Chemistry Reference
In-Depth Information
Co
II
The
NIESST
effect
was
also
studied
in
SCO
compounds,
viz.
X
2
nH
2
O
ð
X
¼
ClO
4
;
n
¼
1
=
2
;
X
¼
Cl
;
n
¼
5
Þ;
where terpy
is the tridentate ligand terpyridine [
65
]. The perchlorate salt shows thermal SCO
with T
1/2
around 200 K and a HS fraction of nearly 100 % at room temperature,
whereas the chloride salt possesses a somewhat stronger ligand field giving rise to
thermal ST at much higher temperatures (the HS fraction starts to rise around
200 K, reaches ca. 20 % at 320 K and obviously would increase further) [
65
].
Conventional Mössbauer absorption measurements were performed on the corre-
sponding systems doped with 5 % Fe
II
, which was found to be in the LS (S = 0)
state at all temperatures under study [
65
]. The emission spectra of the
57
Co-
labelled cobalt complexes were measured using a home-made resonance detector,
which operates as conversion-electron detector with count rates 10-20 times
higher than those of a conventional detector. At room temperature, the nucleogenic
57
Fe ions were found to have relaxed to the stable
1
A
1
LS ground and gave the
same MAS Mössbauer spectrum like the corresponding Fe
II
compound. On low-
ering the temperature a doublet from a metastable Fe
II
-HS state appears in the
MES spectra with increasing intensities. The perchlorate derivative with the
weaker ligand field strength shows, at comparable temperatures, a considerably
higher amount of Fe
II
-HS fraction than the chloride derivative with the stronger
ligand field. For instance, the emission spectra recorded at 100 K displayed in
Fig.
2.35
demonstrate this effect very clearly. Thus, it turns out that the lifetime of
the nuclear decay-induced metastable HS state of Fe
II
is short in strong ligand field
surroundings and long in weak ligand fields. In other words in relation to Fig.
2.14
,
the stronger the ligand field, the larger is the difference between the lowest vib-
ronic energy levels of HS and LS states, and the shorter is the lifetime at a given
temperature. This is known as ''reduced energy gap law'' which holds for all these
NIESST studies [
59
].
57
Co
=
Co terp
ð Þ
2
Fig. 2.35 Mössbauer emission spectra of [
57
Co/Co(terpy)
2
]X
.
nH
2
O(X= ClO
4
-
,n= ;
X = Cl
-,
n = 5) as source material vs. K
4
[Fe(CN)
6
] as absorber (which was kept at 298 K)
recorded at 100 K with a conversion-electron detector. Left X = ClO
4
-
,n= . Right X = Cl
-
,
n = 5 (from [
65
])
