Chemistry Reference
In-Depth Information
Fig. 2.41 Molecular
structure of
Bis(tris(trimethylsilyl)
methyl)Fe
II
, a rigorously
linear two-coordinate
compound of Fe
II
in HS state
with ideal staggered D
3d
symmetry [
75
].
57
Fe
Mössbauer spectrum of
Bis(tris(trimethylsilyl)
methyl)Fe
II
, recorded at
4.2 K in zero applied
magnetic field [
74
]. The
internal magnetic field
derived from the distance
between the two outermost
resonance lines, S
1
-S
2
, is 152
T, the largest field ever
observed in an iron
compound
for the analysis of iron containing materials of technical relevance such as steel,
alloys, pigments, oxides, corrosion products, to name a few. But other ''Mössbauer-
active'' nuclides have also been used. Within the limited scope of this article we must
confine the discussion in the following to only one representative example, viz.
corrosion studies.
Roughly six different iron oxides and oxyhydroxides of iron are known as
corrosion products, which may be formed by corrosion reactions in steel, metallic
iron, and iron containing alloys under different conditions. These corrosion
products can be distinguished by
57
Fe Mössbauer spectroscopy (Fig.
2.42
).
Magnetite, Fe
3
O
4
, is an inverse spinel compound of formula Fe
III
Fe
II
Fe
III
O
4
;
where half of the Fe
III
ions (those outside the square brackets) are in tetrahedral
sites and the other half (inside the square brackets) in octahedral sites. All Fe
II
ions
are in octahedral sites. According to these three kinds of iron ions one should
expect three different resonance signals in the Mössbauer spectrum. This, however,
is not the case. Instead one observes at room temperature two overlapping sextets,
one arising from Fe
III
ions in tetrahedral sites, and the other one is a time-averaged
sextet arising from Fe
II
and Fe
III
ions in octahedral sites with fast electron fluc-
tuations between them (faster than the inverse of the lifetime of the 14.4 keV
nuclear level). The oxides a- and c-Fe
2
O
3
show a magnetically split sextet with a
