Chemistry Reference
In-Depth Information
twofold effect on the magnetic properties, i.e. it lowers the magnetic transition
temperature and reduces the saturation hyperfine field. However, in view of the
high Néel temperature, the sextet-doublet transition still occurs at high tempera-
tures above RT and is therefore not of practical diagnostic use. The reduction of
the saturation value of B, which can already be observed at 80 K in view of the
very small variation of B between 80 and 4 K, is also rather small. At 80 K a
reduction of about 0.04 T per at % Al has been found, whereas at RT it amounts to
0.08 T per at % Al [
92
,
102
].
Similarly as for goethite, a linear relationship of B as a function of Al content and
particle size has been proposed [
102
] which is valid at RT and for concentrations less
than 10 %Al. However, all these results are derived from synthetic samples, mostly
obtained from goethite. Therefore, particularly at RT, the magnetic hyperfine field
will still be largely influenced by morphological effects. Moreover, most preparation
methods, based on the decomposition of oxyhydroxides, result in inhomogeneous Al
substitution [
96
]. A more clear-cut picture for the dependence of the hyperfine field
on Al substitution is obtained for hematites prepared from oxinates [
103
] where a
reduction of 0.061 at RT and 0.032 at 80 K per at % Al is observed.
Somewhat more pronounced effect of Al substitution is reflected in the behavior
of the Morin transition. With increasing Al content the transition temperature T
M
decreases and the transition region becomes significantly broader [
97
]. Moreover
the Morin transition is completely suppressed at about 10at % Al in bulk hematite
[
99
] and even at somewhat lower concentrations (8 at %) for less crystalline
hematite [
100
]. On the other hand, the effect of Al on the Morin transition tem-
perature is smaller in the case of more homogeneous Al substitution in samples
prepared from oxinates [
103
]. Using the aforementioned definition, the Morin
transition temperature for as-such obtained hematite species decreases by 8 K per
at % Al. Because the spectral implications of Al substitution are quite similar to
those of morphological effects, the separation of both effects remains a major
problem and additional techniques are necessary for the characterization of natural
samples.
Another element which is a possibly abundant candidate for substitution of iron
in natural hematite samples is manganese [
8
]. Mn was substituted for iron up to
about 18 % in synthetic samples [
104
], but, it is believed that in natural samples
the substitution is much lower (\5at%)[
105
]. In a study of some Mn-hematites
prepared from Mn-substituted goethites [
106
] a somewhat smaller decrease for the
hyperfine field of the WF phase at RT is observed in comparison with that of
similarly prepared Al-hematites, which is expected in view of manganese being a
magnetic ion. On the other hand, manganese reduces the Morin transition tem-
perature more rapidly, and already at about 4 at % Mn the transition is completely
suppressed [
106
]. A still more drastic effect on T
M
is caused by Ti substitution
which is also abundant in nature. Less than 1 at % Ti completely inhibits the
Morin transition [
27
]. However, Ti occurs rather in high concentration tending
more to the isostructural ilmenite (FeTiO
3
). Another important substitution ele-
ment could be silicon, although little information in that respect is found in the
literature. There are indications that silicon increases T
M
slightly. For example, an
