Chemistry Reference
In-Depth Information
where KV is the energy barrier that has to be bridged in order to change the
direction of the magnetization, with K the effective anisotropy energy density,
V the particle volume and kT is the thermal energy (k Boltzmann's constant). The
factor s
0
is of the order 10
-9
s and is slightly dependent on the magnetization and
the anisotropy. From this relaxation formula, and taking s to be equal to the
Larmor precession time s
L
of the nuclear spin in a local magnetic field, which is
approximately 10
-8
s, the magnetic transition temperature, which is called
'blocking temperature' in this case, can in principle be related to the particle size.
However, due to all kinds of uncertainties, in particular regarding the value of the
anisotropy constant K, it remains difficult to determine this size quantity accurately
from the spectra. Nevertheless, the general spectral behavior observed at a few
different temperatures (e.g. room temperature and 80 K) may already be fairly
indicative for the degree of crystallinity of the compound. Moreover, the values of
the hyperfine parameters, in particular the magnetic hyperfine field, may give
similar information as well.
Another important feature of minerals is the isomorphous substitution for iron
by another element such as aluminum, calcium, silicon, etc. Such a substitution
may alter the hyperfine parameters to some extent and relationships can then be
established enabling an estimate of the degree of substitution from the hyperfine
parameters. Unfortunately, it often happens that both morphological effects and
substitution concurrently give rise to similar changes in the spectrum, thus leading
to ambiguous conclusions. In the next sections the possibilities of the morpho-
logical characterization by the Mössbauer effect will be discussed for some
particular minerals.
3.3 Characterization of Iron Oxides and Hydroxides
Iron oxides and hydroxides are the most important iron-bearing constituents of
soils, sediments and clays. To characterize the samples, i.e. the identification of the
different minerals present and the determination of their morphology and chemical
composition, a variety of standard techniques are commonly used such as X-ray
and electron diffraction, chemical analyses, optical and electron microscopy, infra-
red spectroscopy and thermal analysis (DTA, DTC,…). Most of these techniques
are further applied in conjunction with selective dissolution or other separation
methods in order to obtain more specific information about particular components
in the complex soil system. In addition to all those characterization methods, MS
has proven to be a valuable complementary technique for the study of these kinds
of materials and in particular for the characterization of iron oxides and hydroxides
which are usually poorly crystallized.
Most oxides and hydroxides in soils are indeed known to be less well or rather
poorly crystallized. This feature results in the first place in a lowering of the magnetic
transition temperature yielding a doublet at temperatures were normally a sextet is
