Introduction to Mössbauer Spectroscopy: Part 3


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This section shows how Mössbauer spectroscopy can be a useful analytical tool for studying a variety of systems and phenomena. The spectra have been taken from active research projects and chosen to visually represent the hyperfine interactions presented in Part 2 and how they can be interpreted.

Example Spectra
 
Tin dioxide assisted antimony oxidation

Antimony-containing tin dioxide is an important catalyst for selective oxidation of olefins. Of particular importance in studying these systems to is to know the relative concentrations of the antimony charge states (3+ and 5+) during the catalytic process.

Fig1 shows three 121Sb spectra taken at various stages during the catalytic process: 1) fresh Sb2O3, 2) Sb2O3 calcined at 1000C and 3) the calcined material after catalysis. Firstly the isotope-specificity of Mössbauer spectroscopy picks out only the antimony atoms from the Sn-Sb-O composite. Readily apparent from spectrum 1 is that practically all of the antimony is in a single state (the red component). Comparison with previous experiments shows that the isomer shift for this majority component matches that of Sb3+. The asymmetric shape is due to the quadrupole splitting in this isotope, which has 8 lines (it is a 7/2 to 5/2 transition).


Fig1 shows three 121Sb spectra taken at various stages during the catalytic process: 1) fresh Sb2O3, 2) Sb2O3 calcined at 1000C and 3) the calcined material after catalysis.
Fig1 shows three 121Sb spectra taken at various stages during the catalytic process: 1) fresh Sb2O3, 2) Sb2O3 calcined at 1000C and 3) the calcined material after catalysis.

Fig1 shows three 121Sb spectra taken at various stages during the catalytic process: 1) fresh Sb2O3, 2) Sb2O3 calcined at 1000C and 3) the calcined material after catalysis. Firstly the isotope-specificity of Mössbauer spectroscopy picks out only the antimony atoms from the Sn-Sb-O composite. Readily apparent from spectrum 1 is that practically all of the antimony is in a single state (the red component). Comparison with previous experiments shows that the isomer shift for this majority component matches that of Sb3+. The asymmetric shape is due to the quadrupole splitting in this isotope, which has 8 lines (it is a 7/2 to 5/2 transition).


After calcining the spectrum is now composed of two components of equal area. The second (green) component corresponds to the Sb5+ ion. The component areas give the relative proportion of each site within the compound, in this case 1:1 indicating either Sb2O4 or Sb6O13. After the catalysis in spectrum 3 we can see that the antimony is now all in the 3+ charge state again.

Tin spectra were also recorded, showing a single line spectrum of identical isomer shift during all parts of the process, indicating no change in the tin charge state.

In cases like this basic deductions can be made even without computer analysis: one can simply see one component appear and disappear and the differences in isomer shift are readily apparent. Unfortunately it isn't always quite this obvious!

Off-center tin atoms in PbSnTeSe

Off-center impurities are those which can be displaced from their regular positions in a crystal lattice. They can be considered as existing in an asymmetric double potential well. Such atoms can change their position as the temperature changes. Unfortunately there are often many other phenomena in such systems that can mask the off-centering effect.

Mössbauer spectroscopy provides a good tool for observing this effect. Firstly the movement of the off-center atom within the lattice will change the symmetry of the electric field it is in: hence changing the quadrupole splitting. Mössbauer spectroscopy is also isotope and site specific, meaning we can observe the off-center single component without any masking from other elements or effects.

A compound which was thought to exhibit off-centering is Pb0.8Sn0.2Te0.8Se0.2, with tin as an off-center atom. Spectra are shown in Fig2 from this sample at 200K and 20K. There are two components: one from an off-center site and one from a normal single-potential site. It can be seen in the highlighted region that the small green component develops from a single line to a (broad) doublet. The quadrupole splitting is increasing, indicating the electric field environment around these particular atoms is become more asymmetrical. This is consistent with an atom moving within an asymmetric potential well.

Fig2: 119Sn Mössbauer spectra showing the quadrupole splitting as an off-center atom changes position with a change in temperature

Fig2: 119Sn Mössbauer spectra showing the quadrupole splitting as an off-center atom changes position with a change in temperature

Fig2: 119Sn Mössbauer spectra showing the quadrupole splitting as an off-center atom changes position with a change in temperature



The other component shows no variation in quadrupole splitting. A series of spectra were taken in a temperature cycle and a hysteresis was observed in the values of quadrupole splitting. These results show that tin is an off-center atom in this compound and that there are two tin sites within it: one normal and one off-center.

Uranium/Iron multilayers

Magnetic multilayers are very important in today's technology, particularly in the areas of data storage and retrieval. A recent development is the use of actinides, such as uranium. Uranium in the right environment displays very large orbital magnetic moments, crucial to engineering systems with strong magnetic anisotropy and for magneto-optical applications. As part of this research sputtered Uranium/Iron multilayers have been produced and Mössbauer spectroscopy has been used to investigate the state of the iron within them.

As these samples are sputtered onto a thick substrate we cannot use conventional Mössbauer spectroscopy in Transmission Mode (TM) as the substrate would block the gamma-rays and we would receive no signal at all. There is a technique known as Conversion Electron Mössbauer Spectroscopy or CEMS which records the conversion electrons emitted by the resonantly excited nuclei in the absorber. In TM mode we record the absorption peaks as the gamma-rays are resonantly absorbed and so see dips, whilst in CEMS we record the electrons emitted from those excited nuclei and so see emission peaks. As the electrons are strongly attenuated by the sample as they pass through it most of the signal only comes from the uppermost 1000Angstroms.

Fig3 shows 57Fe CEMS spectra from three Uranium/Iron multilayers of varying layer thicknesses. They are composed of three components: two sextets and one doublet. The hyperfine parameters of the two sextets correspond to alpha-iron, the red component being fully crystalline and the green component being from diffuse and poorly crystalline alpha-iron. The magnetic splitting shows that the iron in these two components is magnetically ordered.

Fig3: 57Fe CEMS spectra taken from U/Fe multilayers of varying layer thicknesses
Fig3: 57Fe CEMS spectra taken from U/Fe multilayers of varying layer thicknesses


Fig3: 57Fe CEMS spectra taken from U/Fe multilayers of varying layer thicknesses


The third component has an isomer shift and quadrupole splitting consistent with previous work on the UFe2 intermetallic. This is paramagnetic at room temperature, as shown by the doublet. It is a doublet and not a sextet even though the intermetallic has a magnetic moment as the moment direction changes much faster in the paramagnetic state than the time it takes Mössbauer spectroscopy to record it and thus the experienced hyperfine field averages to zero.

As the iron layer thickness is increased to 43Angstroms the relative proportion of alpha-iron to UFe2 increases and also the proportion of fully crystalline iron increases. As the iron layer is increased further to 180Angstroms this proportion becomes even greater. We can deduce from this that the thicker the iron layer the greater the proportion of crystalline iron, but more detailed analysis of the component areas compared to the layer thickness shows that the absolute thicknesses of the poorly crystalline iron and UFe2 stay roughly constant.

Mössbauer spectroscopy has easily shown the existence of the three different iron sites within the sample and how their proportion has varied with layer thickness.

Superspin glass transition in Al49Fe30Cu21

The magnetic properties of granular alloys and heterogeneous nanostructures built by ferromagnetic and non-magnetic components attract much attention due both to the fundamental interest of their rich phenomenology and to their potential applications, for instance in magnetoresistive devices and magnetic recording. Of particular interest are superspin glasses but their study is made difficult by the different possible sources for non-equilibrium magnetic behaviour and the mixtures of particle phases within the samples.

Mössbauer spectroscopy, as seen in the previous examples, is very good at distinguishing particular sites or phases within a sample. And as seen in the previous example can show the difference between magnetically ordered and paramagnetic sites. As the superspin glass phase reaches its freezing temperature the atoms become magnetically ordered and this will show up in the spectra as a sextet appearing.

A series of 57Fe spectra were recorded from a ball-milled sample of Al49Fe30Cu21 with decreasing temperature, shown in Fig4. At 40K, above the freezing temperature, there are two components of unequal proportion, both doublets. As the temperature is reduced the smaller component starts to spread outwards into a magnetic sextet. The peaks are broad and diffuse due to there being a distribution of grain sizes within the sample and hence a distribution of magnetic hyperfine fields. Plotting the recorded hyperfine field against temperature can then give the superspin glass transition temperature for this compound.

Fig4: Series of 57Fe Mössbauer spectra showing the superspin glass transition in nanogranular Al49Fe30Cu21
Fig4: Series of 57Fe Mössbauer spectra showing the superspin glass transition in nanogranular Al49Fe30Cu21


Fig4: Series of 57Fe Mössbauer spectra showing the superspin glass transition in nanogranular Al49Fe30Cu21


Mössbauer spectroscopy showed quite readily the onset of the superspin glass 'freezing' and the proportion of the magnetic particles and their surrounding non-magnetic matrix. Analysis of the hyperfine field distribution also proved consistent with that expected for a superspin glass.This section shows how Mössbauer spectroscopy can be a useful analytical tool for studying a variety of systems and phenomena. The spectra have been taken from active research projects and chosen to visually represent the hyperfine interactions presented in Part 2 and how they can be interpreted.


Introduction to Mössbauer Spectroscopy: Part 1

Mössbauer spectroscopy is a versatile technique that can be used to provide information in many areas of science.

Introduction to Mössbauer Spectroscopy: Part 2

Fundamentals of Mössbauer Spectroscopy

Introduction to Mössbauer Spectroscopy: Part 4

Further information