIP, N. Y.) 1975, p. 11.

[3] BAUMINGER, E. R.,-FELNER, I., FROINDLICH, D., LEVRON, D.,

NOWIK, I., OFER, S. and YANOVSKY, R., J. Physique

Colloq. 35 (1974) C6-61.

[4] For a discussion of dislocation defect pinning, see Sosin, A., in Vacancies and Interstitials in Metals, edited by

A. Seeger, D. Schumacher, W. Schilling, and J. Diehl (North Holland, Amsterdam) 1970, p. 729.