Mossbauer Spectroscopy of Europium

Report
Mössbauer Spectroscopy: EuropiumContaining Compounds and High
Pressure studies
Corey Thompson
Technique Presentation
03/21/2011
Mössbauer Effect
Mössbauer Effect - was discovered
by R.L. Mössbauer during his Ph.D
work in 1957 (Nobel Prize in
1961).
• It involves the emission and
absorption of gamma rays in
atoms in solids and forms the
basis of mössbauer spectroscopy.
Mössbauer Spectroscopy
Mössbauer Spectroscopy - probes minute changes in the
energy levels of an atomic nucleus in response to its
chemical environment.
Can provide the following information:
• indication of purity;
• characterize the oxidation state of the atom of interest;
• prove whether one compound has a different structure
from another of the same composition;
• indicate whether two or more nuclei in a polynuclear
compound are in equivalent environments;
• and give information on the magnetic ordering of the
compound.
Mössbauer Spectrometer
Source
Collimator
Doppler Effect
Sample
Absorbance
Detector
-Ve
0
Velocity
+Ve
Suitable Sources
Most important criteria:
• Radioactive isotope emits gamma ray of less
than 150keV (57Fe- 14.4keV or 151Eu –
21.54keV)
• Gamma ray well separated in energy from
other photons
• Half-life of precursor to be long (57Co ~
270days or 151Sm ~ 90 yrs.)
• line-width be small
Sample Collection and Prep.
• Sample is powdered and spread across a
sample holder (use of sucrose or graphite
helps spread sample evenly across
holder)
• Sample is then held in place with
something thin and non-absorbant to γrays such as cellophane or kapton tape.
• Sample amount matters and if chemical
composition is known there is a equation
that can compute how much sample is
needed (use too little then some γ-rays
will not encounter an Eu atom; use too
much can affect area, intensity, width and
etc.)
Mössbauer Spectrum
3 Types of Nuclear interactions that are
observable:
• Isomer (chemical, )shifts
• Quadrupole splitting
• Magnetic (hyperfine) splitting
Isomer (Chemical, ) Shifts
eE
0
sE
γ
aE
γ
aE
γ
> sEγ
gE
0
Source
Absorber
Absorbance
Isomer shifts - results from the electrostatic interaction between
the charge distribution of the nucleus and those electrons
which have a finite probability of being found around the
nucleus(only s electrons have a finite probability of
overlapping the nuclear density; can be influenced by p,d
electrons by screening the s density from nuclear charge;
think SLATER’S RULES and Zeff= Z-S)
• Does not lead to splitting of energy levels but results in a
slight shift of mössbauer energy levels in a compound relative
to the source

-Ve
0
Velocity
 = aEγ - sEγ
+Ve
Quadrupole Splitting
Quadrupole splitting – the interaction of non-spherical or cubic
extranuclear electric fields with the nuclear charge density
resulting in splitting of the nuclear energy levels.
• For half-integral nuclear spins, the quadrupole interaction
results in I + ½ levels for spin I. For integral nuclear spins, the
degeneracy of the nuclear levels may be completely removed
by quadruple interaction to give 2I + 1 levels.
Ex: Fe
3/2
1/2
3/2
Q.S.
1/2
Absorbance
I
mI
Q.S.
1/2
Quadrupole
Splitting
C.S. (from source)
-1
0
+1
Velocity (mm s-1)
Magnetic (hyperfine) splitting
Magnetic splitting – is a result of the interaction between the
nucleus and any surrounding magnetic field. The nucleus spin
I, splits into 2I+1 sublevels. The selection rules mI= 0, 1
give rise to a symmetric 6-line spectrum.
mI
I
6 +3/2
3 5
eE
a
+1/2
3/2
2 4
1
1/2
-1/2
-3/2
-1/2
gE
a
Magnetic
Splitting
+1/2
Absorbance
Ex: Fe
3
4
2
5
6
1
-Ve
0
Velocity (mm s-1)
+Ve
Data Collection and Processing
• Techniques for processing Mössbauer data are
complex and variable.
• There is many software out there to analyze data
such as Mosswinn.
• Software uses a variety of different models to
generate model spectra to compare to the
measured spectra.
• Three different lines shapes are commonly
employed in modeling of spectra but most used
are Lorentzian and Voigt.
Eu-Containing Compounds
• Source – 151Sm ~ 90yrs (SmF3); reference to EuF3
• 151Eu – 21.54 keV Mössbauer transition
153Eu – has 3 Mössbauer transitions (83.37, 97.43,
and 103.18 keV); complex nuclear levels and source
have short half-lives (153Sm ~ 46.7hrs)
Eu-Containing Compounds:
Mossbauer Spectroscopy
• Velocity range -> -30 to +30 mm/s
• Eu2+ Isomer shifts are in the range of -13 to -8 mm/s
• Eu 3+ Isomer shifts are in the range of 0 to 5 mm/s
• Usually see isomer shifts and magnetic hyperfine
interactions (line width is too big to see quadrupole
splitting)
Eu-Containing Compounds:
Quadrupole Interaction
Eu-Containing Compounds:
Magnetic Interaction
MS of EuM2P2, M = Co and Ni
EuNi2P2
no magnetic ordering
Intermediate Valence
EuCo2P2
AFM 67K
Valence is 2+
High Pressure: EuM2Ge2,M = Ni and Pd
Eu2+
Intermediate
Valence
Eu3+
Eu2+
Intermediate
Valence
Eu3+
Conclusion
• Mössbauer spectroscopy is a technique that gives you
information about the nucleus chemical environment.
• Three types of interactions are involved: isomer shifts,
quadrupole splitting, and magnetic splitting.
• Factors that govern the shifts are the chemical environment
(ligands), the nuclear charge (shielding of s density due to
p,d electrons), and etc.
• Can determine oxidation states, high/low spin compounds,
determine magnetic ordering, and indicate whether you
have two different nuclei in a polynuclear compound.

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