MÖSSBAUER STUDIES OF THE Fe-Ni ORDERED PHASE (SUPERSTRUCTURE L10) IN METEORITES

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MÖSSBAUER STUDIES OF THE Fe-Ni ORDERED PHASE (SUPERSTRUCTURE L10) IN METEORITES

J. Danon, R. Scorzelli, I. Azevedo

To cite this version:

J. Danon, R. Scorzelli, I. Azevedo. MÖSSBAUER STUDIES OF THE Fe-Ni ORDERED PHASE

(SUPERSTRUCTURE L10) IN METEORITES. Journal de Physique Colloques, 1980, 41 (C1), pp.C1-

363-C1-364. �10.1051/jphyscol:19801136�. �jpa-00219620�

JOURNAL DE PHYSIQUE Colloque C1, suppl&ment au no 1, Tome 41, janvier 1980, page C1-363

I@SSBAUER STUDIES OF M E Fe-Ni ORDERED PHASE (SUPERSTRUCTURE LIO) I N METEORITES

J. Danon, R.B. Scorzelli and I.S. Azevedo

Centro Brasileiro de Pesquisas Fisicas, Av. WencesZau Braz, 71, Rio de Janeiro, Brasil.

The Fe-Ni ordered phase with supers- tructure Llo was first obtained by neutron irradiation of the disordered alloy (1).

Due to its low ordering temperature (Tc=320C) (2) it has not been possible to obtain the ordered phase by simply cooling since at this temperature the diffusion ra tes of the metallic atoms are very low.

Meteorites are physical systems in which the Fe-Ni alloys have cooled with rates around 1QC per lo6 years. In these conditions phase segregation occurs pract&

cally at equilibrium. Electron microprobe investigations have shown that at the pha se boundaries steep Ni concentration gra- dients occur (3). In 1-2 um thick layer of the interface the Ni concentration may reach 40-SO%, which goes into the domain of stability of the Fe-Ni Llo superstruc- ture (2).

J.M. Knudsen and coll. have shown that the observation of the ordered phase in meteoritic alloys is possible by using Massbauer spectroscopy since the spectrum of the Llo superstructure is unique among the Fe-Ni alloys by exhibiting a marked asymmetry due to a quadrupole interaction superimposed upon the magnetic hyperfine interaction (4).

The superstructure was found to be pre sent in the lamellae of Cape York and Tolg ca meteorites (5) (6) and reported as a ma- jor constituent of the Ni rich ataxite Sag ta Catharina (7)

.

The superstructure Fe-Ni is present in the metallic phase of other meteorites.

Mllssbauer studies of the metallic grains of chondrites have shown the presence of large amounts of the ordered phase (up to 50%) in the LL-chondrites Saint ~6verin and Parambc (8). The ordered phase appears also to be common in L chondrites,although in less proportions. Its presence in H

chondrites, which have a relatively low Ni/Fe ratio was observed only in one case

(Allegan), but not excluded from X-ray studies of the metallic particles of these chondrites.

We were able to extract the Fe-Ni orde red alloy as a single phase constituent from the metallic grains of Saint Sgverin

(8).

Fig. 1 reproduces the MBssbauer Spec- tra of a) Toluca thin lamella, b) Santa Catharina thin slice and c) the ordered phase extracted from Saint Sgverin. All spectra were recorded at room temperature with a 5 7 ~ o source in Rh matrix.

The origin of the ordered phase in me teorites can be attributed to their slow cooling rate. The breakdown of the ordered phase by heating can give important infor- mation on the cooling history of the mete2 rites. We have also evidence that disorde- ring of the superstructure can arise as a consequence oi shock (a), which is a fre- quent event in the meteorites history.

The coexistence of the ordered phase with a paramagnetic y-phase with less than

33% Ni, as is illustrated in fig. l(a) and (b), is of interest for the study of the invar and related phenomena in Fe-Ni alloys.

In this respect it is important to observe that the M8ssbauer spectra at fig. 1 exhibit only minor changes on going from room temperature to liquid helium (9)

.

persistance of the paramagnetic central l&

ne at 4.2K is surprising since the Fe-Ni alloys with less than 33% Ni become ferro- magnetic at these temperatures, due to the martensitic transformation. The observed stability could arise from the formation of an ordered Fe3Ni superstructure, as has been recently suggested (10).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19801136

c1-364 JOURNAL DE PHYSIQUE

References

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3. Buchwald,V.F., Handbook of Iron Mete2 rites, University of California Press

(1975).

4. Petersen,J.F., Aydin,M. and Knudsen, J.M., Phys. Lett. 192-194 (1977).

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(1978).

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277,

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8. Danon,J., Scorzelli,R.B., Souza Azeve- do,I. and Christophe-~ichel-~6vy, M.

Nature submitted to publication.

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174-175 (1979).

10. Chamberod,A., Laugier,J. and Penisson, J.M., J. Mag. Magnetic Materials (in the press).