EVIDENCE FOR DOMAIN FORMATION AT THE HIGH-SPIN(5T2) ⇌ LOW-SPIN(1A1) TRANSITION IN AN ORGANIC COMPLEX OF IRON (II) : 57Fe MÖSSBAUER EFFECT AND MAGNETISM

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EVIDENCE FOR DOMAIN FORMATION AT THE HIGH-SPIN(5T2) � LOW-SPIN(1A1) TRANSITION IN

AN ORGANIC COMPLEX OF IRON (II) : 57Fe MÖSSBAUER EFFECT AND MAGNETISM

B. Kanellakopulos, E. König, G. Ritter, W. Irler

To cite this version:

B. Kanellakopulos, E. König, G. Ritter, W. Irler. EVIDENCE FOR DOMAIN FORMATION AT THE HIGH-SPIN(5T2) � LOW-SPIN(1A1) TRANSITION IN AN ORGANIC COMPLEX OF IRON (II) : 57Fe MÖSSBAUER EFFECT AND MAGNETISM. Journal de Physique Colloques, 1976, 37 (C6), pp.C6-475-C6-478. �10.1051/jphyscol:1976695�. �jpa-00216806�

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JOURNAL DE PHYSIQUE Colloque C6, suppl&ment au no 12, Tome 37, Dtcembre 1976, page C6-475

EVIDENCE FOR DOMAIN FORMATION AT THE HIGH-SPIN(5T2) +

LOW-SPIN('A1) TRANSITION IN AN ORGANIC COMPLEX OF IRON (11)

:

57Fe MOSSBAUER EFFECT AND MAGNETISM

B. KANELLAKOPULOS

Kernforschungszentrum Karlsruhe, D-7500 Karlsruhe, BRD E. KONIG (*), G. RITTER and W. IRLER Institut fur Physikalische Chemie and Physikalisches Institut,

Universitat Erlangen-Nurnberg, D-8520 Erlangen, BRD

R6sm6. - L'ktude Mossbauer et les propribtks magnbtiques de Fe(4-CH3-phen)z(NCS)z, ou phen = 1,lO-phknantroline, montrent la prksence d'une transition progressive spin-fort (5T2)

*

spin-faible (1A1) entre 245 et 120 K. La variation des facteurs de Debye-Waller f i ~ , et fiA, est conforme au modkle de Debye, entre 175 et 250 K pour le spin fort (Oq, = 126,2 K), entre 105 et 225 K pour le spin faible (elAl = 150,2 K). C'est l'indication que dans ces zones de tempbrature le comportement des vibrations du reseau est normal pour chacun des ktats de spin. Par contre les deviations observees en dehors de ces zones de tempkrature sont l'indication de la formation de domaines, dont les consequences sur le mecanisme de la transition de spin sont discutks en detail. Ces rbsultats sont compatibles avec le modkle de spin-flip dkcrit prkckdemment.

Abstract. -On the basis of 57Fe Mossbauer effect and magnetism, the compound Fe(4-CH 3-phen) 2(NCS) 2

where phen = 1.10-phenanthroline shows a continuous high-spin (ST 2)

+

low-spin (1A ¹⁾transition between 245 and 120 K. The Debye-Waller factors f s ~ , and f i ~ , follow the Debye model between 175 and 250 K (059 ⁼126.2 K) and between 105 and 225 K (elAl = 150.2 K), respectively.

Normal lattice vibrational behaviour is thus indicated for the individual ground states within these temperature regions. Deviations encountered outside these regions are considered as evidence for domain formation. The consequences with respect to the mechanism of the spin transition are discussed in detail. The results are consistent with the spin-flip model introduced previously.

1. Introduction. - Thermally induced transitions between the high-spin (5T,) and low-spin (lA,) ground states in organic complexes of iron(I1) have received considerable attention over the last few years [I-31.

Although spin transitions of this type may occur in solution, the majority of results has been collected on solid systems and, therefore, we will confine our discussion to these. Two types of transition may be distin- guished : transitions of type (A) extend over a temperature range of 100-200 K and are continuous ; transitions of type (B) which are associated with a phase change are almost discontinuous and may be characterized by a transition temperature, Tc. It has been demonstrated [4] that the essential difference between the two types of transition is the different amount of cooperative interaction between the iron atoms. More recent investigations are centred around two funda- mental problems : (i) the driving force of the transition ; (ii) the detailed mechanism of the transition.

(*) Author to whom correspondence should be addressed at the Institut fur Physikalische Chemie 11, Universitat Erlangen- Nurnberg, D-8520 Erlangen, Germany.

As far as the first question is concerned, it has been concluded on the basis of heast capacity measurements on systems of type (A) that the entropy change is the driving force of the transition [5]. However, the quantity AS,,,,, consists of at least three contributions, viz.

the configurational, the vibrational and the spin entropy change. The latest estimates available [6]

assign values of comparable magnitude to all three terms and thus it is not much likely that the cause for the transitions can be found in a single source of entropy.

With regard to the mechanism of the transitions, three different models have been proposed : (i) a Boltzmann distribution over the low-lying spin singlet ('A,) and spin quintet (5T2) levels ; (ii) a temperature dependent crystal field model [7] ; (iii) a ^(<spin-flip ⁾⁾ mechanism ,[2, 8, 91. Whereas model (i) is at variance with numerous experimental results [2, 7, 91, model (ii) has been recently disclaimed [5] due to the absence of temperature dependence in infrared frequencies. The spin-flip mechanism of model (iii), although favoured by the present authors since some time, has received little direct support by undisputable experimental

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1976695

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findings. The purpose of this study is to present evidence for the spin-flip mechanism which will be based on a detailed investigation of Debye-Waller factors. The compound chosen is Fe(4-CH3-phen),(NCS), where phen = 1.10-phenanthroline and thus derives by abstraction or addition of a methyl group at the phen ligand from the well-studied high-spin (5T2) S l o w -

spin (lA,) systems Fe(phen),(NCS), [lo] and Fe(4,7-(CH,),-phen),(NCS), [l 11, respectively.

2. Experimental results. - 57Fe Mossbauer effect measurements on the solid compound

were performed between 4.2 K and 309 K employing a spectrometer of constant acceleration type operated in multiscaler mode. Spectra at three typical temperatures, viz. 260,215 and 105 K, are displayed in figure 1,

Velocity (rnrnlsl

FIG. 1.- 57Fe Mossbauer spectra of Fe(4-CH3-phen)2(NCS)z at the temperatures of 260, 215 and 105 K.

full details being given elsewhere [6]. The compound shows a high-spin ( 5 ~ 2 ) low-spin (,A,) transition which occurs over the range 120-245 K with a centre at 215 K. From the observation of separate spectra, the relaxation time between the 5 ~and 'A1 ground 2

states is slow, cf. 25, % o ; ' ( ' ~ , ) [2, 91. It should be noted tfiat the opposite situation of fast relaxation has been recently found in a system containing

iron (11) 1121. The 5T, ground state which is isolated at and above 260 K (Fig. 1) may be characterized by AEQ = 2.88

+

0.02 mm s-l and

the corresponding values for the 'A, ground state being AEQ = 0.37

+

^{0.01 mm}^{s - I} ^and

All values are at 215 K, the temperature variation being found as expected. Note that isomer shifts are quoted with reference to an iron foil standard at 298 K. There is a residual contribution of the 5T2 ground state at cryogenic temperatures, viz.

A ~ ~ = 3 . 0 9 + 0 . 0 2 mm s-l, 61S=

+

I.OO+ 0.03 mm s-l,

residual -

n s ~ , ^-0.056 at 105 K (Fig. 1).

The Mossbauer spectrum shows a texture based line asymmetry which is opposite in the 5T2 and 'A, ground states. It has been demonstrated that the observed texture is essentially pressure induced. From a study of magnetic hyperfine interactions in an external longitudinal field He,, = 40 kG at 4.2 K we obtained VZZ('Al) < 0. In terms of the quantity R, = I,/& [13], it is found R,('A,) = 0.78 _f 0.07 at all temperatures studied. For complete orientation in axial symmetry it is R, = 3 and R, = 315 for k parallel and perpendi- cular to the principal c axis, respectively, where k is the wave vector of the incident y ray. From the obser- vations is follows that, most likely, vzz(5T2) > 0 at all temperatures and R,('T,) -- &,('A,) within experimental uncertainty. It may be of interest to recall that a similar sign reversal of V,, has been found previously in systems showing high-spin ( 5 ~ 2 ) $ low-spin ('A,) transitions 114, 151.

Magnetic measurements between 0.98 and 303.7 K are in full agreement with the results of Mossbauer effect studies. In terms of the effective magnetic moment, a continuous decrease is observed from peff = 5.223 pB at 303.7 K to p,,, = 1.417 pB at 77.60 K which corresponds to the essential part of the high-spin ('T,) z$ low-spin ('A,) transition. An almost temperature independent region down to p,, = 1.348 pB at 16.43 K demonstrates evidence for the residual 5T2 fraction. Below

-

20 K a pronounced decrease of the magnetic moment down to p,,, = 0.608 p~ at 0.98 K sets in. This behaviour is due to depopulation of all paramagnetic states which contribute to peff.

The Mossbauer spectra were decomposed into Lorentzians and Debye-Waller factors were determined for the ground states 5T2 and 'A1, individually, from the suitably corrected areas, viz.

Here, the effective thickness t

<

0.7 and the saturation function L(t) may be well approximated by

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EVIDENCE FOR DOMAIN FORMATION AT THE HIGH-SPIN K? LOW-SPIN TRANSITION C6-477 The absorber contained about 0.071 mg cm-2 57Fe

and the factor Sf, was determined from measurements on thin absorbers of known Debye-Waller factors.

In the expression for t5T2,

the high-spin fraction, n5,,, was taken from the magne- tic data. In conjunction with the corresponding expres- sion, for t l A 1 where nlAl = 1 - nsTz and where all the remaining quantities have the usual meaning, the Debye-Waller factors fsT2 and

A,,

are obtained as shown in figure 2. From the figure, it is evident that

FIG. 2. - Debye-Waller factors for the 5T2 and 1Al ground states of Fe(4-CH3-phen)z(NCS)2 plotted as - In f versus temperature (upper part). Both straight lines for -In fsT, and -In fial pass through zero within 4 0.04. High-spin fraction n5T2, assuming we~f(lA1) = 1.00 PB, versus temperature (lower

part).

both - In fJTz and - In fiAl follow straight lines over a considerable range of temperature. Since both these lines pass through zero, the high-temperature appro- ximation is followed and if, e. g., the Debye model

is applied, the Debye temperatures

OST2 = 126.2 K and O I A , = 150.2 K are obtained. In eq. (4), the mass of the 57Fe atom has been used for M, despite the molecular lattice of the compound studied.

3. Discussion : Domain formation and the spin-flip mechanism. - The result of figure 2 demonstrates that, over the region of temperature where the transition takes place, two different Debye-Waller factors, viz. f5T2 and may be defined. Therefrom it follows that the mean square amplitudes must be different for those lattice modes which are specifically associated with the 5T2 and 'A, ground states of

Based on the x-ray structures of related compounds 116, 171, a change of localized vibrations is expected as consequence of a high-spin (5T2) z$ low-spin ('A,) transition. It may be visualized that this variation may affect lattice modes as well. Similar results to those of figure 2 have been obtained previously for some high- spin (5T2)

*

low-spin ('A,) transitions in compound of iron (11) [9, 181, although the detailed temperature dependence of the Debye-Waller factor is presently the subject of a reinvestigation.

More importantly, in those regions of temperature where the site fraction of the particular state is very low, deviations towards the - ln f values of the alternate state are observed. Thus - In fsT2 departs from the high-temperature limit of the Debye model for

@sT2 ⁼126.2 K below 180 K where n5,, < 0.28, whereas for - lnfi,, this behaviour is found above 225 K where nlA, < 0.20. We conclude that if, due to a high-spin

+

low-spin ('A,) transition, the site fraction of 5T2 or 'A, molecules becomes lower than a limittjng value, the lattice modes corresponding to that particular ground state are progressively changed toward those of the alternate state. This observation is taken as evidence for the formation of individual domains with normal lattice vibrational behaviour by both the 5T2 and 'A, ground state molecules in the respective regions of temperature.

The mechanism of the transition may be now understood on the following, though qualitative, basis : at the absolute zero of temperature, all molecules are in the low-spin ('A,) ground state (') and there exist normal modes of vibration which are charac- teristic for this state. If the temperature is increased, the spin state of individual molecules will be eventually changed by thermal excitation into one of the high- spin (5T2) states. The high-spin (5T2) ground state thus produced is characterized, in general, by a distorted geometry (as compared to the 'A, ground state) and, consequently, by changes in certain normal modes.

The resulting local distortion [16, 171 at one site

(1) In the discussion, it is assumed here that the transition goes to completion at cryogenic temperatures. However, in numerous cases a residual 5T2 contribution has been observed 19, 181.

Apparently, this fraction may be accomodated, within the lattice of 'A1 molecules, without any major structural effects. The detailed influence of the residual 5T2 contribution on the meha- nism of the high-spin (5T2) S low-spin transition is presently under investigation.

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couples via long wave phonons to other sites. This coupling communicates the distortion, and thus the associated spin change, from site to site. The molecular distortion may then produce, stepwise or more or less instantly, a total crystal distortion, and eventually a phase transition, to a lower symmetry structure of different total spin.

If the coupling between molecular sites [4] is weak, distorted arrays, i. e. domains of 5T, ground state molecules, will be produced at first. With further increase of temperature, the domains will combine and, eventually, a distorted lattice of 5T2 molecules will be formed (continuous high-spin (5T,)

=+

low- spin ('A,) transitions ; type ( A ) ) .

If the coupling is strong, the mechanism of the transition may be understood by application of the concept of soft modes [19,20]. Accordingly, as distorted molecules in 5T2 ground states are gradually formed, certain normal modes will be damped and become unstable as T + T,. The phase transition thus occurs because of phonon anharmonicities which drive the particular (soft) modes to zero frequency.

Note that, at a first order transition, a mode may

become undamped at a finite frequency [21] (almost discontinuous high-spin (5T,) low-spin ('A,) transition ; type (B)).

It is obvious that, with respect to the spin change of individual molecules within the solid, the mechanisms discussed here are consistent with the spin-flip model only. This interpretation anticipates or is equivalent to the assumption that the transformation is static and is thus in contrast to the observation on solutions [22; 231.

The Mossbauer results, on the other hand, do not lend themselves to any more precise statement than that of slow relaxation. However, since two sets of Mijssbauer lines are observed (Fig. I), the intensity of one set growing while the other is decreasing, the conclusion of two separate static Fe sites in the transition region (i. e. relaxation extremely slow) is the most obvious one.

Acknowledgements. - The authors appreciate financial support by the Deutsche Forschungsge- meinschaft, the Fonds der Chemischen Industrie and the Bundesministerium fiir Bildung und Wissen- schaft.

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