Chemistry Reference
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Fig. 3.10 Mössbauer
spectrum of a hematite
sample showing
simultaneously AF and WF
phases
-11
-9
-7
-5
-3
-1
1
3
5
7
9
11
Velocity (mm/s)
Pedogenic hematite consists mainly of small crystallites and, similar to goethite,
the spectral features are governed by superparamagnetic relaxation effects. However,
the effective anisotropy constant K of hematite is of the order of 10 4 J/m 3 [ 82 ],
yielding blocking temperatures which are higher than in the case of goethite. Con-
sequently, in most cases the spectra are still magnetically split at RT. Nevertheless,
due to microcrystalline effects, the lines may still be asymmetrically broadened. The
origin of the field distributive and reducing effects is similar to that of goethite,
although, the reduction of B caused by surface effects has been well established in
hematite by MS on 57 Fe surface enriched hematite [ 83 , 84 ] and by surface studies
with conversion electrons MS [ 85 , 86 ]. For very small crystallite sizes with particle
dimension D 8 nm a doublet is observed at RT with a quadrupole splitting D in the
range 0.5-1.1 mm/s [ 87 ], apparently strongly dependent on the particle size.
Morphological effects such as particle size, lattice imperfections and the
presence of micro- and macropores have also a pronounced influence on the Morin
transition. First of all, the transition temperature lowers with decreasing particle
size [ 88 - 90 ] and for particle sizes smaller than about 20 nm the Morin transition is
even completely suppressed, resulting in a single WF phase down to 4 K [ 82 ].
Further, the normally sharp Morin transition becomes a temperature region with
the coexistence of the two phases in which the AF phase diminishes in favor of the
WF phase with increasing temperature. A typical spectrum in this transition region
is shown in Fig. 3.10 .
This region broadens considerably with decreasing particle dimensions and
with increasing structural defects [ 91 ], and the coexistence of both the AF and WF
phase even can extend down to 0 K. Such a case is represented in Fig. 3.11 where
the relative areas (RA) and the quadrupole splitting do not change below 150 K.
The Morin transition temperature can then be defined as that temperature at which
the amount (RA) of AF phase is reduced to half of its initial value at low
temperatures.
The influence of the average particle size on the Morin transition temperature has
been mainly investigated for synthetic samples and is represented in Fig. 3.12 . From
 
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