Chemistry Reference
In-Depth Information
Fig. 3.48
RT spectrum of non-magnetic (a) and magnetic (b) fraction of a dolerite sample
shape of the spectrum. A sextet with low hyperfine field (B = 42 T) and high
isomer shift (d
Fe
= 0.71 mm/s) could be added pointing to a Fe
2+
sextet as
expected for a titanomagnetite (Fig.
3.48
b).
3.7.4 The Strong Means: External Magnetic Fields
The question often arises to what extent one can resolve sextets in a Mössbauer
spectrum. Of course there is no general answer. If sextets strongly overlap, it will
be very difficult to resolve them in a correct way. Moreover, if they possess a
hyperfine-field distribution, it will be practically impossible to discern the different
components, even if other hyperfine parameters like the quadrupole shift or the
isomer shift are significantly different. Only when the lines exhibit distinct
shoulders one can expect another sextet component to be present because strong
anomalies in hyperfine field distributions for the same iron species are rare.
In some cases, Mössbauer measurements with the sample in a high external
field might be a welcome solution to improve the resolution of sextets. Ferri-
magnetic compounds have the advantage that their magnetic moments align in a
more or less strong external magnetic field. Consequently, the external field is
added to the hyperfine field of the site with the smallest magnetic moment and
subtracted from that of the site with the strongest magnetic moment because the
hyperfine field is opposite to the magnetic moment.
A typical example is maghemite with its strongly overlapping sextets of A and
B sites. This spectrum can be simply fitted with 2 sextets, but the resulting hyperfine
parameters will depend on the initial values used in the fit. This ill-posed problem can
only be solved by performing measurements in an external magnetic field, where the
outer sextet from the A sites is well separated from that of the B sites (Fig.
3.49
),
yielding accurate values for both isomer shifts and hyperfine fields.
