MÖSSBAUER STUDY OF BIS (DIMETHYLDITHIOCARBAMATO) IRON (III) HALIDES

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MÖSSBAUER STUDY OF BIS

(DIMETHYLDITHIOCARBAMATO) IRON (III)

HALIDES

Angelos Malliaris, V. Petrouleas

To cite this version:

JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 12, Tome 37, Décembre 1976, page C6-101

MOSSBAUER STUDY OF BIS (DIMETHYLDITHIOCARBAMATO)

IRON (III) HALIDES (*)

Angelos MALLIARIS and V. PETROULEAS (**)

Department of Physics, Nuclear Research Center « Demokritos » Athens, Greece

Résumé. — Trois dérivés halogènes du complexe de fer trivalent Fe(methdtc) 2X (X = Cl, Br, I)

ont été étudiés par effet Môssbauer à l'état cristallin pur ainsi que sous la forme d'un cristal mélangé. Les différents paramètres ont été déterminés par les résultats expérimentaux à l'aide de l'ordinateur électronique et sont comparés avec ceux d'autres complexes connus du dithiocarba-mate. Le dérivé iodé subit une mise en ordre magnétique à T» ^ 1,75 K. Au-dessous de Ts, le spectre Môssbauer est constitué d'une structure magnétique hyperfine ainsi que d'un doublet quadrupolaire. Tous les trois complexes montrent l'évidence d'une dimérisation antiferromagné-tique faible, laquelle est attribuée à un processus de relaxation électronique.

Abstract. — Three halo derivatives of the trivalent iron complex Fe(methdtc) 2X (X = CI, Br, I)

were studied using the Mossbauer technique in the pure crystalline state and in the form of a mixed crystal. Computer fittings of the experimental data allowed determination of the relevant parameters, which were then discussed in relation to other dithiocarbamate complexes. The iodo derivative undergoes a magnetic ordering transition at 7N = 1.75 K. However, below 7V the Mossbauer spectra consist of a magnetic hyperfine structure and a paramagnetic doublet in equi-librium. Evidence of weak antiferromagnetic dimerization in all three complexes is presented and related to the electronic relaxation process.

1. Introduction. — Trivalent iron complexes usually

exist with octahedral symmetry around the metal ion. Depending on the strength of the ligand field they have either an orbital-singlet spin-sextet (weak field) or an orbital-singlet spin-doublet (strong field) ground term. If, however, the iron site symmetry is lowered considerably, other terms, originating from different states of the free ion, can become the ground state of the complex [1] This is the case in the class of iron (III) bis(N,N-dialkyldithiocarbamato) halides (hereafter simply denoted Fe(alkdtc)2X), which have the unusual

for F e+ 3 coordination number five, site symmetry C2v (Fig. 1) and intermediate spin S = f.

FIG. 1. — Molecular structure of Fe(alkdtc) 2X complexes.

(*) Work performed under the auspicies of the Greek Atomic Energy Commission.

(**) Postdoctoral fellow of the Greek National Research Foundation ; Present address : Chemical Biodynamics Lab., Lawrence Berkeley Laboratory, University of California, Berkeley, Ca., U. S. A.

These complexes have been extensively studied [2] because of their remarkable variations in the magnetic properties and paramagnetic relaxation among the numerous members of the series. Thus Fe(ethdtc)2Cl and Fe(ethdte)2I are known to undergo magnetic ordering transitions at 2.45 and 1.9 K respectively [2], whereas other members remain paramagnetic down to liquid He3 temperature. On the other hand, depending on the electronic configuration within the ground quartet manifold, some Fe(alkdtc)2X complexes exhibit moderately resolved paramagnetic hyperfine structure (hfs), while in others all MQssbauer spectra consist of a quadrupole doublet. Structural data and solution pmr shifts have led to the suggestion that there exist Fe-S...S-Fe intramolecular exchange paths responsible for the ferromagnetic transition in Fe(ethdtc)2Cl. However, Fe(ethdtc)2Br with identical crystal structure and spin density on the ethyl groups as the chloro derivative, does not undergo collective ordering [2]. This different behavior has been attri-buted to the effect of ground state Kramers aniso-tropics on the magnetic transition temperature [3].

Despite the considerable volume of studies in the class of iron (III) dithiocarbamate halides, only scattered results of preliminary nature have appeared in the literature concerning the methyl homologs [4], [5], [6]. Here we present a detailed Mossbauer study of three halo derivatives of Fe(methdtc)2X (X = CI, Br, I).

C6-102 A. MALLIARIS AND V. PETROULEAS

2. Experimental. - Fe(methdtc),Cl, Fe(methdtc),Br and Fe(ethdtc),Cl were prepared by the interaction of the corresponding anhydrous halide with the proper ligand in absolute alcohol [7]. For the iodo derivative we employed two different methods of preparation ;

one was the addition of concentrated HI in a benzene solution of Fe(methdtc), and the other the interaction of solid iodine with Fe(methdtc), dissolved in chloro- form [8]. The purity of the compounds was confirmed by C, H and N elemental analysis. Mixed crystals were prepared by crystallization under inert atmos- phere of CHCI, solutions containing the appropriate amounts of Fe(ethdtc),Cl and the guest complex [9].

The electrolytic equilibrium

in solution did not affect the formation of the mixed crystal studied here containing 2

%

mole Fe57(methdtc),C1 and 98

%

Fe(ethdtc), CI. In the case however of 2

%

mole FeS7(methdtc),I in Fe(ethdtc),Cl the lability of the Fe-I bond resulted to a mixed crystal of unknown composition.

The formation of true solid solutions between host and quest complexes was proven by the lack of any magnetic ordering of the mixed crystal at the Curie Temperature T = 2.45 K, of the pure Fe(ethdtc),Cl, and also by the absence of the characteristic quadru- pole doublet of the guest molecule below T,. Several attempts to grow single crystals of Fe(methdtc),X proper for x-ray studies failed, and crystal data are no available for any of the halo derivatives discussed here. However, all known crystal structures of five- coordinate Me(alkdtc),X complexes, where Me= tran- sition metal, and X = halogen or NO, demonstrate some common structural features, i. e. monoclinic systeni, Z=4, similar Me-

-

-Me interionic distances, and nearly identical molecular geometries [6]. Here we

ave assumed that the Fe(methdtc),X complexes conform with the structural characteristics of their homologs, the coordination about the Fef being an approximate square pyramid with the halogen at its apex and the iron atom lying about 0.6

A

above the basal plane of the four S atoms of the dithiocarba- mate ligand (Fig. 1). This assumption was further supported by i. r. spectra which show extensive conju- gation through the ligand and stretching frequencies of the Fe-X and Fe-S bonds characteristic of Fe(alkdtc),X.

3. Results and discussion.

-

3 . 1 Fe(methdtc),Cl.

-

Experimental Mossbauer spectra of Fe(methdtc),Cl polycrystallites are shown in figure 2 along with the theoretical spectra (solid lines) calculated according to

FIG. 2.

-

a)-c) Mossbauer spectra of pollycrystalline Fe(methdtc)pCl ; d ) Spectrum of a mixed crystal containing 2 % mole Fe57(methdtc)~Cl in Fe(ethdtc)zCl. Solid lines

represent computer simulated spectra.

the hamiltonian (1) for the excited state of ~ enucleus, ~ ' and the analogous expression with zero quadrupole term for the ground state (I =

3).

5 1 . 1 2

X

[

_I

-

-

+

- ( I

-

I )

+

g,, b,, I

.

Heff (1) 4 3

I

all symbols have their usual meaning whereas He,, is determined by (2) :

Mossbauer spectra were recorded with a 30 mCi, Rh(CoS7) source in conjunction with a linear velocity drive. Temperatures below 4.2 K were determined by helium vapor pressure thermometry. All isomer shift values are referred to the metallic iron standard at room temperature. Special care was taken to prevent preferential orientation of the microcrystallites when pressed in the absorber holder. Such orientation alters the relative intensities of the absorption lines.

where H, is the hyperfine field per unit spin, equal to

--

220 kG.

In the presence of electronic relaxation effects (Fig. 2a, b, c ) the line shape was computed by the formalism of Blume and Tjon [lo] assuming spin-spin interactions of dipolar and exchange origin.

MOSSBAUER STUDY OF BIS (DIMETHYLDITHIOCARBAMATO) IRON (III) HALIDES FeS7(methdtc),C1 in the lattice of the ferromagnetic

homolog Fe(ethdtc),CI (Fig. 2d), or from the computer fitting of the experimental Mossbauer spectra of pure Fe(methdtc),Cl pollycrystallites (Fig. 2a, b, c). Under the mixed crystal conditions the two complexes pro- duce Mossbauer spectra of about the same total intensity (the abundance of Fe57 in natural iron is only 2

%).

It is obvious therefore from figure 2d that the two spectra coincide, and therefore both deriva- tives have identical hyperfine parameters. The magni- tude of He,, = 330 kG, found in Fe(methdtc),Cl

from the mixed crystal, implies that the molecular field H,,, acts parallel to the crystal field axis

D.

Otherwise H,,, would have caused mixing of the ground Kramers doublet

I

+

3

>

with the

I

+

3

>

state lying at higher energy, the amount of mixing being proportional to HmOI/D. In view of the values of H,,, = 15-20 kG and D =

-

1.4 cm-I, He,, would have a magnitude considerably lower than 330 kG, corresponding to ( S )

<

(3) according to (2). The common direction of H,,, and the D-axis also deter- mines the spin quantization axis and therefore the direction of He,,. Single crystal studies of Fe(ethdtc),Cl, on the other hand, have proved that Hm,, in this complex acts parallel to the major axis of the basal sulfur rectangle [Ill. Also, the low concentration of Fe57(methdtc),C1 in the mixed crystal, and the similar molecular geometry of the two complexes guarantees substitutional replacement of host by guest molecules in the mixed crystal lattice. Under these conditions the orientation of FeS7(methdtc),C1 with respect to H,,, is the same as the orientation of Fe(ethdtc),Cl molecules. Therefore

D

and He,, in Fe(methdtc),Cl are parallel to the major basal sulfur rectangle. This conclusion is further supported by the computer simulated spectra of Fe(methdtc),Cl polycrystallites (Fig. 2a, b, c) which agree with the experimental spectra only if the direction of He,, is assumed perpen- dicular to the V,, axis of the efg, lying along the Fe - C1 bond [ll]. *

The relaxation parameter C determined from the low temperature spectra of pure Fe(methdtc),CI (Fig. 2a, b, c) accounts for all electronic transitions induced by both the dipolar and the exchange mechanism. The value C = 600 Mc/s found, was consistent with all experimental spectra below 4.2 K. It is interesting to compare this value of C with the corresponding value, C = 3 000 Mc/s, found in the paramagnetic region of Fe(ethdtc),Cl [12]. Since the Fe-

-

-Fe interionic distances are expected to be approximately equal in the methyl and ethyl crystals, the five-fold difference in C between the two derivatives must arise entirely from differences in the intermolecular exchange interactions in the two lattices. An approximate estimate of J based on the molecular field theory for six nearest neighbours and ignoring the effect of crystal field terms, yields the value J

-

0.16 K for Fe(ethdtc),Cl [3]. On the other hand, Mossbauer

studies of magnetically perturbed Fe(methdtc),Cl polycrystallites, which will appear elsewhere, suggest the presence of antiferromagnetic dimeric coupling in this crystal, with an estimated exchange interaction J

-

-

0.4 K. Apparently, the exchange interaction is more effective in inducing spin transitions in the former than in the latter case where the coupling is confined within each dimer. It is appropriate to notice here that the exchange coupling is not expected to affect the energies of the ion in the absence of a polarizing magnetic field, applied or internal. However, the nature of the exchange interaction greatly affects the spin-spin relaxation mode. Moreover, our results indicate that for these van der Waals solids with interionic separations of the order of 7-8

A,

the exchange interaction plays a more important role than the dipolar coupling in inducing spin transitions. 3.2 Fe(methdtc),Br. - Contrary to Fe(methdtc),Cl, in bromo derivative the ground state Kramers state has spin

I

+

$

>

and lies approximately 20 K below the next doublet

1

4 $

>

[7]. This strong uniaxial crystal field anisotropy which dominates exchange induces a large depression of the magnetic ordering temperature expected in the absence of the crystal field 131. Indeed, polycrystalline Mossbauer spectra of this compound consist of a simple quadrupole doublet at all temperatures (Fig. 3a).

FIG. 3. - Mossbauer spectra of pollycrystalline Fe(rnethdtc)zBr

at 1.4 K ; a) without applied field ; d ) with transverse applied field of 8.8 kG.

C6-104 A. MALLIARIS AND V. PETROULEAS

common in this class of intermediate spin ferric complexes. Fuller details concerning these dimers will be given in a later publication.

3.3 Fe(methdtc),I. - Polycrystalline Mossbauer spectra of Fe(methdtc),I demonstrate quite different features from the corresponding spectra of the other two halo-derivatives studied here. This crystal under- goes a collective transition evidenced by the abrupt appearrance of hyperfine structure around TN = 1.75 K . Yet the paramagnetic doublet persists well below TN, apparently at an equilibrium with the magnetic

spectrum (Fig. 4). The solid lines of figure 4 are

FIG. 4.

-

Mossbauer spectra of pollycrystalline Fe(methdtc)zI. Solid lines represent the computer simulated spectra.

He,, at various temperatures, to the zero temperature, allows an estimate of the saturation magnitude of the effective field

He,,

-

200 kG consistent with the

expected value of 220 kG for large positive D and

a molecular field transverse to the D axis.

Preliminary studies of magnetically perturbed Mossbauer spectra (Fig. 5) have shown here again the existence of dirneric antiferromagnetic coupling. This coupling however, appears to be stronger, J =

-

0.75 K,

than the coupling in the other two methyl derivatives.

. .

.

.

tn tn Z H = o 95.0

.

<

, p!! % Z .

* , ' "

v. '

.

,,,.,y\c-dCz t _{I .:..*}

.

.

-

7 %, a

.

.-

-. .

2'C+C.

.+.

s : H = ~ . Y K G -J W

..

.

.

. ;. .

.

:C

.:.

i .

.

95.0.-

.

_.

_.

.

.+.

_.

_.

:

1 oao -~,%.t-,*.+,

.

-

x

- -

J:

.

.

.

P'r~,orr~ulrh

i

L_

; ? ; :

, :

.

.

90.0 ( b )

..

.

. .

.

?; - 6 - 4 - 2 0 2 4 6 UELBC I TY (MN/S)

FIG. 5. - Mossbauer spectra of pollycrystalline Fe(methdtc)J :

a) at T = 1.8 K ; b) T = 1.52 K. Applied magnetic field as

indicated.

The persistence of the doublet below TN has been theoretical spectra calculated on the assumption of a observed in other dithiocarbamate derivatives [14], [15] static hyperfine field at right angles to the V,, axis of and has been ascribed to critical superparamagne- e. f. g. The parameters derived from the computer tism [16]. Its explanation is based on the assumption fits are listed in Table I. Extrapolation of the values of that several magnetic domains undergo fast spin

(+) Polar angles in the principal axis system of the e. f. g.

(a) D = - 2.8 K from far i. r, studies [7].

fluctuations around some direction of crystal aniso- tropy. The period of the fluctuation z, is usually expressed by (3)

zF = zO exp(KV/kT) (3)

where z0 lies between and 10-l2 S, K is the

effective anisotropy energy per unit volume, V the domain size, and k is the Boltzamnn constant. Super- paramagnetic effects are observed when z, has an intermediate value between zL, the Larmor precession of the nucleus, and z, the characteristic individual electron spin correlation time. The anisotropy energy KV is different for each superparamagnetic particle depending on its volume V, therefore, according to (3),

z, becomes comparable to zL at different temperatures for the various particles. Consequently, as the tempera- ture of the sample is lowered, the central doublet in the Mossbauer spectrum decreases in intensity while the hyperfine structure becomes more pronounced. Application of an external magnetic field, on the other hand, in the superparamagnetic region induces polarization of the superparamagnetic particles, provided they possess total magnetic moment. This is evidenced by the increase of the intensity of the hyper- fine part of the Mossbauer spectrum at the expense of the central doublet [17].

The temperature and field dependence of the Moss- bauer spectrum have been used as the main criteria of superparamagnetic behavior.

From figure 4 it is obvious that as the temperature of the sample is lowered the intensity ratio of the

doublet to the hyperfine spectrum decreases although the central doublet persists even at the lowest tempe- rature studied. On the other hand, contrary to what is expected for a superparamagnetic material, application of a magnetic field below TN does not reduce the intensity of the central doublet (Fig. 5a), the only effect of the external field being some broadening of the magnetic spectrum.

Fe(ethdtc),I which undergoes magnetic transition at 1.9 K and also has been assumed to demonstrate superparamagnetic effects 11 51 behaves in a similar way as the methyl derivative in the presence of an external field. Above TN there is evidence for antiferromagnetic dimerization whereas below TN the central doublet is not affected by the external field.

Grow et al. [14] have studied several Fe(alkdtc),X

complexes which below TN produce Mossbauer spectra demonstrating both magnetic hyperfine structure and a paramagnetic doublet. These authors have considered superparamagnetic effetcs as a possible contribution to the hfs near TN. They have also pointed out that '

in all cases the parameter D has positive value and have suggested that the superparamagnetism of these complexes is connected with the easy plane anisotropy of the ground state g,

-

g,,

--

4 ; g, = 2. On the other hand, our results suggest that there is a strong possibility that the simultaneous appearance of magne- tic and paramagnetic structures in the Mossbauer spectra of some Fe(alkdtc),X complexes below TN is related to the antiferromagnetic dimerization in these crystals.

References

[I] GRIFFITH, J. S., J. fnorg. Nuel. Chem. 2 (1956) 1.

[2] CHAPPS, G. E., MCCANN, S. W., WICKMAN, H. H. and SHERWOOD, R. C., J. Chem. Phys. 60 (1974) 990. [3] LINES, M. E., Phys. Rev. 156 (1967) 534.

[4] WICKMAN, H. H. and T ~ o z z o ~ o , A. M., Inoug. Chem. 7

(1968) 63.

[5] MALLIARIS, A. and PETRIDIS, D., Chem. Phys. Lett. 36 (1975)

117.

[6] MARTIN, R. L. and WHITE, A. H., Inorg. Chem. 6 (1967) 712.

[7] BRACKETT, G. C., RICHARDS, P. L. and WICKMAN, H. H.,

Chem. Phys. Lett. 6 (1970) 75.

[8] GROW, J. M., Ph. D. Thesis, Oregon State University (1975). [9] MALLIARIS, A., KOSTIKAS, A. and SIMOPOULOS. A., Chern.

Phys. Lett. 28 (1974) 608.

1101 BLUME, M. and TJON, A., Phys. Rev. 165 (1968) 446. [ l l ] WICKMAN, H. H., J. Chem. Phys. 56 (1972) 976. 1121 WICKMAN, H. H. and WAGNER, C. F., J. Chem. Phys. 51

(1969) 435.

[13] MALLIARIS, A. and SIMOPOULOS, A., J. Chem. Phys. 63 (1975) 595.

[14] GROW, J. M., ROBBINS, G. L. and WICKMAN, H. H., present proceedings.

[15] PETRIDIS, D., SIMOPOULOS, A., KOSTIKAS, A. and PASTER- NAK, M., J. Chem. Phys. (in przss).

[16] LEVINSON, L. M., LUBAN, M. and SHTRIKMAN, S., Phys. Rev.

177 (1969) 864.

[17] ROGGWLLLER, P. and KijNDIG, W., Solid State C o m m ~ ~ n . 12