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still shows the typical Fe 2.5+ and Fe 3+ sextets at 100 K which is below the normal
Verwey transition for stoichiometric magnetite [ 35 ].
3.3.9 Maghemite
Maghemite, which is a fully oxidized form of magnetite, has the structural formula
Fe 3 þ
A ½ Fe 3 þ
5 = 3 h 1 = 3 B O 4 where h represents again the vacancies on the octahedral
sites. The corresponding Mössbauer spectrum consists of a somewhat broad-lined
Fe 3+ sextet, which is in fact composed of two non-resolved sextets from Fe 3+ in
tetrahedral and octahedral sites, respectively (Fig. 3.15 a). Only by using an
external field, the hyperfine field and the isomer shift of both sextets could be
accurately determined [ 125 ] (see Sect. 3.6.4 ). Hyperfine parameter values are
given in Table 3.7 .
Maghemite is commonly formed from oxidation of fine-course lithogenic
magnetite, although its abundance in tropical and subtropical regions can also be
explained by the conversion of for instance goethite through fires under reducing
conditions. Also fine-particle magnetite produced by bacteria may lead to
maghemite after spontaneous oxidation. In most of those cases, maghemite pos-
sesses a rather small-particle morphology leading to a superparamagnetic beha-
vior. Maghemite can then appear in the Mössbauer spectrum as a doublet at RT.
From the effective anisotropy constant of about 10 Jm -3 [ 123 ] a superparamag-
netic doublet is expected at RT for particle sizes smaller than about 5 nm. This
Fig. 3.15 Mössbauer spectra of maghemite: well-crystallized maghemite at RT (a) and at 8 K
(b) fitted with A-site and B-site sextets according to parameters derived from external-field
spectra; poorly crystallized maghemite at RT (c) and at 80 K (d) (after da Costa et al. [ 122 ])
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