Corey Thompson Technique Presentation 03/21/2011 Mössbauer Spectroscopy: Europium-Containing 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 Sample Doppler Effect Detector Velocity -Ve +Ve Absorbance

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, ) Shifts 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 Velocity -Ve +Ve Absorbance  = aEγ - sEγ eE0 aEγ aEγ > sEγ sEγ  gE0 Source Absorber

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 mI Velocity (mm s-1) -1 +1 Absorbance Q.S. C.S. (from source) 3/2 I Q.S. 3/2 1/2 1/2 1/2 Quadrupole Splitting

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 Ex: Fe 6 +3/2 3 5 +1/2 I eEa Velocity (mm s-1) -Ve +Ve Absorbance 3/2 2 4 -1/2 1 3 4 -3/2 2 5 6 1 -1/2 gEa 1/2 +1/2 Magnetic Splitting

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 EuCo2P2 no magnetic ordering Intermediate Valence EuCo2P2 AFM 67K Valence is 2+

High Pressure: EuM2Ge2,M = Ni and Pd Intermediate Valence Intermediate Valence Eu3+ 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.