Chemistry Reference
In-Depth Information
potassium by bigger caesium ions. It was speculated that chemical pressure in the
crystal lattice plays a decisive role in this process. To prove the validity of this idea,
57
Fe Mössbauer spectra of K
0.1
Co
4
[Fe(CN)
6
]
2.7
18 H
2
O were recorded at 4.2 K
under applied pressure (Fig.
2.40
). At ambient pressure the sample contains the
paramagnetic [Co
II
(S = 3/2) -NC - Fe
III
(S = 1/2)] building blocks, and the
electronic spins of Co
II
and Fe
III
sites are oriented sufficiently long in one direction
such that the Mössbauer spectrum reflects a well resolved magnetically split sextet
with effective field of 165 kOe at the
57
Fe nucleus. Increase of pressure to 3 kbar
yields a spectrum that shows a dominating singlet (dark grey) characteristic of LS-
Fe
II
with S = 0 at the expense of the collapsing magnetic sextet from remaining LS-
Fe
III
ions. The spectrum recorded under 4 kbar shows only the singlet arising from
LS-Fe
II
ions [
73
].
2.3.4.2 Orbital Magnetism in a Rigorously Linear Two-Coordinate
High-Spin Fe
II
Compound
Reiff et al. have studied the linear two-coordinate HS Fe
II
compound Bis
(tris(trimethylsilyl)methyl) Fe
II
with Mössbauer spectroscopy and observed an
enormously large effective magnetic field at the Fe
II
site of 152 T [
74
]. This is the
largest field ever observed in an iron containing material. The molecular structure
of the compound is shown in Fig.
2.41
[
75
].
The
57
Fe Mössbauer spectrum recorded at 4.2 K in zero applied magnetic field is
also shown in Fig.
2.41
. The authors have plausibly interpreted the origin of this
extremely large field as being due to an unusually large orbital contribution, B
L
,from
electron movement around the molecular axis. In
Sect. 2.2.3
above it has been outlined
that the effective internal magnetic field B
int
at the Mössbauer nucleus observed in a
Mössbauer experiment results from several contributions, the Fermi-contact field B
c
,
the contribution from orbital motion of valence electrons, B
L
, a contribution B
D
,called
spin-dipolar field, and eventually an externally applied magnetic field B
ext
. The term B
c
roughly contributes 12.5 T per electron spin, i.e. in total 50 T in the present case of HS-
Fe
II
with four unpaired valence electrons. B
D
is generally comparatively small and can
be neglected here. Since B
ext
was zero in this experiment, one has found for the orbital
contribution B
L
a value of roughly 200 T (B
c
and B
L
have opposite signs). This sur-
prisingly large orbital contribution arises from the fact that there are no in-plane ligands
(only the axial ligands) to impede the orbital circulation of the electrons within the
doubly degenerate E
g
(d
xy
,d
x
2
-y
2) ground state, which, in addition, does not suffer
appreciably from a Jahn-Teller distortion.
2.3.5 Industrial Applications of Mössbauer Spectroscopy
The eminent capability of non-destructive phase analysis with Mössbauer spec-
troscopy has been used in the multidisciplinary field of materials science, particularly
