Chemistry Reference
In-Depth Information
varied in energy by giving the source velocities of the order of cm/s. This enables
to study the different hyperfine interactions related to the electronic shells of the
Mössbauer active atoms in solids which, in a further stage, yield valuable infor-
mation about structural and magnetic properties of materials. Up to now, the
Mössbauer effect has been observed for nearly 100 nuclear transitions in about 80
isotopes, distributed over 43 elements. Of course, as with many other spectro-
scopic methods, not all of these transitions are suitable for practical studies and
about twenty elements remain for applications. However, it is a gift of nature that
the resonant absorption effect is easily achieved in the iron-57 isotope which has
an abundance of 2.14 % in natural iron. Considering the importance of the element
iron in many branches of science and technology, it is obvious that since 1960, this
new spectroscopic technique has proven to be very useful in the study of all kinds
of iron-bearing materials. In particular, the abundance of iron in the earth's crust
(4th element in wt %) renders this kind of spectroscopy extremely suitable for the
characterization of soil materials and minerals. Moreover, a major advantage of
Mössbauer spectroscopy is the fact that it probes the influences on the iron nucleus
locally. This means that, through the determination of the hyperfine parameters,
not only the different iron-bearing components in a sample can be distinguished,
but also the different ''types'' of iron present in a mineral can be detected. This
leads to a variety of applications in geology and soil sciences such as the quali-
tative and quantitative analysis with respect to the various mineralogical com-
pounds and the determination of the oxidation state and coordination of iron in
minerals.
From experimental point of view, the equipment for Mössbauer spectroscopy
(MS) is nowadays relatively simple and not expensive. Especially, since the data
collection can be achieved by a compact electronic unit connected to a PC, the
latter not necessarily being of high performance, a complete Mössbauer set-up
with low-temperature facilities is cheaper than say, an X-ray diffraction apparatus.
However, most of solid-state laboratories or particularly geological institutes in the
present case, do not possess Mössbauer spectrometers in their standard equipment.
This is often due to the severe rules which are enforced with radioactive source
acquisition and handling. Moreover, the relatively serious recurrent cost of sour-
ces, having a half-life time of 270 days in the case of
57
Fe demands a permanent
operation of the spectrometer, which is not evident in case of limited need. Even
though the application for the characterization of soils and minerals does not
require a deep fundamental physical knowledge, MS is for all the aforementioned
reasons not so popular as a standard technique in the community of geologists and
soil scientists as it should be. Therefore, a sound and permanent cooperation
between Mössbauer laboratories and institutes related to earth sciences is neces-
sary and should still further be promoted.
Another feature that lowers to some extent the general popularity of MS in
comparison with other techniques is related to the spectral analysis. Mössbauer
spectroscopy is not a so-called push-button technique, yielding directly consistent
results after each measurement. As in many techniques, the spectra need to be
refined using home-made or commercial computer programs. But, the analyzing
