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MÖSSBAUER SPECTROSCOPIC STUDIES OF THE
HF INTERACTION IN [Fe(H2O)6-nFn]3-n
COMPLEXES IN FROZEN AQUEOUS SOLUTION
J. Knudsen, S. Mørup
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, suppldment au no 12, Tome 37, Ddcembre 1976, page C6-535
MOSSBAUER SPECTROSCOPIC STUDIES
OF
THE
HF
INTERACTION
IN [Fe(H, O),
-,F,]
3 - nCOMPLEXES IN FROZEN AQUEOUS SOLUTION
J. E. KNUDSEN and S. M0RUP
Laboratory of Applied Physics I1 Technical University of Denmark
DK-2800 Lyngby, Denmark
RBsum6. - La structure hyperfine paramagnktique des spectres Mossbauer des complexes [Fe(H20)6-nFn]3-n dans des solutions aqueuses gel&s, dilukes magnktiquement, a kt6 employee pour la determination des parametres hyperks et des dparations des champs cristallins, A n , des complexes n = 0, 1, 2, 3, 4. On observe que les skparations A n accroissent faiblement de n = 0 B n = 1, mais dkcroissent, dans le cas oh n > 1, jusqu'h A d A o E 0,s. Le champ hyperfin de saturation s'accroit d'h peu prks 3 kG par ligand de fluor.
Abstract. - The paramagnetic hf structure in Mossbauer spectra of [Fe(H20)6-nFn]3-n complexes in magnetically dilute, frozen aqueous solutions has been used in a determination of the hf parameters and cf splittings of the complexes n = 0, 1, 2, 3, 4. The cf splitting A n is found to increase slightly from n = 0 to n = 1, but to decrease for n > 1, until AnlAo = 0.5 for n = 4.
The saturation hf field increases by about 3 kG per fluorine ligand.
1. Introduction.
-
The paramagnetic hyperfine (hf) splitting in Mossbauer spectra is a sensitive tool for the investigation of the electronic structure of ferric complexes. Due to the spherical charge symmetry (L = 0) of the ,S ground state of the high spin ferric ion, the electronic spin-lattice relaxation is generally slow, and a resolved paramagnetic hf splitting may be observed even at 78 K, if the spin-spin relaxation is suppressed by magnetic dilution.Complexes can be studied in an aqueous medium if the solution is frozen into a homogeneous glass. Using this method we have investigated the ferric fluoro complexes, [Fe(H,0),-nF,]3-n, in magneti- cally dilute, frozen aqueous solutions. The formation of these complexes (in water at room temperature) has been studied previously by means of EPR [I]. Here the information is deduced from the width of a broad resonance signal, and it is not very detailed. In contrast, Mossbauer spectra give direct information concerning the hf coupling and the crystal field (cf) spin Hamilto- nian. The dependence of these quantities on the num- ber n of F - ligands is the main topic of this work. 2. Experimental. - The absorbers were frozen from aqueous solutions of Fe(NO,), (90
%
57Fe) and NH,F with 40%
glycerol added as glass former. The relative content of fluorine, R = [NH,F]/[Fe(NO,),], was varied within the limits 0<
R<
12. The iron concentration was 0.030 M for R>
0 and 0.020 M for R = 0. In solutions of low fluorine content the hydro- lysis of iron was prevented by addition of HNO,. Adetermination of pH ensured that only minute amounts of undissociated H F had formed [I]. The solutions were quick-frozen to homogeneous glasses by immer- sion into liquid N,. The solutions with R
<
5.5 were clear immediately before freezing, but for R 2 7.0 a cloudy appearance indicated the formation of a preci- pitate.For each absorber a series of Mossbauer spectra was obtained at 78 K with various magnetic fields, H, applied perpendicularly to the y-ray beam.
A Mossbauer spectrometer of the conventional constant acceleration type was used with a source of 57Co in Pd. Velocities (and isomer shifts) are given relative to metallic iron at room temperature.
3. Results.
-
The Mossbauer spectra show a well resolved hf splitting, but with somewhat broadened lines (- 1 mm/s), often of non-Lorentzian shape. Some of the spectra are shown in figures 1 to 4.For H = 125 G (Fig. 1) the absorbers with R
<
5.5 had similar spectra, composed of magnetically split, six-line contributions. The bar diagram indicates an interpretation of the spectrum of the ferric hexaquo complex (R = 0), corresponding to hf fieldsz
573 kG (A and C) and E 250 k G (B) 12, 31. For R 2 7.0 the spectra (not shown) exhibit a broad, central line at 0.5 mm/s in addition to the six-line spectra. This dip does not disappear upon application of H = 6 200 G.It is presumably associated with the precipitates in these absorbers.
The spectra obtained in 1 G show a gradual change
C6-536 J. E. KNUDSEN AND S. MBRUP
with the fluorine content for R
<
5.5 (Fig. 2). The asymmetry is present mainly as broad lines at about six lines A and C of the 125 G spectra are found also at+
5 and-
9 mmls. At 6 G the spectrum is unaltered, 1 G, but the lines B are not. Instead, for R = 0, an but for increasing field the asymmetry disappears and at H = 60 G the spectrum is nearly identical with the 125 G spectrum. For increasing R the asymmetric contributions of the 1 G spectrum are gradually replaced by a broad, central line at 0.5 mm/s. By application of a small field H<
125 G this central dip disappears, and the spectra become similar to those of the absorber with R = 0. The field at which the dip disappears increases with increasing R, for R = 5.5 it can still be traced at H = 60 G.For H > 125 G a spectral evolution takes place with increasing field, but the changes proceed at different rates for different absorbers. This is illustrated in figures 3 and 4, where spectra of the absorbers R = 0 and R = 3 can be compared.
VELOCITY (MM/S)
c 1
-
FIG. 1.
-
Mossbauer spectra of absorbers of various composi- ~20
tions R, obtained at 78 K with a transverse magnetic field of P 3
125 G . L1 0 m = 1 . ----% .ww- ' . R=O * 1000 G . . %
.
.
-.
. .
.-
.
:
'.
.
.-
:.. -.
8 \ C-
cV?#&- , 1500 G , :.
j*:.
Err..
? .. . .
:...
.
F"..
:.
.
:>
..-
r *.*:.
". . . .
2 ::..
. d N f l a . L : 4%'
-
$>
'
4000 G , ' f ?.
.-
. . .
-.
A .I
. . . .
f?,!
8800 G ,,. . .
-
.
6.:.:
.-
f.;.
. . .
. . .
. . .
.
# 'r'.
-16 -12 -8 -4 0 4 8 12 16 VELOCITY (MM/S)FIG. 3. - Mossbauer spectra of the absorber with R
-
0 obtained at 78 K with various applied transverse fields.-16
VELOCITY (MM/S) at - 9 and
+
10 mm/s and the lines at - 5 andFm. 2.
-
MBssbauer spectra of absorbers of various composi-+
mm/s changes 0.g7 (f 0.03) at 125 G to tions R, obtained at 78 K without an applied field. 0.50 (f 0.02) at 6 200 G for all the absorbers.-12 -8 -4 0 4 8 12
'UDIES OF THE HF INTERACTION C6-537
-16 -12 -8 -4 0 4 8 12 16
VELOCITY (MM/S)
FIG. 4.
-
Mossbauer spectra of the absorber with R = 3obtained at 78 K with various applied transverse fields.
From the actual positions, x,, x,, x,, x,, of the four intense lines at
-
9,-
5,+
6, and+
10 mm/s, the following hf parameters are deduced : the hf splitting, x,-
x,, in the 6 200 G spectra, the isomer shift, 6 = (x,+
x,+
x,+
x6)/47 calculated for each absorber as an average from 8-10 spectra at various fields, and the quadrupole shift,in the 125 G spectra. These parameters are shown in figure 5 as a function of R. The quadrupole shifts calculated from the 1 G spectra are quite similar to those obtained at 125 G. The 6 200 G spectra have
E R 0.
According to calculations based on equilibrium constants given in the literature [I, 41, the composition range investigated corresponds to the successive formation of complexes with n = 0, 1, 2, 3, and 4 fluorine ligands. The calculations assume that (1) the distribution of complexes does not change during the cooling, and (2) it is not influenced by the presence of the glycerol or the electrolyte. The results are indicated in figure 5, where the divisions of the upper abscissa correspond to the calculated 50
%
ranges of the various complexes n. These ranges are not very sensitive to minor deviations in the equilibrium constants from an insufficient fulfilment of the two assumptions.DOMINATING COMPLEX ( n ) 0 1 2 3 4
I l l I
1
FIG. 5. - Hf parameters and relative cf splittings in absorbers of ,~arious compositions R, deduced from the Mossbauer spectra.
4. Discussion.
-
Paramagnetic hf split Mossbauer spectra of high spin ferric complexes ( S =3)
are usually described in terms of a spin Hamiltonian (omitting terms independent of the electronic state) :The first term represents the cf interaction, where for simplicity only quadratic contributions have been included. Left alone, this term splits the ,S ground state into three Kramers doublets A, B, and C of total separation A . The second term is the electronic Zeeman interaction, and the third term is the magnetic hf interaction.
C6-538 J. E. KNUDSEN AND S. MBRUP the hf interaction couples a given Kramers doublet to
the nuclear state, and the Mossbauer spectrum can generally not be described by simple means. In a so- called stabilizing field (H = 125 G) where
each of the three Zeeman split Kramers doublets gives rise to a magnetically split, six-line spectrum with a hf field dependent on the cf spin Hamiltonian. With a strong applied field, A 3 2 pH, the electronic spin is quantized in the field direction 2, S, = f
3,
+
8,
++.
Each of these states contributes a six-line spec- trum with hf field H, S,/S in the 2-direction, H, = - ASIg,fin
being the saturation value of thehf
field.
The stabilized spectrum, indicated by the bar dia- gram in figure l , is expected in the model defined by eq. (I), if 2 N
+.
In this case the Kramers doublets A and C have highly anisotropic and nearly identical gyromagnetic tensors, while the doublet B has nearly magnetic isotropy. The (coincident) six-line spectra Aand C are clearly resolved for all absorbers. The resolution of the spectrum B decreases with increasing fluorine content R, but the intermediate B-lines are visible for all absorbers. The main features of the stabilized spectra of all the absorbers may thus be explained by the Hamiltonian (1) with 2
-
3.
(The influence of the quartic terms of the cf Hamiltonian is discussed elsewhere [3].)The dependence on R of the 1 G spectra (Fig. 2) seems contradictory to this conclusion, However, this dependence only concerns the spectrum of the doublet B. Due to its magnetic isotropy, the spectrum of this doublet is extremely sensitive to weak magnetic fields 121, such as the fields from the nuclear moments of the ligands. Each fluorine ligand gives probably rise to a field H
--
10 G via the ligand hf interaction 15, 61, whereas a water ligand contributes only a dipolar field, H-
2 G. This difference may possibly explain the dependence on R in the 1 G spectra and the field dependence for H<
125 G.The total cf splitting A depends on R. This appears from the differences in the spectral evolutions with increasing applied field H > 125 G, which reflect the relative importance of A and 2 pH. The relative splitting, ARIAo, can be deduced either from a direct comparison of spectra as in figures 2 and 3, or by
considering the field dependence of the line intensity ratio, 1,/12, that reflects the partial alignment of the hf fields. Both methods yield similar results, shown graphically in figure 5. From the value A, = 0.7 cm-I [3], the absolute cf splitting A , can then be obtained. The monofluoro complex has a cf splitting A slightly larger than A,, but for n > 1, A decreases until A , = 0.5 A,. For n > 1 the relative values are consis- tent with the EPR results [I], but the result for n = 1 is not. The absolute values of A are about 5 times larger than those obtained by EPR in liquid solution. Cf splittings A 9 A, might be expected for the ferric fluoro complexes from their low symmetry. In fact, from symmetry arguments alone an axial cf Hamilto- nian
(A
= 0) was expected for n = 1, instead of the Hamiltonian with 2-
+.
Unfortunately, a generally accepted, detailed theory of the cf splitting of a 'S state ion does not exist [7].The observed quadrupole shift for R = 0, E
z
0, isin agreement with the octahedral symmetry of the ferric hexaquo complex, but very small shifts are observed for n
>
0(I
EI
% 0.05 mm/s) too. The lack of a substantial line broadening in the strong field spectra shows that the quadrupole interaction is small.The hf field in the strong field spectra increases from 580 kG for n = 0 to 592 kG for n = 4, as shown in figure 5. Ignoring the cf splitting for H = 6 200 G this hf field is identified with the saturation field H, (the decrease observed for n = 1 may possibly be attributed to an insufficient decoupling of the cf field splitting, which has its maximum value for n = 1). The increase in H, with the exchange of H 2 0 with F - is expected from the lower covalency of the F--Fe3+ bond compared to the H,O-Fe3+ bond. The low increase observed, 1:3 kG per fluorine ligand, then
reflects the proximity of F- and H,O in the nephelau- xetic series [8].
The observed isomer shifts are typical of high spin ferric complexes. The increase from 0.50 to 0.53 mm/s for 0
<
n<
4 follows the same trend as the hi? field. In conclusion, it may be said that even if the cf splitting and the hf coupling in the ferric fluoro complexes show a discernible dependence on the number of fluorine ligands, the similarity of the properties of the ligands F- and H 2 0 , especially concerning the hf parameters, is most striking.References
[I] LEVANON, H., STEIN, G., and Luz, Z., J. Chem. Phys. 53 [5] TINKHAM, M., Proc. R. SOC. A 236 (1956) 535.
(1970) 876. [6] HALL, T. P. P., HAYES, W., STEVENSON, R. W. H., and
[2] AFANAS'EV, A. M., GOROBCHENKO, V. D., DEZSI, I., LUKA- WILKENS, J., J. Chem. Phys. 38 (1963) 1977.
SHEVICH, I. I., FILIPPOV, N . I., Zh. Eksp. Theor. Fiz. 62 [7] NOVAK, P. and VELTRUSK?, I., Phys. Stat. Sol. (b) 73 (1976) (1972) 673. [Sov. Phys. JETP 35 (1972) 355.1 575.
