ELECTRIC HYPERFINE INTERACTIONS OF IRON IN DIMERIC STATE IN METALS

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ELECTRIC HYPERFINE INTERACTIONS OF IRON IN DIMERIC STATE IN METALS

B. Sawicka, J. Sawicki

To cite this version:

B. Sawicka, J. Sawicki. ELECTRIC HYPERFINE INTERACTIONS OF IRON IN DIMERIC STATE IN METALS. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-576-C2-578.

�10.1051/jphyscol:19792201�. �jpa-00218580�

JOURNAL DE PHYSIQUE Colloque C2, suppl6ment au no 3, Tome 40, mars 1979, page C2-576

E L E C T R I C H Y P E R F I N E I N T E R A C T I O N S OF I R O N I N D I M E R l C STATE I N METALS

B.D. Sawicka and J.A. ~awicki*

I n s t i t u t e of Nuclear Physics, Cracow 31342, Poland

X I n s t i t u t e of Physics, JagielZonian University, Cracm 30059, Poland

RCsum6.- On prdsente les rdsultats d'une Ctude systgmatique d'expbriencesd'implantation de 5 7 ~ e dans les mgtaux des series de transition 3d, 4d et 5d, 1 partir d'observations en spectromgtrie Mzssbauer par Clectron de conversion. On observe une diminution de la densitd blectronique sur le fer des dimsres, avec le nombre d'6lectrons p6riphCriques des atomes de la matrice d'accueil.

On a aussi mesurC l'effet quadrupolaire sur les dimsres. La formation de dimsres apparait c o m e hautement probable dans les rbseaux cfc alors qu'on observe une distribution au hasard dans les rgseaux cc.

Abstract.- The systematics for data on 5 7 ~ e implanted into hosts of the 3d, 4d, and 5d metal se- ries are presented based on conversion electron Gssbauer measurements. The decrease in electron density at the iron nuclei in iron dimers with the number of outer electrons in the host was observed in each series. The quadrupole coupling in dimers was also determined. Iron dimers are formed with an increased probability in fcc lattices, while in bcc lattices the random distribu- tion of impurities seems to be obeyed.

1. Introduction.- The technique of 5 7 ~ e implanta- tion combined with conversion electron ~zssbauer spectroscopy / I / can provide unique information concerning the electronic densities and electric field gradients at iron nuclei in isolated iron atoms and their simple associations in various hosts. In particular, the difference between iron in monomeric and in dimeric states in metals indi- cated by their different isomer shifts is of con- siderable interest /2/.

This work completes our previous studies of 5 7 ~ e implants in aluminium / 3 / and in transition metals 141.

2. w r i m e n t a l technique.- The samples were obtai- ned by implantation of 70 keV 5 7 ~ e + ions into tar- gets kept at room temperature. Doses of 10' at/cm2 and 5x10' at/cm2 were usually applied to give ave- rage iron concentrations of about 3 at. % and 1.5 at. %, respectively. Some samples were studied after annealing at various temperatures. The conversion electron ~Gssbauer spectra were recorded at room temperature. A careful least square fitting proce- dure was applied in order to obtain the parameters of the spectra. A summary of experimental data obtained for cubic hosts is given in table I. The data for non-annealed samples were only included.

3. Results and discussion.- In the case of random distribution of impurities at the concentrations studied iron atoms should occur mainly in the mono- meric (no nn Fe) and dimeric (one nn Fe) state. For

instance, at 3 at. % the probability of monomer occurrence is 0.78 in a body centered cubic lattice and 0.69 in a face centered cubic and a hexagonal closely packed lattices. The dimer probabilities are 0.19 and 0.26.

In the Miissbauer spectra of 5 7 ~ e in most cu- bic hosts such as V, Cr, Cu, Nb, Mo, Ag, Ta, W and Au superpositions of a single line due to iron mono- mers and a quadrupole-split doublet caused by iron dimers usually could be observed. The relative occur rence of monomers determined from a fraction of a single line area agrees with those for a random distribution in many cases (e.g., V, Cr, Nb, Mo, and Ta). In other hosts such as Cu, Ag and Au, the spectra indicate an enhanced formation of iron asso- ciations which are similar to those observed for Fe in an A1 lattice /3/. Then the process of iron asso- ciation increases during annealing of the samples.

In figures1 and 2 the experimentally determi- ned monomer occurrences in cubic metal hosts are compared with the calculated ones. It is notable that usually iron impurities in bcc lattices obey statistical rules, but in fcc lattices they do not.

This can be associated with an additional possibili- ty for iron to occupy relatively large interstitial spaces in fcc lattices and, possibly to form there dumbbell-like configurations. Also, an implantation- enhanced diffusion of iron via interstitial posi- tions in fcc lattices seems to be more effective than in the case of bcc lattices.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792201

Table I : Mijssbauer spectra parameters for ''~e implanted in metals with cubic lattice structure.

I I I 1

I I the single line I the doublet 1

host

;

^E(%)

;

^(Fe

-

monomer) I ^I (Fe

-

dimer)

:

^ss/s

I

I

; IS^

^(mmls)

:

^{W (mmls) ISd (m/s)}

:

^QSd

( ~ 1 ~ ) i

^{W (mm/s)}

:

1 I

,

¹ I

1 I

,

I I t 1

1 1 I I I 1 1

V 1.5

:

^-0.157(5)

:

^0.4

:

^-0.10(5)

:

^0.6

:

^0.4

:

^0.9

I 1 1 I I 1 I

Cr 3 - 0 . 2 1 0.5

:

^{0.19(5) 0.6 0.7 0.67}

I 1 I 1 1 1 I

Ni 3 0.03(1) 0.40

:

^I 1 ¹ I ^I I 1

I I I 1 1 1 1

Cu

:

^{0.3 0.23(1) 0.3}

:

^{0.23(8) 0.8 0.4 0.51}

1 I I 1 t

I I I

:

^{1.5 0.230(5) 0.3 0.7}

:

^{0.4 0.30}

I 1

:

³

:

^{0.225(5) 0.3}

:

^{0.21(2) 0.7 0.5 0.24}

I I I 1 I I 1

Nb 3 -0.03(2) 0.6

:

^{-0.17(3) 0.6}

:

^{0.4 0.75}

I 1 I I 1 I I

Mo 3 0.04(1) 0.4

:

^{-0.04(2) 0.8 0.4}

;

^0.75

1 1 I I I I 1

Rh 3 0.107(5) 0.40

;

^I 1 ¹ I ^I I 1

I I 1 1 I I I

Pd

:

^{3 0.194(5) 0.46}

:

^{7 1 1} 1 ^I I I

I I I 1 I I 1

Ag

:

^{1.5 0.517(5)}

:

^0.3

;

^0.38(2)

i

^0.8

:

^{0.5 0.38}

1 I I 1 1 1

Ta

:

^{3 0.09(8) 0.7}

:

^0.07(9)

:

^{0.9 0.6 0.72}

1 1 1 I 1 1 I

W 3

;

0.12(4) 0.4

;

^0.05(4)

:

^{0.7 0.7}

:

^0.2

3 0.144(5)

j

^0.43

i

^{1 I t I t I I 1 r 1}

1 I 1

~t

:

³

;

0.324(5)

i

^0.46

:

^{7 7 I} I ¹ I 1

1

.

^{I 1 1}

Au 3

:

^{0.632(5) 0.28 0.48(3)}

:

^{0.7 0.5 0.41}

I I I

,

r 1 1

E

= average iron concentration, IS, and ISd = isomer shift values relative to metallic iron,

QSd = quadrupole splitting, W = linewidth, Ss/S = the single line contribution to the total area of the spectrum.

Fig. 1 : Plot of the occurrence of monomeric iron vs.

implanted iron concentration in body centered cubic Fig. 2 : Explanation as in figure 1 but for face lattices as determined from the single line contri- centered cubic lattices.

bution in the spectra. Solid lines represent the mo- o

-

points were obtained for splat-cooled samples nomer occurrence calculated for the cubic lattice (from ref. 191).

(upper line) and the random dense packing of hard spheres (lower line).

In such a case, a more random distribution of impu- rities should be obtained for splat-cooled samples than for the samples produced by ion implantation.

Such an effect was indicated for Fe in A1 (Fig. 2).

For a few hosts, usually close to iron in the table of periodic properties of the elements, even statistically permittable dimers were not distin- guished. The single-line spectra for cubic lattices

(Rh, Pd, Ir and Pt), the quadrupole-split spectra for the hexagonal lattices (Ru, Re and 0s) or the

magnetically-split sextuplets (Fe, Co and Ni) were then observed. In these hosts the hyperfine parame- ters for iron in monomeric and dimeric states are apparently close to one another.

In figure 3 the systematics of the isomer shifts for monomeric and dimeric iron are presented versus the number of outer electrons. The isomer shifts ISm for iron monomers agree' well with the data for diluted iron alloys. The observed charac- teristic dependences may be explained by the volume

C2-578 JOURNAL DE PHYSIQUE effect and the s-d electron charge transfer effect

/5,6/.

2 4 6 8 10 12 14

NUMBER OF OUTER ELECTRONS

Fig. 3 : Systematics of the isomer shifts for mono- meric and dimeric iron vs. the number of outer elec- trons in various host elements. Arrows at IS, points indicate in which direction they have to be shifted to get the ISd values. Hatched points in ISd

-

^no

dimers were distinguished.

Systematic data for the isomer shifts ISd for iron in dimeric states in metals are presented here for the first time. The systematics indicate a lar- ge decrease in electron density at the iron nuclei in dimers formed in different hosts with the in- crease in a number of electrons of the host element atoms. Since the ISd dependence observed is very si- milar to the ISm dependence after correction for the volume effect /5/, it can be stated that the electron density at iron nuclei in dimers is large- ly sensitive to the s-d electron transfer effect.

The isomer shifts ISd are usually closer to the shift value for pure iron than the shifts ISm for monomers which suggests that the electron density at the iron nuclei in dimers is more iron-like than in the case of isolated iron impurities.

The quadrupole splittings QSd observed for iron dimers in cubic metals appear to be very large.

Their values lie between 0.6 m / s and 0.9 mm/s. The QSd splittings are associated with the different effective charges and radii of iron and host atoms, and they depend on the iron concentration as was found for Fe in A1 /3/. The systematic studies of the concentration dependence and the measurement of the sign of the quadrupole coupling in the external magnetic field are necessary in order to clarify the nature of the electric field gradients in dime-

ric iron impurities in metals. Calculations of efg's similar to those carried out for dimeric iron in ncr ble gas matrices /7/ would be of great value.

In conclusion, it can be said that iron atoms are easily accepted in the substitutional si- tes of the host lattice. They are distributed ran- domly as long as the atomic radius and electronega- tivity of the iron atom are near those of the host atoms. In different cases iron dimers are formed with an increased probability, especially in hosts with face centered cubic lattices. The electron den- sity at the iron nuclei in a dimeric state of iron atom depends systematically on the electronic struc- ture of the host element, and is mostly due to the s-d electron transfer.

The results presented in this work will be thoroughly discussed elsewhere /8/.

References

/I/ Sawicka, B.D., Sawicki, J.A., "Proc. Int. Conf.

on Mijssbauer Spectroscopy", Bucharest 1977, Vol.

2, Invited Papers p. 35

-

70, edited by D. Barb, D. Tarina.

/2/ Sawicka, B.D., in "Workshop in Chemical Applica- tions of M6ssbauer Spectroscopy", Seeheim 1978, Abstract, p.107

-

108.

/3/ Sawicka, B.D., Sawicki, J.A., Stanek, J., Drwiega, M., Hyperfine Interactions,

5

⁽¹⁹⁷⁸⁾

147.

/4/ Sawicka, B.D., Sawicki, J.A., Stanek, J., Phys.

Lett.

A59

(1976) 59.

/5/ Ingalls, R., Solid State Commun.

14

(1974) 1 1 . /6/ Watson, R.E., Bennett, L.H., Hyperfine Interac-

tions

4

(1978) 806.

/7/ Trautwein, A., Harris, F.E., Phys. Rev. Bl (1973) 4755.

/8/ Sawicka, B.D., Report INP-Cracow ~ ~ 1 0 3 0 / P L (1978).

/9/ Nasu, S., Gonser, U., Shingu, P.H., Murakami, Y.,

3 . Phys. F

4

(1974) L24.