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Mössbauer spectroscopy
Enver Murad, D-95615 Marktredwitz, Germany
Introduction
In 1958, Rudolf Ludwig Mössbauer, a young German physicist, published two papers on
“Nuclear resonance fluorescence [absorption] of gamma radiation in Ir191” 1. Both the
radiation source (191Os, the decay of which to 191Ir is accompanied by the emission of
gamma rays) and absorbing material (an Ir foil) were bound in solids. By slightly shifting
the source energy Mössbauer was able to bring about a strong resonant absorption of the
129 keV gamma ray, which increased as the temperature was reduced.
Both papers were written in German, and international acclaim was initially subdued.
Mössbauer’s discovery was even met with skepticism by some colleagues, and it was not until
scientists at the Los Alamos and Argonne National Laboratories – on the basis of a bet ! –
repeated his experiment and later extended the range of known Mössbauer-active nuclides to
more common elements such as Fe and Sn that the
phenomenon became generally accepted. A further
major breakthrough came in 1960 when Kistner
and Sunyar 2 showed that the Mössbauer Effect
allows measurement not only of the magnetic
hyperfine field and quadrupole splitting in
hematite, but also of an isomer shift relative to the
source. In 1961, Mössbauer was awarded the
Nobel Prize in Physics for his discovery.
To date, the Mössbauer effect has been
observed for 50 elements and 80 nuclides. About
two thirds of the publications making use of Mössbauer spectroscopy have been concerned
with 57Fe. We will be concerned exclusively with this nuclide here.
Physical Background
As indicated above, Mössbauer spectroscopy stands for the recoil-free emission and resonant
absorption of gamma rays in solid materials. Prior to Mössbauer’s discovery, the observation
of gamma-ray resonance had been hampered by
the Brownian motion that causes the energies of
both the source and absorber to have rather broad
distributions of energy in fluids. The maxima of
these distributions furthermore differ by the
energies of recoil during emission and absorption
(see graph at right). Their overlap is therefore only
minor, but increases with temperature because the
distributions broaden. However, when the emitting
and absorbing atoms are fixed in a crystalline structure, there is no thermal broadening and a
certain fraction of emission and absorption events occurs without recoil at well-defined
energies 3. The degree of absorption furthermore increases as the temperature is reduced 4.
Mössbauer parameters
Resonant absorption occurs when the atoms in the emitting and absorbing matrices are in
identical environments. In the more common case that the emitting and absorbing materials
2
differ, resonant absorption will occur at slightly
´
different energies. The difference, the isomer shift,


S
δ (also called “center shift”); see the (simplified) I = _3
2
diagram for 57Fe at right, is a function of the sA
electron densities at the emitting and absorbing
57Co
nuclei 5. The maximum of resonant absorption can
1 2
123456
be determined by moving the source over a range
of velocities, making use of the Doppler Effect to
57Fe
vary the gamma-ray energy. Energies in
_
1
Mössbauer spectroscopy are therefore given in I = 2
mm/s, and the velocity vs. intensity scan produces
a Mössbauer spectrum. The isomer shift is sensitive to the oxidation state and can therefore be
used to quantify Fe2+/Fe3+ ratios.
Next, the interaction between the electric quadrupole moment of an ellipsoidal nuclear
charge distribution and the electric field gradient (EFG) in a non-spherically symmetric
electric field splits the degeneracy of the excited state into two levels that are separated by the
quadrupole splitting (Δ). The Mössbauer spectrum will then consist of a doublet. The
quadrupole splitting can be broken down into two contributions: a valence contribution from
the atom itself and a lattice contribution from neighboring atoms. High-spin Fe3+, with a
cubically symmetric 3d5 valence electron configuration, lacks an EFG contribution from the d
electron shell, so that high-spin Fe3+ ions have a relatively low quadrupole splitting that is
4
proportional to the site distortion 6. High spin Fe2+
th
Fe
ions have a large EFG contribution from the 6
Fe
d electron, giving typical splittings of 1.5-3 mm/s
3
Fe
Fe
(with a maximum of > 4 mm/s), whereas the
splittings are reduced in the low spin states (see
Fe
2
Fe
Fe(III)
figure at right),.
Isomer shifts and quadrupole splittings
Fe
1
of Fe-bearing phases thus vary systematically as a
function of Fe oxidation and spin states and
Fe(II)
Fe
0
coordination (see figure at right), so that the
-0.5
0.5
1.0
1.5
0.0
Mössbauer parameters can be used to
Isomer shift (mm/s)
“fingerprint” an unknown phase.
Finally, under a magnetic field (whether applied or internal), the nuclear levels split
into (2I+1) components. The allowed transitions (ΔI = 0, ±1) between the ground and excited
levels then generally lead to a sextet for 57Fe. This is one of the most convenient methods for
measuring internal, i.e. magnetic hyperfine fields (Bhf). The magnetically-split excited levels
can be furthermore shifted by the quadrupole interaction. For Fe3+, the resultant shifts can be
considered merely a perturbation of the magnetic splitting (broken lines in the diagram at the
top of this page; quadrupole shifts are indicated by the symbol Δ’) 7.
60
At the magnetic ordering temperature (the
Curie temperature for ferromagnetic and
ferrimagnetic materials and Néel temperature for
antiferromagnetic materials), Mössbauer spectra
40
thus change from a singlet or, more commonly, a
doublet above to a sextet below the ordering
Gt
temperature. The variations of magnetic hyperfine
20
fields as a function of temperature can be readily
monitored by Mössbauer spectroscopy. The figure
Hm
at right shows the variations of magnetic
0
0
200
400
600
800
1000
hyperfine fields of hematite and goethite as a
T (K)
_
5
2
_
3
2
_
1
2
[6]
2+
[8]
[5]
2+
3+
[4]
[4]
2+
3+
[6]
[6]
3+
[5]
[6]
Bhf (T)
[sq]
-10
-5
0
5
10
-10
-5
0
5
10
2+
2+
3
function of temperature. The broken and dotted vertical lines indicate room temperature
(295 K) and the boiling point of liquid nitrogen (77 K) and the insets display Mössbauer
spectra of hematite and goethite at 295 K.
The theoretical line shape for resonance is a Lorentzian function, and this should
therefore be the shape for Mössbauer spectra. Deviations from Lorentzian line shape provide
an indication of imperfections in the material under study or point to shortcomings in the
fitting procedure (for example when the existence of multiple Fe sites in the absorbing
material or a complex composition of this are not taken into regard).
Non-ideal behavior
Many natural materials have non-ideal properties (”real samples”), such as those that might be
due to structural defects, small particle size, or foreign element substitution. Such deviations
from perfection are often reflected in Mössbauer spectra. Numerous physical phenomena can
further complicate Mössbauer spectra, leading to equivocal or controversial interpretations.
Magnetically-ordered materials possess nominally static hyperfine fields below their
ordering temperatures. In small particles, however, the magnetization can change its direction
spontaneously. This collective phenomenon, superparamagnetism, involves all spins in a
magnetic domain, and the nucleus senses a reduced or eventually zero magnetic field if it
occurs rapidly enough 8. Superparamagnetism causes magnetic order to disappear at room
temperature in, for example, goethites smaller than 15 nm and hematites smaller than 8 nm.
Mössbauer spectra of these minerals then bear some resemblance to those in the paramagnetic
state. The effects of superparamagnetic relaxation 9 can be offset by reducing the sample
temperature, and because many minerals formed in the weathering environment (soils and
clays) have particle sizes in this range, their Mössbauer spectra must often be taken at low
temperatures, e.g. those of boiling liquid nitrogen (77 K) or liquid helium (4.2 K), in order to
observe magnetic order.
Another widespread “imperfection” that can strongly affect magnetic hyperfine fields
is the substitution of foreign elements in a crystal
Ionic radii
structure. One of the most common and best
3+
3+
documented substitutions is that of Al for Fe .
Their ionic radii are relatively similar (see graph at
right) and Al is more common in the earth’s crust
than Fe. Al can therefore replace up to ⅓ of the Fe
content of goethite and up to 1/6 of the Fe content
of hematite. Because Al3+ is diamagnetic, i.e.
magnetically “dead”, such substitutions constitute
a strong perturbation of the magnetic properties of vi 2+
Fe : 0.78, viFe3+: 0.645 nm ivFe2+: 0.63, ivFe3+: 0.49 nm
viAl3+ : 0.535 nm
ivAl3+ : 0.39 nm
Fe oxides, lowering the ordering temperatures and
reducing the hyperfine fields at all temperatures.
Initially it was believed that the hyperfine field reduction would allow a determination of the
degree of Al substitution in Fe oxides. Magnetic hyperfine fields, however, react similarly to
other perturbations: thus small particle size, even in the absence of superparamagnetism, has a
similar effect.
At Fe concentrations of a few percent or less (“magnetically dilute” systems), reduced
energy exchange between neighboring magnetic atoms in paramagnetic materials can lead to
a slow electronic relaxation rate. If the relaxation rate is slower than the lifetime of the excited
nucleus, the nucleus senses a magnetic field (slow paramagnetic relaxation). At low
temperatures, the resultant Mössbauer spectra often resemble spectra of magnetically-ordered
systems, but differ from these in the quadrupole shifts and rather broad resonant lines.
4
Measurement of Mössbauer spectra
The singular component of all Mössbauer
Source Absorber
HV
spectrometers is the drive, which imparts a
Drive
Detector
range of velocities to the source to modulate
Preamp
Vel
the gamma-ray energy over the desired span.
signal
Display
In the standard (transmission) arrangement
Power
Amp
supply
(see graph at right), the resultant range of
Drive waveform Sync
PC/MCA
generator
gamma-ray energies falls upon the material
SCA
to be studied (the “absorber”) and the
transmitted radiation is recorded by a detector. Most other constituents of a Mössbauer
spectrometer are standard electronic components such as waveform generators, single- and
multichannel analyzers, amplifiers and power supplies.
Excited Mössbauer sources decay to the ground state by emitting not only gamma
rays, but also conversion electrons, associated Auger electrons and X-rays. Electrons have
penetration depths to ~ 100 nm, X-rays have penetrations of 1-10 µm and gamma rays have
penetrations to 100 µm or more (depending on their energy and the sample). Suitable
detectors can therefore simultaneously characterize samples at up to three depths.
As mentioned above, it is often necessary to take Mössbauer spectra below room
temperature (and less frequently at higher temperatures). Modern cryostats that enable the
former measurements to be made are available in several geometries and many can cool
samples down to almost any desired temperature. To enable comparison with published data,
and also for convenience, spectra are often taken at 77 or 4.2 K (the boiling points of liquid
nitrogen and helium, respectively, at a pressure of 1 atmosphere). Occasionally the
application of external magnetic fields is necessary to better characterize samples; an example
for such an application is given below.
The evaluation (fitting, plotting) of Mössbauer data is a complex, occasionally
controversial procedure. Numerous computer programs that can do this are currently in use. It
must, however, be borne in mind that fitting Mössbauer spectra is a purely mathematical
procedure, and models that are used must be supported by the physical realities 10. It is
furthermore obvious that straightforward interpretations of 57Fe Mössbauer spectra of any
substance are only possible if this contains no other Fe-bearing components that mask the
resonances of the material that is to be studied, or if such components can be removed without
affecting the material in question.
Selected mineral data
Fe oxides and oxyhydroxides (Fe oxides sensu lato) 11
Oxygen and Fe are the most abundant and fourth most abundant elements by mass in the earth’s
crust, respectively. It is therefore not surprising that compounds consisting of these two
elements are common in nature. The oxides, oxyhydroxides and a (single) hydroxide of Fe3+ are
often grouped together under the general term “iron oxides”, and the natural occurrence of nine
such Fe oxides sensu lato has been reported 11. Most Fe3+ oxides are structurally related, having
Fe atoms located in the octahedral interstices of oxygen atom arrangements that are either
hexagonal close-packed (α forms, sequence ABAB) or cubic close-packed (γ forms,
sequence ABCABC). In magnetite and maghemite, which belong structurally to the spinel
group, part of the Fe is also in tetrahedral coordination.
A major distinction between Fe oxides and the majority of other Fe-bearing minerals
is that the Fe oxides order magnetically at relatively high temperatures. Thus magnetic order
can be observed for some Fe oxides at room temperature and for the majority of Fe oxides at
5
Bhf (T)
77 K. Because the Mössbauer parameters of the various Fe oxides in their magnetically
ordered states differ significantly 12, these may serve to discriminate between the individual
oxides. Deviations from crystalline perfection or ideal chemical composition can usually also
be recognized by reduced magnetic ordering temperatures and hyperfine fields, and can lead
to distributions of Mössbauer parameters rather than precise, well-defined parameters.
Hematite, α-Fe2O3, is a common mineral in nature and perhaps the Fe oxide that has
been most frequently studied by Mössbauer spectroscopy. Hematite has the highest magnetic
ordering temperature and the highest magnetic
Bhf (0) = 54.2 T
60
TM  264 K
hyperfine field of all Fe oxides. Between the Curie
temperature and the Morin transition the spins lie
40
close to the basal plane but are tilted away from
this by a small angle. This results in a weak
TC = 955 K
magnetic moment and thus weak ferromagnetic
afm
pm
wfm
Bhf c
Bhf  c
20
character. At the Morin temperature the spins
change their direction (spontaneously at 264 K for
pure, well-crystallized hematite) so that they are
0
nearly parallel to the c axis, leading to
0
200
400
600
800
1000
T (K)
antiferromagnetic character. Because the principal
axis of the electric field gradient is and remains oriented parallel to the c axis, the quadrupole
splitting changes from +0.40 mm/s above the Morin transition to −0.20 mm/s below 7. For
poorly crystalline and most foreign-element substituted hematites, the Morin transition is
spread out over a range of lower temperatures and becomes completely suppressed at particle
sizes below 20 nm or Al substitutions ≥ 10 mol %. Such hematites therefore remain in the
weakly ferromagnetic state at all temperatures.
Goethite, α-FeOOH, is another very common Fe oxide. Well-crystallized, pure
goethite has a Néel temperature of 400 K. Most goethites, however, have lower Néel
temperatures and display asymmetrically broadened resonant lines when magnetically
ordered. The magnetic hyperfine field of goethite is lower than that of hematite at all
temperatures, which allows these two minerals to be readily distinguished in samples, such as
soils, that contain both (see graph in section on Mössbauer parameters).
Akaganéite, nominally β-FeOOH (actually β-Fe(OH)1-x(Cl,F)x) occurs very rarely in
nature. Despite this fact, Mössbauer spectra of akaganéite have attracted a lot of attention
because of their complexity in both the paramagnetic and magnetically-ordered states. The
Mössbauer spectrum of paramagnetic akaganéite consists of a broad doublet. Careful analysis,
however, shows this to consist of several doublets rather than just one doublet broadened due
to high site distortion. Mössbauer spectra of magnetically-ordered akaganéite, which have a
distinctly asymmetric appearance, similarly need to be fitted with at least three sextets with
differing magnetic hyperfine fields and quadrupole splittings.
Lepidocrocite, γ-FeOOH, has the lowest Néel temperature of all Fe oxides (77 K).
However, even well crystallized, pure lepidocrocites display coexisting doublets and sextets
over a relatively wide range of temperatures. This persistence of a doublet has been attributed
to various causes, among others particle size effects and excess structural water.
The ferrimagnetic Fe oxides, magnetite, (Fe3O4), and maghemite, (γ-Fe2O3), have
relatively high Curie temperatures and are therefore often magnetically ordered at room
temperature. The Mössbauer spectrum of stoichiometric magnetite (Fe3+[Fe2+Fe3+]O4)
consists of two sextets resulting from Fe on the A and B sites; the isomer shifts indicate these
to result from Fe3+ and nominally Fe“2.5+” (because of electron dislocation), respectively.
Partial oxidation of magnetite leads to a reduction of the “2.5+” site resonance paralleled by a
gain in the intensity of a new sextet attributed to octahedrally-coordinated Fe3+. The
tetrahedral and octahedral Fe3+ components are difficult to distinguish in the absence of an
externally applied magnetic field because their isomer shifts are quite similar.
-10
-5
0
5
10 -10
-5
0
5
10
-10
-5
0
5
10
6
Transmission (%)
Transmission (%)
The A and B site magnetic hyperfine fields in
100
magnetite and maghemite will add to and subtract from
an external magnetic field that is applied parallel to the
97
gamma-ray direction, so that the two sites can then be
readily distinguished and quantified. The graph at right
94
shows Mössbauer spectra of a non-stoichiometric
magnetite taken at 160 K without (top) and with a
91
100
magnetic field of 6 T applied parallel to the gamma-ray
direction (bottom). The spectra not only reveal the
98
separation of the A and B site resonances, but also a
96
substantial (albeit incomplete) suppression of the 2nd and
5th (ΔmI = 0) peaks.
94
Below the Verwey transition at ~ 120 K, electron
92
dislocation in magnetite is inhibited. Magnetite then
-14
-7
0
7
14
Velocity (mm/s)
displays a very complex Mössbauer spectrum consisting
of several poorly-distinguishable sextets.
Ferrihydrite, ~ Fe5HO8·4H2O, is a classic example of a superparamagnetic mineral.
Ferrihydrite is the Fe oxide that has been (and occasionally still is) termed “amorphous iron
(hydr)oxide” or, more simply but just as erroneously, “Fe(OH)3”. Ferrihydrite occurs in
spherical particles about 7 to 2 nm in size that order magnetically between about 120 and
15 K. These magnetic ordering temperatures are determined by particle size and should not be
confused with Néel or Curie temperatures; they are termed magnetic blocking temperatures
and defined as the temperatures at which the (super-)paramagnetic and magnetically ordered
components of a Mössbauer spectrum have equal areas. Because of its poor crystallinity,
ferrihydrite has Mössbauer parameters that tend to be “smeared out”. Mössbauer spectra of
ferrihydrite – both when superparamagnetic and magnetically ordered – must therefore be
100
fitted with distributions of parameters rather than
specific, well-defined parameters.
96
Ilmenite, Fe2+TiO3, has a structure that is related
92
to that of hematite. These two minerals form a
continuous solid solution series at high temperatures (and
88
often occur exsolved at lower temperatures). Ilmenite has
100
a low quadruple splitting of ~ 0.6 mm/s at room
temperature which increases markedly with decreasing
96
temperature. At 4.2 K ilmenite is magnetically ordered
with a relatively low hyperfine field of about 5 T.
92
Because of the high quadrupole splitting, the two ΔmI ±
½ → ± ³/2 transitions are shifted to the highest velocities,
88
giving the spectrum a skewed appearance (see graph at
-3
-2
-1
0
1
2
3
right).
Velocity (mm/s)
Fe-bearing clay-sized phyllosilicates (clay minerals sensu stricto)
By analogy to the Fe oxides, the phyllosilicates are also prone to isomorphous substitution, Fe
replacing elements such as Si, Al and Mg. Phyllosilicates, like almost all Fe-bearing silicates,
order magnetically at lower temperatures than Fe oxides. Mössbauer spectroscopy can therefore
help to distinguish between Fe bound in oxide and clay mineral structures in samples of
complex mineralogy. 57Fe Mössbauer spectroscopy can furthermore serve to determine the
oxidation state of Fe in phyllosilicates and in favorable cases also the Fe coordination, making
this technique suitable for the characterization of their Fe contents. Because the quadrupole
splitting of Fe3+ is determined by external charges, its magnitude will furthermore give a direct
7
,  (mm/s)
indication of the site distortion. In the following discussion, exemplary data on clay minerals
from three groups will be presented: 1:1 minerals (kaolinite), non-expandable 2:1 minerals
(illite) and expandable 2:1 minerals (the smectite minerals montmorillonite and nontronite).
Fe can substitute for Al on the octahedral sites of kaolinite, Al4Si4O10(OH)8. However,
because the Fe3+ ion is larger than Al3+ (see graph above), this substitution is more limited
than Al substitution in the Fe oxides, although Fe substitutions in kaolinite of up to several
mole-percent have been observed (especially in kaolinites formed in soils). The determination
of the Mössbauer parameters of Fe in kaolinites is often complicated by their low Fe contents
(leading to the development of slow paramagnetic relaxation) and by the presence of
associated minerals. Spectral contributions from associated Fe oxides (that can dominate the
spectra even when present only in minor amounts) can be prevented by removing these using
selective dissolution procedures; alternatively, they can be taken into account separately by
taking spectra at low temperatures (an example is given below). Mössbauer spectra taken at
4.2 K have allowed the identification of goethite in kaolins of complex mineralogy at
concentrations as low as 0.1 %.
Fe3+ dominates the Mössbauer spectra of most kaolinites. “Best values” for Fe2+-free
kaolinites are a temperature-independent quadrupole splitting of ~ 0.5 mm/s and an isomer
shift at room temperature of 0.35 mm/s relative to metallic Fe, thus resembling many
(super)paramagnetic Fe oxides. However, many kaolinites contain noticeable proportions of
Fe2+, which has a quadrupole splitting of ~ 2.5 mm/s and an isomer shift of ~ 1.1 mm/s at
room temperature.
2.0
The Mössbauer spectra of kaolinite change
Neoformed
radically upon firing, reflecting properties of both
silicates
the original sample and the reactions that take
1.5
place during firing. The most striking change is a
Kaolinite
Metakaolin
substantial increase in quadrupole splitting upon
1.0

dehydroxylation of kaolinite, leading to the
formation of (highly disordered) metakaolin, and
0.5
the subsequent formation of high-temperature

phases (see graph at right). These variations allow
0.0
an assessment of the temperature and redox
0
200
400
600
800
1000
1200
Firing temperature (°C)
conditions during firing and can, for example, be
used for reconstruction of the conditions during the productions of archaeological ceramics.
Illite, a clay-sized 2:1 mica with more Si and less K than muscovite, is a common
constituent of soils, clays and shales. Illites may contain Fe in concentrations ranging from
< 1 % to over 8 %, and a typical composition is K0.75(Al1.75R0.25)(Si3.5Al0.5)O10(OH,F)2, where R
is a cation such as Mg2+ or Fe2+.
As in kaolinites, slow paramagnetic relaxation occurs in Fe-poor illites and must be
accounted for when fitting their Mössbauer spectra. Octahedrally-coordinated Fe3+ in illite has a
quadrupole splitting of ≥ 0.6 mm/s (i.e. higher than that of Fe3+ in kaolinite), and both the Fe3+
and Fe2+ quadrupole splittings vary inversely with the Fe contents. Distinguishing the cis and
trans-OH coordinated Fe sites by Mössbauer spectroscopy is not generally possible, but the
presence of tetrahedral Fe3+ can be observed in some Fe-rich samples.
While the production of high-quality porcelain requires clays to be as free from minerals
other than kaolinite as possible, most clays used for the production of structural clay products
(bricks, tiles, etc.) contain significant proportions of other phyllosilicates, for example illite.
Two properties of illite have a strong effect on the firing behavior of such complex clays: the
alkali (K) content, which acts as a flux, and the generally significant Fe content, which lead to
the formation of (colored) Fe-bearing silicates and hematite in the final product. The firing
reactions of illite can be readily monitored by Mössbauer spectroscopy. Thus the graph below
shows features such as the disappearance of Fe2+ and the formation of magnetically ordered
8
Transmission (%)
Transmission (%)
hematite in an illite containing ~ 5 % Fe as a
function of firing temperature. The displayed
spectra were taken at room temperature; at 4.2 K
part of the intense doublet in the high-firing
temperature range is replaced by a sextet,
indicating that the doublet contains contributions
from superparamagnetic hematite as well as
genuinely paramagnetic (silicate-bound) Fe3+.
Smectites are clay-sized 2:1 phyllosilicates
with negative layer charges between 0.6 and 0.2 per
formula unit (O10(OH)2) that can accommodate a
variety of materials in their interlayers.
Montmorillonite, ideally M(Al2-xMgx)Si4O10(OH)2, is a dioctahedral smectite with a layer
charge that arises mainly in the octahedral sheet. Montmorillonites with Fe contents from < 0.05
to > 20 wt.-%, corresponding to 0.01 – 1.70 Fe3+ per formula unit have been described. Montmorillonites with more than 0.6 Fe3+ per formula unit
100
are termed ferrian or Fe-rich.
298 K
A feature that is often neglected in Mössbauer
99.8
studies of Fe-poor montmorillonites is – once again –
99.6
the influence of slow paramagnetic relaxation.
Paramagnetic relaxation shows up clearly in spectra of
100
Fe-poor montmorillonites at 4.2 K (see figure at right),
but is not overtly apparent at room temperature. It is,
99.6
however, conceivable that the outer doublet (i.e. the
difference between the dotted doublet fitted to the
99.2
room-temperature spectrum and the measured data in
the upper spectrum at right) of two-doublet fits that
98.8
SAz-1
4.2 K
have repeatedly been used to fit the Fe3+ resonance of
-10
-5
0
5
10
montmorillonites may, in the case of Fe-poor samples,
Velocity (mm/s)
result from this phenomenon.
Nontronite is the Fe3+-rich end-member dioctahedral smectite with the theoretical
formula MFe2(Si4-xAlx)O10(OH)2, and thus a predominantly tetrahedral negative charge,
although nontronites from different localities may vary widely in chemical composition.
Mössbauer spectra taken at low temperatures
100
(≤ 120 K) have revealed a common association of
296 K
90
nontronites with Fe oxides, in particular goethite, that
are superparamagnetic at room temperature. The
80
resonant peaks of these minerals will in general be
unresolvably superimposed upon each other, making
70
meaningful interpretations of the spectra difficult if not
100
impossible. Thus the room-temperature spectrum of the
95
Garfield nontronite H33a at right shows no evidence of
goethite, whereas the sextet that can be observed at
90
77 K indicates ~ 5 % of the Fe content of the sample to
be bound in goethite.
85
H33a
77.3 K
Magnetic order has been observed in nontronites
-10
-5
0
5
10
at temperatures between 4.2 and 1.3 K. Mössbauer
Velocity (mm/s)
spectra taken under external magnetic fields have
indicated Garfield nontronite to have a Néel temperature of about 20 K. Frustration due to
tetrahedral Fe3+, however, reduces the magnetic ordering temperature to below 7 K.
9
Intercalation (”pillaring”) of both montmorillonites and nontronites with Fe-rich materials
significantly raises the magnetic ordering temperatures.
Complex materials
Transmission (%)
Transmission (%)
Transmission (%)
Mössbauer studies of materials formed at the earth’s surface (sediments, soils, clays, etc.) is
100
often demanding: besides the complex mineralogy of
295 K
the materials themselves, the individual components
98
often display one or more of the features of non-ideal
96
behavior outlined above, making an interpretation of
94
their spectra correspondingly challenging.
92
120 K
Soils formed under moderate climatic conditions
100
thus do not only exhibit the expected intricacy of spectra
98
due to complex mineralogy, but may in addition display
96 78 K
100
superparamagnetism down to temperatures well below
98
77 K, indicating the Fe oxides to have particle sizes in
96 4.2 K
the nanometer range. As an example, the figure at right
100
shows Mössbauer spectra of a rubified soil from the
98
northern Alpine foothills. Note the increase in sextet
96
area and concurrent attenuation of the doublet between
-10
-5
0
5
10
Velocity (mm/s)
78 and 4.2 K.
The values of commercial clays are strongly dependent not only on the essential
mineral(s), but also on the possible presence of
100.0
ancillary components. Thus kaolins that are used for
99.8
paper coating must be free from pigmenting minerals
such as Fe oxides, and possibilities to account for such
99.6
components have been mentioned above. As an
99.4
example, the figure at right shows Mössbauer spectra
100.0
of a commercial kaolin taken at 4.2 K before (top) and
99.8
after (bottom) treatment with Na dithionite. The sextet
that disappears as a result of this treatment identifies
99.6
the removed constituent as goethite; the amount of Fe
removed by this treatment (0.07 wt.-%) corresponds to
-10
-5
0
5
10
a goethite content of 0.11 % (the remaining sextet is
Velocity (mm/s)
due to paramagnetic relaxation).
100
The distinction of the components contributing
90
to the Mössbauer spectra in the above example was
facilitated by the markedly different magnetic fields of
80
oxide and silicate-bound Fe. Such distinctions become
70
Untreated
more complicated if the constituents of a mixture have
100
similar magnetic hyperfine fields, and may make
90
physical or chemical treatments necessary. As an
80
example, the graph at right shows Mössbauer spectra of
70
an Fe-rich “pipe stem” (a root void that has been filled
3 x 2h oxalate
60
with Fe oxides) at 4.2 K. Spectra were taken prior to
100
and following different treatments with acid Na oxalate,
98
which selectively removes poorly-crystalline Fe oxides.
96
All samples show complete magnetic order, but even a
24 h oxalate
94
cursory inspection shows distinct differences:
92
increasingly intense treatments cause the hyperfine field
-10
-5
0
5
10
Velocity (mm/s)
distributions to narrow and the average hyperfine field
10
to increase. A concurrent change of the average quadruple splitting from -0.10 to -0.23 mm/s
identifies the removed constituent – not surprisingly – as ferrihydrite (the broad lines of which
initially mask the goethite sextet) and the residual component as goethite.
Summary: strengths and weaknesses of Mössbauer spectroscopy
Strengths
Weaknesses
Sensitive only to Fe (“sees” only 57Fe)
Sensitive only to Fe (no matrix effects)
Sensitive to oxidation state
Determination of coordination may be
controversial
Allows distinction of magnetic phases
Paramagnetic phase data may be ambiguous
Very sensitive to magnetic phases
Magnetic parameters sensitive to diamagnetic
element substitution and relaxation
Non-destructive
Relatively slow
Resolution limited by uncertainty principle If possible, also use other techniques ▼
Often a combination of Mössbauer spectroscopy with other techniques can help solve
problems that cannot be resolved using Mössbauer spectroscopy alone.
Useful web sites
Mössbauer Effect Data Center, Asheville, N.C., U.S.A. (http://orgs.unca.edu/medc).
Comprehensive services to the Mössbauer community including an archive of Mössbauer
publications (> 46.000 to date !), data and personnel searches, publication of the “Mössbauer
Effect Reference and Data Journal” (10 issues annually) and handbooks on specific materials
(e.g. minerals) and nuclides.
Mössbauer Effect “Community” Site (http://www.mossbauer.org). Topical collection of data
on Mössbauer spectroscopy with a variety of links to relevant web sites.
Mars Mineral Spectroscopy Database (http://www.mtholyoke.edu/courses/mdyar/database)
Freely accessible collection of Mössbauer spectra of a variety of minerals taken over ranges
of different temperatures.
IBAME Web Site: (http://pecbip2.univ-lemans.fr/%7emoss/webibame). Introduction to
Mössbauer spectroscopy.
11
Appendix: references and notes
1
Mössbauer RL (1958) “Kernresonanzabsorption von Gammastrahlung in Ir191”, Die
Naturwissenschaften 45, 538-539; “Kernresonanzfluoreszenz von Gammastrahlung in Ir191”,
Zeitschrift für Physik 151, 124-143.
2
Kistner OC & Sunyar AW (1960) “Evidence for quadrupole interaction of Fe57m, and influence of
chemical binding on nuclear gamma-ray energy”, Physical Review Letters 4, 413-415.
Mössbauer line widths are limited by the natural line width (Γ) that ensues from the Heisenberg
uncertainty principle. Γ is given by Γ· ≥ ħ, where  is the mean lifetime of excited gamma state
and ħ is Planck’s constant divided by 2π. For 57Fe, the mean life of the 14.4 keV excited nuclear
level is 141 ns and Γ is each 4.67·10–9 eV for emission and absorption. This corresponds to 0.097
mm/s, leading to a theoretical minimum line width of 0.194 mm/s. Line widths attained in practice
(W) are somewhat larger, and values of 0.22 – 0.23 mm/s are considered good. The ratio of the
14.4 keV 57Fe line width to the transition energy amounts to ~ 3·10−13, i.e. this line is extremely
well defined (narrow).
4
Conservation of momentum requires some emission and absorption events to occur with recoil.
At a first approximation, the “recoil-free” fraction is given by f = exp [–k2 <x2>], where k is the
wave vector of the gamma ray and <x2> the mean square amplitude of vibrations along the
gamma-ray direction. <x2> decreases and f therefore increases as the temperature is reduced.
Typical values of f for selected Fe oxides and silicates range from ~ 0.65 − 0.85 at room
temperature and ≥ 0.85 − 0.95 at 80 K 13.
4π 2
2
2
5
Ze [
Ψa(0)

Ψs(0)
] RδR
The mathematical expression for the isomer shift, δ, is given by δ 
52
2
where Z is the atomic number, e is the change of the electron, |a(0)| and |s(0)| are the electron
densities at R = 0 inside the nuclei at the absorber and source, respectively, R is the nuclear radius,
and R is the change in nuclear radius during the transition from the excited to the ground state.
3
6
Quadrupole splitting. The interaction between the electric quadrupole moment and the electric field
gradient splits the excited state into two levels separated by  = ½ eQ Vzz (1 + 2/3), where e is
the charge of the electron, Q the nuclear quadrupole moment, Vzz the electric field gradient along
the principal axis and η an asymmetry parameter defined as (Vxx −Vyy)/Vzz.
7
The combined effects of a high magnetic hyperfine field and a much smaller quadrupole interaction
for an axially symmetric electric field gradient is given by Δmag = Δpm (3cos2θ − 1), where Δmag and
Δpm are the quadrupole splitting in the magnetically ordered and paramagnetic states, respectively,
and θ is the angle between the principal axis of the electric field gradient and the magnetic field.
8
The energy required to reverse the magnetization of a particle depends on its volume V and magnetic
anisotropy constant K. As particles become smaller, the energy barrier KV becomes comparable to
the thermal energy kBT. The frequency for spontaneous magnetization reversal is then given by
f = f0 exp (−KV/(kBT)), where f0 is a poorly-defined frequency in the range 109 − 1011 Hz.
9
Relaxation (French “trainage”, lagging behind) is a generic term for the time-dependent response of a
system to external stimuli. In the case of superparamagnetism, the minimum time required for the
nucleus to detect a hyperfine field is the nuclear (“Larmor”) precession time; for 57Fe this is 30 ns
(34 MHz) in a 50 T hyperfine field. Fluctuations of the electron spin must remain constant for at
least one Larmor precession period for the hyperfine field to be observed.
10
Caveats: Mössbauer spectroscopy is a deceptively simple technique which, however, is riddled with
pitfalls. Fits to Mössbauer data are not always equivocal (“non-unique solutions”), and the
application of other techniques may be more often necessary than is generally realized to arrive at
sound interpretations. Some examples of possible pitfalls and nonsense are given below.
An apparently trivial error that can nevertheless be observed from time to time is the overinterpretation of spectral data. As mentioned above, fitting is a purely mathematical procedure in
which deviations between the experimental and fitted spectra are minimized by a least-squares
procedure. Spectral fitting thus provides no information on whether a model is physically
appropriate or not, and no more components should be fitted to a spectrum than is reasonable on
12
the basis of the spectral quality and physical relevance. The “goodness of fit” parameter, χ², is
furthermore known to improve when fitting poorer quality data, where the discrepancy between
the true and modeled function is less apparent (which should, of course, not be taken as a
recommendation for collecting spectra of poor quality; in fact fewer spectra of better quality are
often preferable to more spectra of mediocre quality).
The magnetic field (~ 55 T) resulting from slow paramagnetic relaxation in Fe-poor materials
at low temperatures has occasionally been mistaken as resulting from magnetically-ordered
hematite (Bhf(0) ≈ 54 T). Particular caution is necessary when studying samples that have been
doped with small amounts of 57Fe. Careful analysis, however, shows hematite to have a distinctive
quadrupole shift (either +0.41 or −0.20 mm/s, depending on whether the hematite has or has not
passed though a Morin transition) and significantly narrower resonant lines than resonances
ensuing from paramagnetic relaxation.
In early Mössbauer work, relatively high quadrupole splittings and low hyperfine fields of
soils that resembled those of paramagnetic and magnetically-ordered akaganéite, respectively,
lured some research groups into deeming akaganéite to be a common constituent of soils. Later
work, however, indicated that the paramagnetic spectra more probably result from ferrihydrite and
the reduced hyperfine fields observed in the magnetically ordered material to be rather due to Al
substitution in goethite. Although the akaganéite “problem” thus has long been settled, it does
show how tenuous assignments based solely on selected Mössbauer parameters can be. A more
recent debate relates to Mössbauer data collected by the spectrometers on the Mars Exploration
Rovers: while Fe2+ doublets with high quadrupole splitting were initially taken as resulting from
olivine, subsequent work including detailed analyses of infrared spectra indicated that these
doublets might originate from Fe2+ sulfates with similar Mössbauer parameters that have also been
observed in terrestrial acid drainage environments 14.
Finally, there have been some abortive attempts at dating of laterites and archaeological
artifacts by Mössbauer spectroscopy. The laterite “dating” papers were based on the well-known
fact that the quadrupole splittings of Fe oxides, notably hematite, increase as particle sizes
decrease. Reduced Fe3+ quadrupole splittings have therefore been implied to indicate the hematites
to have matured, and thus to have higher ages. Besides the fact that these studies were not
accompanied by detailed investigations of the Fe mineralogy, there is no reason to assume that
hematites would recrystallize in the course of time (unless they underwent a dissolution/
recrystallization stage, but that would make them younger anyway). A later careful study of
laterites from a single profile in Australia indeed did not reveal any dependence of Fe mineralogy
on age 15. “Dating” of archaeological artifacts, in contrast, has been based on apparently varying
asymmetries of paramagnetic Fe3+ doublets. Inspection of the data provides another explanation:
in samples that contain both Fe3+ and Fe2+, the low-velocity peaks of paramagnetic Fe3+ and Fe2+
are nearly coincident and therefore have a higher dip than the separate high-velocity Fe3+ and Fe2+
peaks. When spectra are taken in a relatively narrow velocity range (≤ 2 mm/s) the latter peak may
be missed, and different Fe2+/Fe3+ ratios could then be mistaken for varying degrees of asymmetry
of an Fe3+ doublet.
13
De Grave E & Van Alboom A (1991) Evaluation of ferrous and ferric Mössbauer fractions. Physics
and Chemistry of Minerals 18, 337-342.
14
Bishop JL, Dyar MD, Lane MD & Banfield JF (2004) Spectral identification of hydrated sulfates on
Mars and comparison with acidic environments on Earth. International Journal of Astrobiology 3,
275-285.
15
St. Pierre TG, Webb J & Butt CRM (1990) Laterite mineralization near Kalgoorlie, Western Australia:
dating by Mössbauer spectroscopy? Hyperfine Interactions 57, 2279-2294.
Acknowledgments: These notes have benefited significantly from helpful reviews by Janice J. Bishop
and M. Darby Dyar.
13
11
12
Fe oxides and oxyhydroxides that have been observed in nature
Mineral
Occurrence
Hematite
Composition
Structure
Space
Group
Unit-Cell Dimensions (Å)
a
b
c
very common α-Fe2O3
corundum
R3c
5.034 a
Magnetite
common
Fe3O4
inverse spinel
Fd3m
8.396
Maghemite
common
γ-Fe2O3
disordered spinel Fd3m or
P422
8.3474
8.347
diaspore
Pnma
9.956
3.0215
4.608
β-FeOOH b
hollandite
I2/m
10.600
3.034
10.513
Lepidocrocite common
γ-FeOOH
boehmite
Bbmm
3.071
Feroxyhyte
very rare
δ’-FeOOH
disordered CdI2
P3m1
2.93
4.56
Ferrihydrite
common
Fe5HO8
4H2O
disordered corundum
2.955
9.37
Bernalite
extremely rare Fe(OH)3
disordered ReO3 Immm
7.544
Goethite
very common α-FeOOH
Akaganéite
very rare
13.752 a
25.01
12.52
3.873
7.560
7.558
Mössbauer parameters of Fe oxides and oxyhydroxides that have been observed in nature
Mineral
TN, TC
MAG a
Bhf
(K)
δ/Fe
Δ
Δ
Bhf
Room Temperature
4.2 K
Hematite
955
wfm
51.8
0.37
-0.20
53.5
or 54.2 b
-0.20
0.41
Magnetite
850
fim
49.2
46.1
0.26
0.67
0.02
0.02
50.6
36 - 52 c
0.00
1.18-(-0.79)
Maghemite
 950
fim
50.0
50.0
0.23
0.35
0.02
0.02
52.0
53.0
0.02
0.02
Goethite
400
afm
38.0
0.37
-0.26
50.6
-0.25
Akaganéite
299
afm
–
0.38
0.37
0.55
0.95
47.3
47.8
48.9
-0.81
-0.24
-0.02
77
afm
–
0.37
0.53
45.8
0.02
Feroxyhyte
450
fim
41
0.37
-0.06
53
52
0.0
0.0
Ferrihydrite
115 d
25 d
spm
–
–
0.35
0.35
0.62 e
0.78 e
50 e
47 e
-0.07
-0.02
Bernalite
427
wfm
41.5
0.38
0.01
56.2
0.01
Lepidocrocite
Bhf in Tesla; δ and Δ in mm/s.
a
Magnetic character: weakly ferromagnetic (wfm), ferrimagnetic (fim), antiferromagnetic (afm),
speromagnetic (spm).
b
For afm hematites (that have passed through a Morin transition).
c
Several magnetic B-site subspectra below the Verwey transition at ~ 120 K.
d
Range of superparamagnetic blocking temperatures which vary as a function of crystallinity.
e
Maximum probabilities of quadrupole-splitting and hyperfine-field distributions.