TRANSFERRED MAGNETIC FIELDS AT 129I AND COVALENCY EFFECTS IN TRANSITION METAL-DIIODIDES

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TRANSFERRED MAGNETIC FIELDS AT 129I AND COVALENCY EFFECTS IN TRANSITION

METAL-DIIODIDES

J. Sanchez, J. Friedt, G. Shenoy

To cite this version:

J. Sanchez, J. Friedt, G. Shenoy. TRANSFERRED MAGNETIC FIELDS AT 129I AND COVA-

LENCY EFFECTS IN TRANSITION METAL-DIIODIDES. Journal de Physique Colloques, 1974,

35 (C6), pp.C6-259-C6-261. �10.1051/jphyscol:1974635�. �jpa-00215792�

TRANSFERRED FIE1 DS AND CO VAL ENC Y.

TRANSFERRED MAGNETIC FIELDS AT '1

AND COVALENCY EFFECTS IN TRANSITION METAL-DIIODIDES

J. P. SANCHEZ, J. M. FRIEDT and G. K. SHENOY Laboratoire de Chimie Nucltaire, Centre de Recherches Nucltaires

BP 20, 67037 Strasbourg Cedex, France

R6sum6.

Le dkplacement isomkrique, l'interaction quadrupolaire ainsi que le champ magnk- tique transfkre au site de I'iode ont kt6 mesurks par spectroscopie Mossbauer de 1291 dans les com- posks antiferromagnktiques MnI2, FeIz, Co12 et NiI2. La correlation entre la valeur des paramktres hyperfins et la covalence des liaisons est discutke.

Abstract.

Using the 27.8 keV Mossbauer resonance in 1291, the isomer shift, the quadrupole interaction and the transferred magnetic field have been measured in antiferromagnetic MnI2, FeI2, CoI2 and NiI2. The values of the hyperfine parameters are utilized to discuss the covalency in these compounds.

The covalency effects in chemical compounds can be demonstrated through the measurement of hyperfine (hf) interactions. In particular, the transferred hf magnetic fields a t the ligand nucleus are sensitive to the covalency [I]. In this paper we are concerned with the covalency effects in MI, (M = Mn, Fe, Co, Ni) compounds elucidated through the measurement of the isomer shift, the quadrupole interaction and the trans- ferred hf magnetic field using the 27.8 keV Mossbauer resonance in ','I.

The iodides of Mn, Fe and Co crystallize with a simple - CdI, type structure whereas NiI, has a CdC1, structure [2]. In both of these structures each metal ion is surrounded by a distorted octahedron of iodine ions. The I- ion is bonded to the three equiva- lent metal ions through sp3 hybridization, the fourth orbital being occupied by a lone electron pair.

The magnetic susceptibility measurements [3,4] have shown that these iodides become antiferromagnetic.

The nature of the magnetic ordering has been establish- ed through neutron diffraction studies in the cases of MnI, and FeI, [5,6].

The Mossbauer resonance spectra of these iodides were measured using the 27.8 keV resonance in 12'1 in the temperature range from 77 to 1.6 K. At 77 K all the compounds are paramagnetic (see Table I for values of NCel temperatures) and the spectra clearly indicated only a quadrupole interaction at the iodine nucleus. In the antiferromagnetic state of the compounds, the spectra showed the presence of a hf magnetic field at the

12'1

nucleus. These spectra were analyzed by least- squares fitting the data with sum of Lorentzians whose positions and intensities were determined by the value and the relative orientation of the hf magnetic'field

Isomer shvt, Quadrupole interaction and the angle between the eq,

and the Jield, 0, TransferredJieId at '''1 in MI, (M = Mn, Fe, Co, Ni) compounds at 4.2 K Isomer shift (") e29z Q

Transferred

mm/s mm/s field

Compound (-+ 0.05) (k 0.2) kOe (+ 5)

-

MnIz ("1 - 0.08 2.87 140

FeI, - 0.02 3.50 A : 191

B : 96

CoI, 0.01 4.78 130

NiI, 0.18 4.68 263

0

degree TN

(+ 5 ) K

-

90 3.2

0 10

40

90 12

63 75

(") Relative to ZnTe source at 4.2 K.

( b )

1 mm/s = 22.45 MHz.

(") Values at 1.7 K.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1974635

C6-260

P. SANCHEZ,

M. FR .IEDT AND G. K. SHENOY

with respect to the principal electric-field-gradient (efg) axis. The pure quadrupole spectra measured in the paramagnetic region were analyzed using a transmis- sion integral [7]. In figure 1 we show a typical magnetic spectrum of NiI, measured at 4.2 K. The results of the

-12 - 8 - 4 0 4 8 12

V E L O C I T Y ( M M I S )

analysis are given in table I. In the case of FeI, the spectra at 4.2 K could be satisfactorily fitted only assuming the presence of two superposed magnetic patterns with equal quadrupole interactions but diffe- rent magnitudes and orientations for both hf fields.

The magnitude and the direction of the hf magnetic field in the antiferromagnetic region depends on various factors. Primarily, it is determined by the amount of covalent spin density transferf, and fp from the magne- tic transition metal ion to the iodine 5s and 5p orbitals, respectively. The dipolar contribution is usually small enough to be neglected.

The field H, due t o f , arises from the Fermi contact term and is isotropic

Here p(0) is the 5s electron density at the iodine nucleus.

The 5p electrons produce an anisotropic field Hp, and along the M-I bond direction the field can be written as

where

is the covalent transfer along* the bond and

< ^{r - 3} > is an average for the 5p electrons. Using accepted values of p(0) and < r - 3 > ^{(*), Hps} is estimated to be 450 kOe for fpa

1 and H, _{is about} 25 times this value forf,

1. Thus the direction of the observed field at iodine nucleus is determined by the relative magnitudes and directions of Hs and HP,. This in turn also depends on,f, and

Utilizing the quadrupole interaction and the isomer shift, it is possible to deduce the ionicity of the M-I

(*)

p(0)=150 x 1024, < ^r-3 > ^=121.5 x 1024 cm-3 [ref. 121.

bond in the Townes and Dailey approximation 19, 101.

We notice from table I1 that the ionicity of the M-I bond decreases with increasing Z of the metal atom.

This trend is in agreement with the usual chemical arguments and also with the behaviour in similar fluo- rides [I I].

Values of

h,, h,, the ionic character of M-I bond, and f, in MI, compounds (*)

Ionic

Compound f,, hs h , character fs

-

- ^{X )}

- ^%)

- ^%)

MnIz 4.0 0.055 0.65 77 0.59

0.52

FeIz

site) 4.9 0.065 0.75 73 0.80 CoIz

0.087 0.91 67 0.54 (a)

7.6

NiI2 6.5 0.084 1.00 64 1.1

(a) Ref. [12].

( b )

WINDSOR, C. G., GRIFFITHS,

H. E. and OWEN,

Phys.

(1963), 373.

(*)

hs and hp are the number of holes in the

and

shells respectively.

The value offpe can be obtained from the analysis of the quadrupole interaction [8]. The efg in these com- pounds is a sum of that produced by the lattice charges and by the asymmetric distribution of the p electron, the principal axis being the C-axis of the crystal which coincides with the direction of the lone-pair. The lattice contribution is however considerably smaller than that measured experimentally [9] and hence will not be considered.

In table I1 we have given the value of

which represents the excess hole density. This is about 4%

in MnI, and 6.7 % in CoI,. The latter value compares well with the value of 7.6 % determined from EPR of Co2+ ions in CdI, [12].

In none of the compounds is the transferred field along the C-axis (Table I). Thus Hpa is not the domi- nant field. The

values permit us to estimate Hpe which is of the order of 30 kOe. This is smaller than the measured values (Table I). Thus most of the observed field arises from the isotropic contact term, H,, whose direction is basically determined by the direction of cation moments, and their arrangement among the three near neighbours. Accounting for this, we obtain the transferred field due to each cation to be 47 kOe in MnI,, 64 kOe in FeI, (A site), 96 kOe in FeI, (B site), 43 kOe in CoI,, and 88 kOe in NiI,. The value off, has been determined for the Co-I bond from the EPR [12]

to be 0.54 f 0.1 %. We have utilized this value and the hyperfine fields given above to scale thef, values in other compounds [13]. We estimate it to be 0.59 %

for Mn-I bond, 0.80 % for Fe-I bond (A site), and

1.1 % for the Ni-I bond. The trend in the h, values

(Table IT) deduced from the isomer shifts and the

quadrupole interactions does not totally agree with the neglected the contribution of

to the measured field.

above trend inf,. This perhaps reflects the uncertainties The different values of 9 (Table I) and two values for in h, andf, values. Hence the present analysis should the field in FeI, in fact further emphasize the impor- be considered tentative particularly because we have tance of f p w contribution.

References

[l] OWEN, J. and THORNLEY, J. H. M., Rept. Prog. Phys. 29 [7] SHENOY, G. K. and FRIEDT, J. M., Nucl. Instrum. Methods

(1966) 675.

[2] WYCKOFF, R. W. G., Crystal Structures (Interscience [81 BERSOHN, R. and SHULMAN, R. G., J. hem. phys. 45 (1966)

Publishers N. Y.), 1963, vol. 1. 2298.

[3]

GOODENOUGH, J. B., Magnetism and the Chemical Bond [9] BARNES, R. G., SEGEL, S. L. and JONES, W. H., Jr.,

(Interscience Publishers, N. Y.), 1963, Table 8. Phys. 33 (1962) 296.

[lo] PASTERNAK, M., Symp. Faraday Soc. 1 (1967) 119.

[41

UITERT, L.

H.

R' C' and [I11 BASCH, H., VISTE, A. and GRAY, H. B., J. Chem. Phys. 44 RUBIN, J. J., J. Appl. Phys. 36 (1965) 1029. (1966) 10.

151 CABLE,

WILKINSON, M.

WoLLAN, E. 0. and [12] WINDSOR,

H. M., GRIFFITHS, J. H. E.

KOEHLER, W. C., Phys. Rev.

and OWEN, J., Proc. Phys. Soc. 80 (1962) 803.

[6] GELARD, J., FERT, A. R., MERIEL, P. and ALLAIN, Y., Solid 1131 RINNENBERG, H. H. and SHIRLEY, D. A., Phys. Rev. Lett.

State Commun. 14 (1974) 187.