MAGNETISM AND HYPERFINE INTERACTIONS IN (Np1-xUx)6Fe

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MAGNETISM AND HYPERFINE INTERACTIONS IN (Np1-xUx)6Fe

Z. Hadari, J. Gal, I. Nowik

To cite this version:

Z. Hadari, J. Gal, I. Nowik. MAGNETISM AND HYPERFINE INTERACTIONS IN (Np1-xUx)6Fe.

Journal de Physique Colloques, 1979, 40 (C2), pp.C2-185-C2-187. �10.1051/jphyscol:1979265�. �jpa-

00218663�

MAGNETISM AND HYPERFINE INTERACTIONS IN ( N p ^ ^ g F e

Z. Hadari, J. Gal and I. Nowik*

Nuclear Research Center, Negev, Beerscheva, Israel

*"Racah Institute of Physics, The Hebrew University, Jerusalem, Israel

Abstract.- Magnetic susceptibility and Mossbauer studies of the 59.5 keV transition of 2 3 7Np and the 14.4 keV transition of S7Fe in (Npj-xUx)6Fe were performed. The susceptibility and the Mossbauer ex- perimental results prove that the actinides and the iron do not carry a localized magnetic moment.

The samples do not order magnetically down to 1.9 K. The 57Fe spectrum consists of a quadrupole dou- blet, eqQ = 16(2) Mc/s, and isomer shift - 0.29(6) mm/s, relative to Fe metal. The 2 3 7Np Mossbauer spectra, for all temperatures and x values (T < 77 K, x = 0, 0.5, 0.8, 0.9) are composed of two pure non-axial quadrupole subspectra of relative intensities 2:1. In Np6Fe, eqiQ = 4450(50) Mc/s, n = 0.46(2) and eq2Q = 900(100) Mc/s, r) = 0.9(1). These results agree very well with the known crystal structure (tetragonal 41/mem) which contains Np ions in two low symmetry sites. The isomer shift of the two subspectra (-16(1) mm/s and -21(2) mm/s relative to NpAl2) corresponds to pentavalent neptu- nium. N p5 + are non Kramers' ions and since they are located in very low symmetry crystalline fields, their being nonmagnetic is not surprising. On the other hand, the phenomenon that iron in an inter- metallic compound is nonmagnetic is not that common.

The increasing interest in intermetallic com- pounds of actinides with other transition series ele- ments has led to extensive studies of magnetic and other properties of such systems /l/. Here we pre- sent magnetization and Mossbauer studies of the sys- tem (Npx_xUx)6Fe. The (Npj-xUjjOsF6 compounds have a tetragonal unit cell (14 mem). The actinide ions are located in two low symmetry sites (16(k) of symmetry m and 8(h) of symmetry mm). The iron is located in the high symmetry site 4(a) (symmetry 42) /2,3/.

The 2 3 7Np Mossbauer studies show that Np in

(Npi-xUxHFe is pentavalent. The Np ion is nonmagne- tic down to 1.9 K. The quadrupole interactions in the two sites are nonaxial. All these results are expected for a N p+ 5 3Hi, state in sites of low sym- metry crystalline fields.

Susceptibility measurements of U6Fe show that the U nor the Fe have a magnetic moment. This result is consistent with the Mossbauer observations.

Mossbauer studies of 59.5 keV transition of

2 3 7Np in (Np1_xUx)6Fe at temperatures 1.9 to 77 K were performed. The source used was Am^zhl:Th. All experimental details about preparation of absorbers, cryogenics and electronic equipment used, were simi- lar to those described in a previous publication /4/.

The spectrum of NpeFe, at 1.9 K is shown in fgure 1.

Fig. 1 : Mossbauer spectrum of 2 3 7Np in NpgFe at 1.9 K. The solid lines are the two theoretical sub- spectra and their sum.

JOURNAL DE PHYSIQUE Colloque C2, supplément au n° 3, Tome 40, mars 1979, page C2-185

Résumé.- Des études Mossbauer de la transition à 59,5 keV du23fap et de la transition à 14,4 keV du

5 7Fe ainsi que des mesures de susceptibilité magnétique ont été faites sur le système ^P1--Sfix) gFe.

La susceptibilité magnétique ainsi que les résultats Mossbauer montrent que les actinides et le fer ne possèdent pas de moment magnétique localisé. Les échantillons ne s'ordonnent pas magnétiquement à aussi basse température que 1,9 K. Le spectre du S7Fe comporte un doublet quadrupolaire, eqQ = 16(2) Mc/s et un déplacement isomérique de - 0,29(6) mm/s par rapport au Fe métallique. Les spectres Mossbauer du 2 3 7N p , à toutes les températures et pour toutes les valeurs de x (T < 77 K, x = 0; 0,5;

0,8; 0,9) sont composés de deux sous-spectres purement quadrupolaires non-axiaux et ayant une inten- sité relative de 2:1. Pour le Np6Fe, eqxQ = 4450(50) Mc/s, n = 0,46(2) et eq2Q = 900(100) Mc/s, ri = 0,9(1). Ces résultats sont en excellent accord avec la structure cristalline connue (tétragonale 41/mcm) où les ions de Np se trouvent dans deux sites de faible symétrie. Le déplacement isomérique des deux sous-spectres (-16(1) mm/s et -21(2) mm/s par rapport au NpAl2) correspond au neptunium pentavalent. Les ions N p5 + ne sont pas des ions Kramers et, puisqu'ils sont situés dans des champs cristallins de faible symétrie, leur manque de magnétisme n'est pas surprenant. D'autre part, pour le fer, ce phénomène est beaucoup moins courant dans les composés intermétalliques.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979265

c2-186 JOURNAL DE PHYSIQUE

Table I : Hyperfine Interaction Parameters Derived from ~Essbauer Spectra of 2 3 7 ~ p and 5 7 ~ e in (Npl-xUx)6Fe

:

Temperature :Quadrupole interaction:

-

I I Eta

:

^Isomer

:

^Intensity

Compound K I Mc/s I I I mm/ s Ratio

I

I

:

^{eqlQ eq2Q ni n2 : S I SP}

;

^12/11

i i ^{I I} I

NpsFe

:

^{1.9 :4450(50) 900(10o)}

:

^{0.46(2) o.9(1)}

:

^{-16(1) -21(2)}

:

^0.62(5)

I I I 1 I

1 1 I I I

(NPo.~UO.~)~F~ 4 - 2 ~4900(100) 1200(150)

:

^{0.47(2) 0.9(1)}

;

^{-16(1) -20(2)}

:

^0.68(5)

I I 1 I

1 I 1 I

UsFe ^I 295 ^{I I I 1 1 I}

(Fe spectrum)

:

^I 1 ¹⁶⁽²⁾ I ^{I 0 I} 1

1 I 1 I

-0.29(6)

j

. . I

(a)~elative to NpA12 for 2 3 7 ~ p and relative to Fe metal for 5 7 ~ e .

A least square fit to a theoretical spectrum compo- were only 0.04 % and 50 million counts per channel sed of two subspectra yields the hyperfine interac- had to be accumulated until one could derive hyper- tion parameters given in table I. fine interaction parameters (Table I). It is clear The fact that pure quadrupole spectra are observed why at this stage only U6Fe at 295 K was measured.

proves that Np6Fe is not ordered magnetically even The spectrum consisted of a pure quadrupole doublet.

at 1.9 K. One also observes that the 2 3 7 ~ p in NpsFe The magnetic susceptibility of U6Fe was measu- spectrum is composed of two non-axial quadrupole red in the temperature range 4.2 to 300 K using a subspectra of relative intensities 0.62. These re- PAR vibrating sample magnetometer. A flat curve was sults are consistent with the crystal structure of obtained. The magnetic moment of the sample in a Np6Fe in which the Np ions occupy two low symmetry magnetic field of 10 kOe was less than 5~10-~ernu/~, sites (16(k) and 8(h). The relative abundance of the proving that both the uranium and iron are nonmagne- Np ions in the two sites is 0.5. However, the rela- tic. From general systematics of other families of tive intensity of the observed two subsepctra is intermetallics of actinides /6/ one can conclude 0.62, which proves that the recoil-free fraction for that also in NpsFe the iron does not have a magnetic Np ions in the (h) site is larger by 20 % than for moment. This conclusion is also supported by the those in the (k) site. 2 3 7 ~ p in (Npl-xUx)6Fe Mgssbauer studies which do not

The observed isomer shift of the two subspec- indicate any magnetic order even at 1.9 K. The spec- tra in Np6Fe (table I) indicates that the Np ions tra depend very little on x, indicating that no are in the pentavalent state 151. This result is change occurred to the Fe ion. Any change to the Fe also consistent with the conclusions derived from ion would have led to changes in the conduction band volume considerations 121. Since Np5+ (5f

',

^{3 ~ are ~ )} which would affect the 2 3 7 ~ p isomer shift. The phe- non Kramers' ions and in Np6Fe they are located in nomenon of nonmagnetic iron in an intermetallic very low crystalline fields which remove all ionic compound is not very common, though it has been ob- degeneracies, it is not surprising that they are served in other actinide-iron compounds 161.

nonmagnetic. Our magnetization and Mijssbauer studies prove

The studies of Np6Fe at temperatures up to that the intermetallic system (Np,-,UX),Fe is non- 77 K and the studies of (Npl-,UX),Fe for x = 0.5, magnetic down to 1.9 K. This and the observed hyper- 0.8, 0.9 yield similar spectra to that shown in fi- fine interaction parameters given in table I are gure 1. The hyperfine interaction parameters change consistent with expectations for the behaviour of very little as a function of temperature and x Np+' ions in low crystalline fields.

(Table I) proving that the Np ion configuration for

dilute Np in UsFe is the same as that in Np6Fe. We are very grateful to S. Fredo for technical Since the 14.4 keV gamma ray from 5 7 ~ e is help in performing the material preparation and strongly absorbed by the high Z actinides, it is M;issbauer measurements.

extremelydifficult to obtain 5 7 ~ e assbauer spectra of (Np,-,UX),Fe. We used a high activity source (150 mCi of cos7;%) and a Harwell proportional counter detector capable to stand high counting ra- tes.Since the absorber had to be of less than 30 mg/cm2 (~0.02 mg 57~e/cm2) the observed effects

References

/I/ Darby, J.B. and Freeman, A.J., Editors "The actinides;

electronic structure and related properties",(Academic Press) 1974.

/2/ Giessen, B.C., Roof, R.B., Russel, A.M. and Elliott, R.O., J. Less-Common Metals

53

(1977) 147.

/3/ Dwight, A.E., In "Developments in the Structural Che- mistry of Alloy Phases", Giessen, B.C., (Editor, Plenum Press, N.Y.)1969, p. 181.

/4/ Gal, J., Hadari, Z., Atzmony, U., Bauminger, E.R., Nowik, I. and Ofer, S., Phys. Rev.

88

(1973) 1901.

/5/ The dependence of the isomer shift on valency is dis- cussed by Dunlap, B.D. and Kalvius, G.M. in Reference / I / .

/6/ Gal, J., Kroupp, M., Hadari, 2. and Nowik, I., Phys.

Rev.

16

B (1977) 39 13; Solid State Commun

20

⁽¹⁹⁷⁶⁾

515.