Chemistry Reference
In-Depth Information
asymmetric spectrum at RT has been found for a Si-rich hematite sample from
Elba, pointing to the presence of the AF phase above 265 K [
90
].
In conclusion, the characteristic hyperfine parameters and the high magnetic
transition temperature T
N
and blocking temperature T
B
for hematite render MS
very suitable for the identification of this material in natural samples. Moreover,
when used in conjunction with other techniques, MS yields some indicative
information about morphological or substitutional properties through the deter-
mination of the magnetic hematite phases present in the sample. Although small-
particle effects are not so pronounced as in the spectra of goethite, the spectra of
hematite may still show asymmetric lines, particularly at room temperature.
Hence, appropriate fitting procedures such as the ones based on a hyperfine field
distribution are recommended, especially when the hematite sextet overlaps with
other sextets.
3.3.7 Fe-Ti Oxides
Related to hematite, ilmenite, FeTiO
3
, has a similar rhombohedral structure.
Alternating planes are occupied by Ti
4+
and Fe
2+
, breaking down the strong
magnetic interactions that occur in hematite. It is therefore a weak antiferromagnet
with T
N
= 55 K [
107
]. Although the whole range between hematite and ilmenite,
Fe
2-x
Ti
x
O
3
can be synthesized, natural samples of ilmenite possess a relatively
high amount of Ti (0.75 \ x \ 0.95). Ilmenite with a higher degree of stoichi-
ometry was found in lunar samples [
108
]. At room temperature a doublet is
observed with a high isomer shift, d
Fe
= 1.1 mm/s and a low quadrupole splitting
D = 0.71 mm/s. These typical hyperfine parameters for ilmenite, make it rela-
tively easy to discern this oxide in the Mössbauer spectrum. At 80 K the quad-
rupole splitting is larger and amounts to about 1.0 mm/s. At 5 K only a very low
magnetic hyperfine field (*4 T) is observed due a strong opposite orbital con-
tribution of Fe
2+
[
107
,
109
].
Deviation from stoichiometry implies the presence of Fe
3+
which is clearly
observed in the spectra by an asymmetry, namely the left absorption line is broader
and deeper. The spectra can then be analyzed with two doublets: one from Fe
2+
having hyperfine parameters very close to those of pure ilmenite, and one from
Fe
3+
with typical hyperfine parameters for trivalent iron [
110
] (see Table
3.5
).
Another well-known mineral is pseudobrookite, which consists of a complete
solid solution series between ferrous FeTi
2
O
5
and ferric Fe
2
TiO
5
. Also in this case,
the Mössbauer spectra reveal iron species that vary from Fe
2+
over mixed valences
to Fe
3+
. However, the structure contains two different sites (8f and 4c) where iron
can be present, resulting in two doublets for each iron valence. The hyperfine
parameters according to Guo et al. [
111
] are summarized in Table
3.5
.
