Chemistry Reference
In-Depth Information
Fig. 3.2 Quadrupole
splitting (D) values at RT
versus coordination number
for low-spin (II, III) and
high-spin (2+, 3+) Fe in
compounds and minerals
Fig. 3.3 Trans and cis
configuration of a O
4
(OH)
2
octahedron
z
z
TRANS
CIS
y
y
x
x
O
2-
OH
-
whereas it is usually much lower than 40 T for Fe
2+
due to the reducing effect of
the orbital contribution in B which is generally more pronounced for Fe
2+
than for
Fe
3+
. However, the Fe
2+
magnetic spectra are often very complex due to the
perturbation of a relatively strong quadrupole interaction and/or to structural and
magnetic disorder effects.
3.2.3 Morphological Effects and Isomorphous Substitution
In soils, iron oxides and oxyhydroxides are commonly poorly crystallized and as such
occurring as agglomerates of ultra fine particles. In various techniques, such a
behavior becomes a perturbing factor in the characterization of iron minerals. In
X-ray diffraction, for instance, the line width of the reflections may increase con-
siderably, often inhibiting a clear-cut assignment and also hindering an accurate
determination of the other well-crystallized components. In MS the poor crystallinity
is mainly reflected by a superparamagnetic behavior, which results in a magnetic
transition (sextet-doublet transition), that occurs at much lower temperatures than in
the well-crystallized material. However, at very low temperatures the small-particle
system still reproduces to a large extent the hyperfine parameter values of the bulk
compound so that from the point of view of qualitative analysis the compound can
still be relatively well determined. The superparamagnetic fluctuations of the par-
ticles' magnetic moments are described by a relaxation time given by
s
¼
s
0
exp KV
=
kT
ð
Þ
ð
3
:
3
Þ
