Mossbauer Spectroscopy: Fundamentals and Applications

Mossbauer Spectroscopy also referred to as nuclear Gamma Resonance Spectroscopy is a versatile tool in material sciences. It involves the absorption of a gamma ray photon by a nucleus, causing it to transition. The idea of gamma-ray resonant absorption is first proposed by Kuhn in 1929 and observed for the first time by Mossbauer in 1958.

This is a powerful technique that provides detailed information about the properties of materials. It is based on the Mossbauer effect, which involves the resonant absorption and emission of gamma rays by atomic nuclei.

Mossbauer spectroscopy is being applied in materials sciences, geology, environmental science, biology, and medicine. For example, it is used to study the magnetic properties of composite materials. It also provides information about various properties of the sample including its atomic and electronic structure, valence state, and coordination number. This information is then used in the development of new materials, including high-performance magnets, catalysts, and semiconductors.

Working principle of Mossbauer spectroscopy

Mossbauer spectroscopy is a spectroscopic technique that uses the resonant absorption and re-emission of gamma rays to study the atomic and electronic structure of solid crystals. The basic principle of Mossbauer spectroscopy is the resonance absorption of gamma rays by nuclei in a solid sample. This resonance absorption occurs when gamma rays of a specific energy match the energy difference between two nuclear states.

This can be explained in terms of energy by the following expression:

Eγ-ray emission = Etransition – ER

where,

• • Eγ-ray emission = the energy of the emitted γ-ray
  • ETransition = the energy of the nuclear transition
  • ER = the energy of the recoil.

The above illustration shows a schematic of the vibrational energy levels in a solid. The left side of the illustration shows that the recoil energy (ER) of a gamma photon is sometimes not enough to reach the next higher energy level, resulting in a low probability for vibrational mode excitation. The variable “f” represents the fraction of events where no excitation takes place during a zero-phonon transition. This means that the gamma-ray is emitted without any energy loss to the solid.

Mossbauer Effect

The Mossbauer effect is a nuclear phenomenon where a nucleus emits or absorbs gamma radiation without recoil. This is possible due to the interaction of the nucleus with the surrounding electrons. This effect is utilized to study minerals containing iron, in particular. In addition, this effect has valuable contributions in the field of physical, chemical, biological and earth sciences.

Atoms have a much greater effective mass in the solid matrix. So, the recoil mass of the nuclei becomes the recoil mass of the whole or entire matrix. It is important to note here that the mössbauer effect has been observed for about 100 nuclear transitions in 80 nuclides in nearly fifty elements.

However, not all of these transitions are suitable for actual exploitation.

What is recoil?

The recoil effect occurs when a body releases a high-energy particle or projectile. The releasing body experiences a back-kick or push, which is a result of momentum conservation. When a gaseous atom or molecule emits a quantum of energy, E, the emitted quantum will have momentum equal to E/c, where c is the speed of light.

To conserve momentum, the emitter will recoil in the opposite direction with momentum, P, that is equal in magnitude to the emitted quantum’s momentum.

P = M.V_R= -E/c

where V_R is the recoil velocity and the negative sign shows that its direction is opposite.

E_R= P₂/2M = E₂/2Mc²= (E_t-E_R)2/2Mc²≈ E_t/2Mc²

Since ER is small. If M increases the recoil energy can still be lower. So the source and the absorber are fixed on a larger lattice to increase mass.

Instrumentation of Mossbauer Spectrophotometer

The basic components of a Mossbauer spectrometer include an energy source, collimator, sample holder, and detector. These components and their function are briefly discussed below:

Energy sources of Mossbauer spectroscopy

The energy source used in the mossbauer spectrometer is a gamma source that emits gamma radiation during de-excitation. For the spectroscopic technique, we need monochromatic radiation such as gamma rays. These rays are produced due to the energy difference between the nuclear levels. Additionally, gamma rays frequency can be altered as per requirement by using the Doppler effect.

It is of great importance that the energy of the nuclear transition must be in the range of 10-150 keV, because high enough to produce a useful gamma-ray photon. But not so high as to cause recoil. The lifetime of the excited state must be long enough to produce a reasonably broad emission range. For instance, the best source has a duration of approximately 0.001 to 100 nanoseconds. Moreover, the source isotope should have a  stable ground state and a high cross-section area.

Working principle

The sample is placed in a beam of gamma rays. When the gamma rays irradiate the sample some portion is absorbed to excite the nuclear energy level.

It is to be noted that the mechanism employed here for absorption is to move either the source or absorber. This can be accomplished by moving the source closer to or farther away from the sample while altering its velocity linearly over time.

As an example, a velocity of 1 mm/sec towards the sample increases the energy of the emitted photons by around ten times the natural linewidth in the case of ⁵⁷Fe. The velocity unit “mm/sec” is commonly used as the “energy” unit in Mossbauer spectroscopy.

In some cases, the source can also be kept stationary and the sample oscillated, as in synchrotron Mossbauer spectroscopy.

Detectors

The arrangement of the detector in relation to the source and sample determines the experiment’s geometry. Therefore, transmission or backscatter modes are the most frequently used.

The common gamma detectors are as follows:

1.  proportional counters
2. scintillation detectors
3. semiconductors such as silicon and germanium.

Shifts in Mossbaour spectroscopy

The most common shifts in this spectroscopic technique are isomer shift, center shift, and chemical shift. The possible effects observed are listed below:

• The Et value is affected by the interaction between the nucleus and electrons present around it.
• This interaction is due to the different sizes of the nucleus in the ground and excited states.
• The change in the nuclear radius from the ground state to the excited state is represented by ΔR.
• Z represents the atomic number of the nucleus.
• The change in electrostatic energy on decay can be calculated using the formula:

δ= (ε_o/5) (Ze²R²)(Δ R/R)[|ψ_s(abs)|2-|ψ_s(source)|2]

where:

• • ε_o represents the permittivity of free space.
  • e represents the electronic charge.
  • ψ_s(abs) represents the s orbital wave function of the absorber.
  • ψ_s(source) represents the s orbital wave function of the source.
• The s electron wave functions have their maxima at the nucleus, thus s electron density has a great effect on the isomer shift.
• Changes in p and d orbital occupancies affect the s electron through screening, resulting in a smaller effect on the isomer shift.
• If ΔR/R is positive, the isomer shift is also positive, and vice versa.
• The isomer shift is related to the oxidation state of the metal.

Chemical shifts in iron compounds

The isomer shift of Fe cannot be used to determine its oxidation state (O.S) in a molecule. Because the O.S of Fe often differs by only one unit charge. Note that, the electrons involved in the O.S of Fe are from d orbitals, which leads to a smaller effect on the s electrons. However, varying spin states, which depend on the ligands present, also affect the isomer shift value.

Mossbauer spectroscopy and Iron analysis

Mineralogists frequently use mossbauer spectroscopy to assess the valence state of iron, which occurs as iron oxide (FeO), Ferrous ion (Fe²⁺), and ferric ion (Fe³⁺). They also use it to identify the coordination polyhedron occupied by iron atoms. Scientists also apply this method to determine the redox ratios in glasses and rocks.

The precursor of ⁵⁷Fe is ⁵⁷Co which decays to ⁵⁷Fe* with a half-life of 270 days, making it a highly stable precursor. Of the ⁵⁷Fe*, 9% decays directly to the ground state with the emission of a γ photon of energy 136.32 keV. On the other hand, 91% of ⁵⁷Fe* decays to another excited state with the emission of 121.91 keV energy.

This lower excited state has a lifetime of 99.3 ns, making it more stable. It then decays to the ground state with the emission of 14.41 keV energy. This transition satisfies all the conditions for Mössbauer spectra except for the second condition, but it is compensated by a larger absorption cross-section.

However, other elements that can be studied for Mössbauer spectra include ¹¹⁹Sn, ¹²¹Sb, ¹²⁵Te, ¹²⁹I, ¹²⁹Xe, and ¹⁹⁷Au.

Limitations of Mossbauer spectroscopy

The only viable source of gamma radiation is from excited nuclei of the same isotope undergoing radioactive decay. There is no way to adjust the energy of the emitted gamma photon, and the energies involved are on the order of kilo-electron volts (keV), which are much higher. Some drawbacks of this technique are listed below:

• Inherently a bulk technique requires powders spread thinly across an absorber to get optimal results.
• Recent advancements have made it possible to run small samples but it is still not ideal for tiny samples.
• The Mössbauer multiprobe modification is used to study single grains in thin sections or single crystals but is still not ideal for extremely small samples.
• Most rock-forming minerals contain Fe²⁺ in octahedral coordination and have very similar Mossbauer parameters, making it difficult to distinguish between minerals.
• The parameters vary as a function of temperature and cation substitution, which makes it challenging for mineral identification.
• Not ideal for mineral identification, especially for extraterrestrial applications.

Applications of Mossbauer spectroscopy

The Mossbauer spectrometer is used to identify minerals through a combination of isomer shift and quadrupole splitting measurements. These parameters help determine the valence state and site occupancy of iron-57 in a given site and mineral. In magnetic phases, the hyperfine field can also be used in conjunction with the isomer shift and quadrupole splitting. In such cases, the magnetic field value, usually in Teslas, can aid in the identification of some phases.

Strengths of Mossbauer spectroscopy

Mossbauer spectroscopy is considered the “gold standard” for determining the valence state of iron and identifying various iron oxides. Additionally, it is well-suited for determining the coordination number of iron atoms.

Related Resources

• Atomic Absorption Spectroscopy (AAS)
• Infrared Spectroscopy: Principle, Instrumentation & Applications
• X-ray Absorption Spectroscopy: Principle, instrumentation and Applications
• Spectrophotometers

Concepts Berg

Which radiation is used in Mossbauer spectroscopy?

Mossbauer spectroscopy uses gamma radiation.

Which elements are Mössbauer active elements?

The Mossbauer active elements are those elements with a nucleus that has non-zero spin and that can emit or absorb gamma radiation in a nuclear transition. The most commonly used Mössbauer active element is 57Fe. Other elements that are sometimes used in Mössbauer spectroscopy include 119Sn, 121Sb, 125Te, 129I, 129Xe, and 197Au.

what are the advantages of Mossbauer spectroscopy?

1. High resolution and sensitivity
2. Non-destructiveness
3. Quantitative analysis
4. Versatility
5. Insight into magnetic properties
6. Easy to interpret results

What are the parameters of Mossbauer spectroscopy?

The parameters of Mossbauer spectroscopy include isomer shift, quadrupole splitting, magnetic field (in the case of magnetic phases), coordination number, and valence state of the elements being studied.

How are the parameters of Mossbauer spectroscopy used in the analysis?

These parameters are used to identify the chemical composition, bonding environment, and coordination geometry of the elements in a sample. By analyzing the changes in these parameters over time, temperature, or pressure, researchers can obtain valuable information about the physical and chemical properties of the sample.

Additionally, the Mössbauer effect can be used to determine the temperature dependence of physical properties and to monitor changes in the sample environment.

References

• Radiochemical analysis section (nist.edu)