Chemistry Reference
In-Depth Information
Fig. 3.4 RT Mössbauer spectra of poorly crystallized goethite fitted with two doublets (a) and a
relatively well crystallized goethite (b) with corresponding hyperfine field distribution (c)
form, consisting of conglomerations of small, usually needle-shaped crystallites.
Consequently, the spectral behavior is mainly governed by superparamagnetic
relaxation (Eq.
3.3
) with an anisotropy constant K of the order 10
3
Jm
-3
. The spectra
of goethite exhibit simultaneously a sextet and doublet over a wide temperature
range, which can be attributed to the non-uniformity of particle morphology within
the goethite sample. A doublet spectrum is obtained at room temperature for mean
crystallite sizes smaller than about 15 nm. Such a doublet consists of somewhat
broadened lines (Fig.
3.4
a) indicating the presence of a distribution in the quadrupole
splitting with an average value of about 0.55 mm/s (Fig.
3.8
). This doublet resembles
that of many other paramagnetic or superparamagnetic Fe
3+
-bearing species and is
therefore practically not useful for identification of goethite. However, the
magnetically split spectrum of poorly-crystalline goethite, commonly obtained at
lower temperatures, is usually recognized by its asymmetrically-broadened lines
(Fig.
3.4
b). Moreover, its average hyperfine field B
av
is usually lower than that of
well-crystallized goethite and decreases with increasing temperature in different
ways depending on the particle size. Therefore its value at a certain temperature
