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Submitted on 1 Jan 1979
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MÖSSBAUER EFFECT STUDY OF
OXALATO-BRIDGED Fe(II) CHAIN COMPOUNDS
P. Gubbens, A. van der Kraan, J. van Ooijen, J. Reedijk
To cite this version:
P. Gubbens, A. van der Kraan, J. van Ooijen, J. Reedijk. MÖSSBAUER EFFECT STUDY OF
OXALATO-BRIDGED Fe(II) CHAIN COMPOUNDS. Journal de Physique Colloques, 1979, 40 (C2),
pp.C2-328-C2-330. �10.1051/jphyscol:19792116�. �jpa-00218486�
JOURNAL DE PHYSIQUE Co[[oque C2, supplt2ment au n O 3, Tome 40, mars 1979, page C2-328
MOSSBAUER EFFECT STUDY OF OXALATO-BRIDGED F~(I
I)
CHAIN COMPOUNDS P.C.M. Gubbens, A.M. van der Kraan and J.A.C. van OoijenX and J. ~eedijk*I n t e r w z i v e r s i t a i r Reactor I n s t i t u u t , De Zft, t h e Netherlands
i Department of Chemistry, University o f Techno Zogy, D e l f t , the Netherlands
R 6 s d . - L'ordre magndtique tri-dimensionnel des composds en forme de chaine du Fe(I1) obdissant B la formule gCn6rale FeLzOx, avec Ox = oxalato-dianion et L = H20 et des imidazoles (substitudes), a Qtd 6tudid par l'effet Mgssbauer.
Abstract.- The 3-dimensional magnetic ordering of Fe(1I) chain compounds of general formula FeLzOx with Ox = oxalato dianion; L = water and (substituted) imidazoles has been studied by assbauer spectrometry.
As a part of a research program on the magne- tic properties of poly-nuclear coordination com- pounds, a system of Fe(I1) chain compounds has been prepared and investigated with magnetic susceptibi- lity measurements and Gssbauer spectrometry in or- der to obtain further insight into the mechanisms of superexchangeinteractions between paramagnetic ions.
The coordination compounds have the general formula FeLzOx, with Ox : oxalato-dianion; L = water and (substituted) imidazoles (12).
Barros and Friedberg / l / have shown that a rather strong interchain interaction exists in the Fe(H20)zOx compound. In order to eliminate this in- teraction (substituted) imidazole compounds are stu- died.
In the compounds with L = H20 the Fe(I1) ions are linked together through tetradentate bridging planar oxalato anions. The octahedron around the
~ e ion is completed by two additional ligands (L). ~ + In this way a linear chain is formed. In the case of L = 2MIz (M = methyl) and Zn instead of Fe, X-ray diffraction studies of a single crystal show a zig- zag chain structure due to a cis location of the ligands 121. Furthermore from X-ray powder diagrams it follows that the Fe(I1) compound is isomorph with the Zn compound and one H20 molecule per 2 formula units was added in order to stabilize these com- pounds. For L = Iz and DMIz (DM = 1-2 dimethyl) the trans or cis location of the ligands has not yet been e~tablished.
he
susceptibility measurements have been carried out in the temperature region of 4.2-
80 K at a field strength of 14 kOe. The molar susceptibi- lity versus temperature curves of these compounds have a very broad maximum as shown in figure 1 for Fe(Iz),Ox, and they obey the Curie-Weiss law aboveabout 65 K, yielding 8-values listed in table I.
Fig. 1 : Molar susceptibility versus temperature.
From the observation of both the maxima in the X-T curves and the negative 8-values, it is concluded that the Fe(I1) ions in these compounds exhibit an antiferromagnetic interaction within a chain. In figure 1 also a fit of the experimental data is gi- ven following the interpolation scheme of Weng /3/.
This interpolation scheme uses the Hamiltonian -25
S(S+I) iC. S S. for an arbitrary spin S. It J i' J
is noticed that only the high temperature part of the curve can be described accurately, whereas in the low-temperature region theory and experiment seen to deviate. The latter can presumably be caused by interchain coupling effects which become more impor- tant at low temperature.
In order to study the interchain coupling ef- fects Mgssbauer spectrometry have been performed down to T = 1.7 K. In the figures 2 and 3 some of the measured spectra at T = 1.7 K are shown. It ap- pears that in these compounds the magnetic hyperfine interaction is of the same order as the electric qua- drupole interactions. The positions of the absorption lines are determined by a least squares fitting pro- cedure of Lorentzian lines.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792116
Table I : Parameters deduced from the Gssbauer effect measurements
Fig. 2 : M;jssbauer spectra of Fe(1z)lOx and Fe(DMIz)20x.
From the line positions we have deduced the diffe- rent interaction parameters using the program of van Dongen Torman et al. /4/ and ~';ndi~ / S / . The parame- ters deduced are summarized in table I. The rather complex spectrum of Fe(2MIz),Ox.?H20 1 at T = 1.7 K (see Fig. 3) could only be analyzed by assuming two different Fe sites. Different effective hyperfine fields at these two sites have been found as shown in table I and figure 3, while the electric quadru- p d e splittings are the same. The difference in the effective hyperfine fields is most likely due to the presence of one H20 molecule per two Fe(I1) ions.
The small effective hyperfine fields observed in these Fe(I1) chain compounds are due to large orbital and anisotropic contributions 161. The sign of the effective field has been determined by mea- surements in an external magnetic field of 50 kOe.
For all the compounds the sign is negative. The hy- perfine field in Fe(H20)zOx is in agreement with the
Fig. 3 : Gssbauer spectra of F ~ ( ~ M I Z ) ~ O X . ~ 1 H20 at different temperatures and external field. The two hyperfine field spectra at T = 1.7 K are indicated with I and 11.
result of Barros et al. / 7 / .
The 3-dimensional ordering temperature TN, due to the interchain-coupling has been determined from the vanishing hyperfine splitting of the Gssbauer spectra and they are also given in table I. From these results it follows that for larger ligands TN decreases, which means that the interchain-coupling decreases and subsequently the chainbecomes a more
"ideal" one-dimensional antiferromagnet. In the vi- cinity of the transition from a 3-dimensional to a
C2-330 JOURNAL DE PHYSIQUE I-dimensional magnetic behaviour relaxation effects
are observed as it follows from the shape of the Fe(2MIz)20x.p20 Mksbauer spectrum at T 1 = 4.2 K gi- ven in figure 3. The small peak at the centre of the spectrum of this compound at T = 80 K is due to a small Fe3+ contamination.
The sign of the electric quadrupole splittings has been determined from Gssbauer measurements in an external field of 50 kOe at T = 80 K and it is given in table I. An example of such a measurement has been shown in figure 3. The large electric qua- drupole splitting found for Fe(2MIz)Ox.PzO compared 1 to the other azole compounds may indicate that the ligands in the latter are located in a trans confi- guration, although it can not be proven quantitati- vely.
From the magnitude and sign of the electric quadrupole splittings, the anisotropy contribution to the effective hyperfine field can be calculated.
In this way one gets the order of magnitude of the large orbital contribution which yields information about the gS values. However for a quantitative cal- culation of these values it is necessary to know the covalency effects on < r-3 h
References
/l/ Barros, S. de S. and Friedberg, S.A., Phys.Rev.
141 (1966) 6-87.
/2/ Van Ooijen, J.A.C. and Reedijk, J., Trans. Met.
Chem.
2
(1978) 90./3/ Weng, C.Y., Thesis Carnegie Mellon Inst. of Techn. (1968).
/ 4 / Van Dongen Torman, J., Jagannathan, R. and Troos-
ter, J.M., Hyperfine Int.
1
(1 975) 135.151 ~Gndig, W., Nucl. Instrum. Methode
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(1967) 219.161 Marshall, W. and Johnson, C.E., J. de Physique et le Radium
23
(1962) 733.171 Barros, F. de S., Zory, P. and Campbell, L.E., Phys. Lett.
