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the radiation source. WinNormos for Igor Pro software has been used for the
quantitative evaluation of the spectral parameters (least-squares fitting to Lorentzian
peaks). The minimum experimental line widths were 0.20 mms
-1
. The temperature of
the samples was controlled by an MBBC-HE0106 MÖSSBAUER He/N
2
cryostat within an
accuracy of ±0.3 K. Isomer shifts were determined relative to
D
-iron at 298 K. The
following spectrum was obtained:
T
rel
1.000
0.998
-6
-4
-2
0
2
4
6
v
/ mms
-1
Figure 33
57
Fe Mössbauer spectrum of (THA)
5
[SiW
11
O
39
Fe],
Type: doublet,
G
= 0.53(1) mms
-1
,
'
E
Q
=
0.84(1) mms
-1
,
*
FWHM
= 0.42(1) mms
-1
.
Acquisition of a proper spectrum turned out to be difficult for the Fe-Keggin cluster as
the eleven tungsten atoms (atomic number of 74!) per cluster effectively shield the iron
nucleus from the gamma radiation, decreasing the effective cross section and increasing
radiation loss due to
Compton scattering
. Therefore only a very thin film of the ionic
liquid could be used, considerably decreasing the amount of active iron which resulted
in a very weak signal. Yet it was possible to detect a response for {W
11
Fe
III
} when
measured for five days. The value for the isomer shift was found to be
G
= 0.53 mms
-1
.
In addition quadrupole splitting of
'
E
Q
= 0.84 mms
-1
is observed which can be
attributed to minor extend to the non-cubic ligand symmetry surrounding the Fe-
centre. The main reason however should be found in an unsymmetrical electron
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