Chemistry Reference
In-Depth Information
Table 3.5
Representative hyperfine parameters for Fe-Ti oxides
Mineral
T (K)
Iron
B (T)
2e or D (mm/s)
d
Fe
(mm/s)
Fe
2+
Ilmenite FeTiO
3
RT
-
0.71
1.1
Fe
2+
Ilmenite (non-stoichiom)
RT
0.65-0.70
1.0-1.1
Fe
3+
Fe
1+x
Ti
1 - x
O
3
0.3-0.5
0.3
Fe
2+
Pseudobrookite (ferrous)
RT
8f
1.10
2.16
Fe
2+
FeTi
2
O
5
4c
1.06
3.15
Fe
2+
Pseudobrookite (Intermed.)
RT
8f
1.1-1.2
1.6-2.1
Fe
2+
Fe
1+x
Ti
2-x
O
5
4c
1.04-1.06
2.8-3.1
Fe
3+
8f
0.38-0.41
0.54-0.58
Fe
3+
4c
0.34-0.39
0.85-1.00
Fe
3+
Pseudobrookite (ferric)
RT
8f
0.38
0.57
Fe
3+
Fe
2
TiO
5
4c
0.38
0.92
3.3.8 Magnetite
Magnetite (Fe
3
O
4
) is a ferrimagnetic spinel oxide with a Néel temperature of
858 K. It naturally occurs often in a fairly crystallized form and its presence can
readily be recognized in a Mössbauer spectrum. Magnetite has the following
structural formula
2
B
O
4
where the octahedral (B-site) ferrous and
ferric ions merge into Fe
2
:
5
þ
due to a fast electron hopping in pairs above the
so-called Verwey transition ([125 K). Consequently, the RT spectrum of mag-
netite exhibits two partly resolved sextet patterns (Fig.
3.13
) resulting from tet-
rahedral (A site) Fe
3
þ
with B = 49.1 T, 2e = 0 mm/s, d
Fe
= 0.28, and octahedral
(B site) Fe
2
:
5
þ
with B = 46.0 T, 2e = 0 mm/s, d
Fe
= 0.66 mm/s. The latter sextet
possesses a somewhat broader linewidth (C * 0.5 mm/s) because it is in
fact composed of two B-site sextets with B = 45.6 T, 2e = 0.18 mm/s,
d
Fe
= 0.66 mm/s, and B = 46.0 T, 2e =-0.05 mm/s, d
Fe
= 0.66 mm/s respec-
tively, as a result of two different possible directions of the magnetic hyperfine
field with respect to the local B-site EFG principal axes [
112
]. However, from the
ferric A-site component, magnetite is readily recognized by MS at RT and for
characterization purposes a two-sextet fitting is adequate. For ideal magnetite the
sextet area ratio S(B)/S(A) has to be 2:1. In practice, this ratio is somewhat lower
and amounts to about 1.8:1 at RT. This is related to the Mössbauer fraction f of the
Fe
2.5+
on the B sites being somewhat lower than that of Fe
3+
on the A sites.
However, deviations from this ideal ratio are often observed. The main reason
is that magnetite may partly be oxidized by replacing Fe
2
þ
by Fe
3+
and introducing
vacancies. Because the electron hopping occurs in pairs, this oxidation does not
result in another intermediate Fe valence on the octahedral sites, but in a decrease
of the Fe
2.5+
component and the appearing of a B-site Fe
3+
sextet for which the
hyperfine parameters do not appreciably differ from those of the A-site sextet.
Together with the introduced vacancies, a decrease of the Fe
2
:
5
þ
B-site sextet area
and an apparent increase of that of the A-site sextet are observed. Therefore, one
cannot speak anymore about one A-site and one B-site sextet—a mistake that is
A
Fe
3
þ
Fe
2
:
5
þ
