Vij D.R. Handbook of Applied Solid State Spectroscopy
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A powerful demonstration of the use of Mössbauer spectroscopy to study
electronic valence is provided by ytterbium. The typical 4f electronic
configuration for rare earth ions in compounds is 3+. Yb occurs at the end of
the 4f series and Yb
3+
has the 4f
13
configuration, which is magnetic. However,
by gaining an extra electron to form Yb
2+
, a full 4f electronic shell is obtained
and Yb
2+
is nonmagnetic. In Figure 5.33 we show the
170
Yb spectra of
intermetallic YbMn
2
Ge
2
and YbMn
2
Si
2
[57]. The Yb ion in YbMn
2
Ge
2
is
divalent and therefore nonmagnetic. The spectrum of YbMn
2
Ge
2
at 4.5 K is a
typical nonmagnetic triplet. YbMn
2
Si
2
has trivalent Yb and is consequently
magnetic. The spectrum of YbMn
2
Si
2
at 40 K is also a quadrupole-split triplet,
being obtained well above the magnetic ordering temperature of the Yb
3+
sublattice. However, the quadrupole splitting in the silicide is much larger
than in the germanide and is typical of a Yb
3+
state. The series of
YbMn
2
(GeSi)
2
spectra at 4.5 K allows one to determine accurately the relative
amounts of Yb
2+
and Yb
3+
across this series.
Figure 5.33 Top Left––
170
Yb spectra of YbMn
2
Ge
2
(4 K) and YbMn
2
Si
2
(45 K). Right–– 4.5 K
spectra of the YbMn
2
Ge
x
Si
1–x
series. Bottom Left–– Fraction of Yb
2+
in the YbMn
2
Si
x
Ge
1–x
series [57].
5.3 Applications 247
5. Mössbauer Spectroscopy
5.3.8 Industrial Applications
57
Fe Mössbauer spectroscopy is used in numerous industrial applications. We
have used
57
Fe to study the direct reduction process in steel-making [58, 59].
In Figure 5.34 we show the RT
57
Fe Mössbauer spectrum of an Australian
iron ore raw material, used as the starting material in our study. About 95% of
the spectral area is a magnetically split sextet with a hyperfine field of 51.5 T.
This corresponds to hematite (D-Fe
2
O
3
). The remaining 5% of the spectrum is
a paramagnetic, quadrupole-split doublet due to wüstite (Fe
x
O, with x slightly
less than 1). We also show the spectrum of this iron ore material after it has
been reduced in H
2
at 600°C for 13 minutes. The spectrum of the reduced
material contains about 90 wt%D-Fe, with traces of magnetite (Fe
3
O
4
).
Figure 5.34 RT
57
57
Fe Mössbauer spectroscopy also allowed us to follow the reduction
process quantitatively. In Figure 5.35 we show the RT spectra of the iron ore
after various reduction times [58, 59]. After two minutes, the hematite is gone
and the sample comprises D-Fe, magnetite and wüstite. In Figure 5.35 we also
show the relative amounts of these three phases as a function of reducing
time.
5.3.8.1 CEMS (Conversion Electron Mössbauer Spectroscopy)
As mentioned in Section 5.2, conversion electron Mössbauer spectroscopy is
ideally suited to the study of surface phenomena. It is well known that many
amorphous materials, which are malleable in the as-quenched state, become
quite brittle upon heating to a critical temperature below the crystallization
temperature. Thus, the embrittled materials remain amorphous. It is believed
that the embrittlement phenomenon is related to changes in the surface of the
amorphous ribbons, possibly involving the propagation of surface micro-
cracks into the bulk material. In an attempt to investigate the surface
Fe spectra of raw iron ore (a) before and (b) after reduction in hydrogen [58].
248
magnetism of an embrittled amorphous material we carried out both
transmission and CEMS Mössbauer spectroscopy on amorphous Fe-Si-B. In
Figure 5.36a and 5.36b we show
57
Fe transmission and CEMS Mössbauer
spectra of as-quenched amorphous Fe-Si-B [60]. In both cases, the spectra are
broad, magnetically-split sextets. The most significant difference between the
two spectra is the relative intensity of lines 2 and 5.
As discussed in Section 5.1, the intensity of these two lines, relative to the
other four, provides a measure of the magnetic texture of the sample. The
transmission spectrum samples the bulk material and the relative line
intensities indicate a random arrangement of magnetic moments. In contrast,
the enhanced intensities of lines 2 and 5 in the CEMS spectrum, which
samples the surface of the material, indicates significant magnetic texturing of
the surface with the Fe magnetic moments lying preferentially along the plane
of the amorphous ribbon samples (see Section 5.1). In Figure 5.36c and 5.36d
we show the spectra of an annealed sample of Fe-Si-B (360°C for 5 min). The
annealed sample is still amorphous, as indicated by the broad magnetic
spectra, but is now brittle. Changes in the CEMS spectra indicate structural
relaxation on the surface of the annealed sample, which probably causes the
embrittlement.
Figure 5.35 Left–– Reduction time-dependence of the RT spectra. Right–– Relative amounts of
D-Fe, Fe
3
O
4
, and Fe
x
O in reduced iron ore as a function of reducing time [58, 59].
5.3 Applications 249
5. Mössbauer Spectroscopy
Figure 5.36 RT
57
Fe Mössbauer spectra of as-quenched amorphous Fe-Si-B (spectra a and b)
and embrittled Fe-Si-B (spectra c and d), obtained in transmission and CEMS modes [60].
5.3.8.2 CEMS––Thin Films
Figure 5.37 shows room temperature CEMS spectra for a 316 stainless steel
sheet covered by increasing thicknesses of D-Fe. As the iron overlayer gets
thicker, fewer conversion electrons from the stainless steel substrate are able
to escape, and the spectrum is dominated by the D-Fe sextet. A plot of the
relative area of the 316 stainless steel component as a function of iron
overlayer thickness shows that the CEMS signal derives from two distinct
conversion electron populations. The softer K-electrons (7.3 keV) have a
penetration depth of 52(8) nm and are lost quite rapidly, while the L-electrons
Figure 5.37 Left–– RT
57
Fe Mössbauer CEMS spectra of a 316 stainless steel sheet covered by
increasing thicknesses of D-Fe. Right–– Relative area of the stainless steel spectral component
as a function of D-Fe layer thickness [Perry and Ryan, unpublished].
250
A comparison of XBS and CEMS data for a 540-nm thick iron overlayer
(Figure 5.38) shows that the 6.4-keV Fe-K
D
X-rays are far more penetrating
(penetration depth = 3.5(2) Pm) and provide information from quite deep into
the sample. The combination of CEMS and XBS measurements allows surface
and bulk contributions to be distinguished and is particularly valuable in
corrosion and other surface-damage studies where the samples are too thick
for conventional transmission work.
Figure 5.38
57
Fe Mössbauer XBS (top) and CEMS (bottom) spectra of a 316 stainless steel
sheet covered by a 540-nm layer of D-Fe [Perry and Ryan, unpublished].
5.3.8.3 SEDM (Selective Excitation Double Mössbauer) Spectra
Figure 5.39 shows a conventional Fe transmission spectrum (top) followed by
SEDM spectra obtained by setting the constant velocity drive (source) on each
of lines 1, 2, and 3 in succession. This illustrates that SEDM actually leads to
the population of a specific hyperfine state in the target nucleus and is not
simply a scattering or fluorescence technique. Driving line 1 yields emission
only at line 1 as the
2
3
I
m state has one allowed transition. Driving line 2
populates the
2
1
I
m excited state and there are two allowed decays (and
hence two lines in the SEDM spectrum) to the
2
1
I
r m ground states,
yielding lines 2 and 4. Driving line 3 leads to a more striking demonstration
that the nuclear selection rules play a major role in determining the SEDM
pattern as the emission at the line 5 position is actually stronger than that at
line 3 where the source energy is centered. Note also that no “forbidden”
transitions are observed for any of the SEDM patterns.
5.3 Applications 251
(13.6 keV) are more penetrating (~ 300 nm) and yield a signal from quite
deep below the surface.
5. Mössbauer Spectroscopy
Figure 5.39 Left–– RT
57
Fe Mössbauer transmission and SEDM spectra of D-Fe. Right–– RT
57
Fe Mössbauer transmission and SEDM spectra of Fe
80
B
20
metallic glass [61].
Figure 5.39 shows the same experiment carried out on a chemically and
structurally disordered sample (Fe
80
B
20
metallic glass). The SEDM emission
lines follow the same pattern seen for D-Fe and the severe disorder associated
with the glass simply leads to some line broadening. It is interesting to note
that the SEDM lines are slightly sharper than those observed by transmission
as only a limited subset of the iron environments present in the glass are
excited by the source at any given constant velocity.
Figure 5.40 shows the SEDM patterns expected if the nuclei in the sample
are subject to a rapidly fluctuating magnetic field, due for example to
superparamagnetic moment fluctuations [61]. As the fluctuation rate
increases, a new line appears at position 6 even though the sample is being
driven only at position 1. This arises because if the field reverses rapidly
enough, the excited nucleus is unable to follow the change and is now in a
spin-up orientation, but in a spin-down field. This effectively converts the
created
2
3
I
m state into a
2
3
I
m state, and the subsequent decay
appears at the line 6 position. More rapid fluctuations increase the intensity of
the line 6 emission at the expense of the line 1 signal and also lead to a
broadening of both lines.
Figure 5.41 shows precisely this behavior for a sample of fine particle
magnetite in oil (a ferrofluid) [61]. As the temperature is increased,
superparamagnetic fluctuations appear and the field reversals lead to an
emission signal at the line 6 position. Figure 5.41 also shows the fitted
relaxation rates derived from these spectra.
252
Figure 5.40 Calculated
57
Fe Mössbauer SEDM spectra in the case of a rapidly fluctuating
magnetic field [61].
Figure 5.41 Left––
57
Fe Mössbauer SEDM spectra of a fine-particle suspension of magnetite in
oil. Right–– Relaxation rates derived from the SEDM spectra [61].
Figure 5.42 shows a particularly clear demonstration of the power of
SEDM. The metallic glass, a-Fe
92
Zr
8
, undergoes two magnetic ordering
transitions as a result of exchange frustration [62]. The upper one establishes
long-range ferromagnetic order, while the lower transition (at T
xy
) marks the
ordering of transverse spin components. Conventional Mössbauer
measurements cannot easily detect any changes at T
xy
due to the pre-existing
magnetic order combined with the inevitable chemical disorder from the
glassy matrix. However, SEDM has been used to detect the fluctuations that
occur in the vicinity of T
xy
and SEDM patterns obtained at T
xy
while driving at
the line 1 (top) and line 2 (bottom) positions are shown in Figure 5.42. The
derived relaxation rates are in perfect accord with values obtained
independently on the same sample using PSR.
5.3 Applications 253
5. Mössbauer Spectroscopy
Figure 5.42 Left––
57
Fe Mössbauer SEDM spectra of amorphous Fe
92
Zr
8
. Right–– A
comparison of relaxation rates derived from SEDM and PSR experiments [62].
5.4 CONCLUDING REMARKS
Mössbauer spectroscopy is now a well-established analytical technique and
transmission
57
Fe spectroscopy is routine. Despite its age (nearly 50 years)
significant advances are still being made in this research area. Selective
excitation double Mössbauer spectroscopy, for example, is more difficult to
perform than the standard transmission technique but provides a unique
insight into dynamic processes. Another recent development in this technique
involves the use of synchrotron radiation. Clearly, Mössbauer spectroscopy
will continue to play a vital role in the study of materials.
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