What is Mössbauer spectroscopy?

The Mössbauer effect is useful in a wide range of sciences including physics, chemistry, biology and geosciences, especially those which focus on iron (Fe). It can be used to determine oxidation state, coordination environment and mineral composition of solid samples such as soils, sediments or rocks.

This website has been designed to provide a general overview of Mössbauer spectroscopy to non-specialists, especially those interested in 57Fe. Overtime, more information will be added to this page however for the time being, it has been split into different sections which focus on:

Theory | Sample Preparation | Instrumentation | Hyperfine Parameters

Theory

The Mössbauereffect was first discovered by Rudolph Mössbauer who was able to demonstrate the recoilless emmission and absorption of gamma rays in solid samples. For this, he was awarded a Nobel prize in Physics 1961 and ever since the effect bears his name. The theory behind the effect is reasonably simple. In short, a radioactive isotope (e.g. 57Co) undergoes decay and emits a photon, or gamma (γ) ray. This gamma ray is then absorbed by an atom which has a resonant energy, and in doing so the absorbing nucleus is excited. The absorbing nuclus then returns to its ground state and emits another photo which is detected by the detector.

There are approximately 20 elements that can be studied by Mössbauer spectroscopy including iron, tin, antimony, tellurium, iodine, gold, nickel, ruthenium, iridium, tungsten, krypton, xenon, many of the rare earth elements, neptunium. Despite such a range of possible elements available, most studies focus on the use of Fe due to various reasons including cost, half-life of the radioactive source, and accesibility.

Sample Preparation

Sample preparation is crucial for ensuring good quality spectra are collected in a reasonable time frame. The sample must be fixed so as to enable recoilless absorption and emission of γ-radiation. For transmission configuration, the sample must be thin and homogeneous. The 57Fe concentration should also be high enough to obtain a useable spectruma within 1 day. Longer measurement time may be required for environmental sciences.

Two standard approaches are often used for preparing samples including:

1. Filtration (0.44 µm filter) of wet sample 2. Dry powder in plexiglas cap

Instrumentation

In general, the Mössbauer effect requires a moving source (to impart the Doppler shift) and a necessary detector. In principle, the detectors can be placed behind the sample, as is the case with transmission geometry measurements, or behind the source as for the miniaturized Mössbauer spectrometer (MIMOS II) on board of Martian rovers which do not have the advantage of someone to prepare their samples.

In the vast majority of laboratory based studies, transmission mode is the preferred type of measurement.

Hyperfine Parameters

Hyperfine parameters are the values which are determined through fitting Mössbauer spectra. In general, there are three main parameters which are usually considered including the Isomer shift (δ), Quadrupole Splitting (Δ) and Hyperfine Field (Bhf). Each parameter provides a range of data

Isomer shift

The isomer shift is highly sensitive to the oxidation state of the mineral under investigation. The isomer shift is defined relataive to a calibration, commonly metallic Fe(0) which is defined to have a value of 0 mm/s. Change the value in the box below and see what happens!

δ (mm/s) =

You should have realised that changing the value of δ shifts the plot along the x-axis.

Quadrupole Splitting

The quadrupole split is also highly sensitive to the oxidation state of the mineral under investigation. Change the value in the box below and see what happens!

ΔEQ (mm/s) =

You should have realised that changing the value of δ shifts the plot along the x-axis.

Hyperfine field

The hyperfine field is an indicator of magnetization, i.e. the atoms within the mineral are magnetically ordered. This is seen due to the emergence of up to six peaks. Change the value in the box below and see what happens!

Bhf (T) =

You should have realised that changing the value of Bhf above 1 T will break the singlet into six different lines. As Bhf is increased, these 6 lines become increasingly spaced apart. The intensity of the lines also appears to decrease as the area under the curve is now spread over multiple peaks. Also notice the velocity range of the spectrum. At the moment this box is restricted to the range of -6 mm/s to +6 mm/s which means that only spectra with hyperfine field of ~33 T (i.e. metallic Fe) or less can be viewed in full. This is why spectrometers are commonly set up to run between -12 mm/s to +12 mm/s.