Mössbauer Spectroscopy: Europium-Containing Compounds and High Pressure studies

Corey ThompsonTechnique 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 Sample

DetectorDoppler Effect

Velocity-Ve +Ve0

Abso

rban

ce

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 γ-rays 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, ) ShiftsIsomer 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

eE0

gE0

sEγ

aEγaEγ > sEγ

Source Absorber Velocity-Ve +Ve0

Abso

rban

ce

= aEγ - sEγ

Quadrupole SplittingQuadrupole 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.

I

3/2

1/2

mI

3/2

1/2

1/2Quadrupole Splitting

Q.S.

Velocity (mm s-1)-1 +10

Abso

rban

ceQ.S.

C.S. (from source)

Ex: Fe

Magnetic (hyperfine) splittingMagnetic 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.

I

3/2

1/2

mI

-1/2

Magnetic Splitting

+1/2

-1/2

-3/2

+1/2

+3/2

eEa

gEa

Velocity (mm s-1)-Ve 0 +Ve

Abso

rban

ce

Ex: Fe

2

3

1

4

5

6

1

2

3

4

5

6

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 orderingIntermediate Valence

EuCo2P2

AFM 67K Valence is 2+

High Pressure: EuM2Ge2,M = Ni and Pd

Eu2+

Eu3+

Intermediate Valence

Eu3+

Eu2+

Intermediate Valence

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.