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Methods for Structural and Chemical Characterization of Nanomaterials 117

Figure 4.8 Resonant Mössbauer absorption in the presence and absence of a magnetic field.

different interactions that may occur between the nuclei and accompa-
nying electrons and the information that may be derived from each can
be summarized as follows:

■ The shift due to monopole interactions provides information about the
coordination number, valency and spin state of the studied atom.
■ The quadrupole shift provides information about the site distortion.
■ The magnetic hyperfine field provides information about the valence
and magnetic properties of the compound.
Accordingly, inspection of Mössbauer spectra can provide informa-
tion regarding the crystallographic nature of minerals and the local
scale evolution of a target atom. For iron nanoparticles Mössbauer spec-
2 3
troscopy is a very efficient technique for probing the Fe /Fe ratio and
the nature and size of the different oxide phases.
Sample preparation. In performing Mössbauer spectroscopy, samples must
be prepared either as powders or as homogeneous thin-films (e.g., 5 mg/cm 2
of Fe). As with the case of powder-XRD it is important to avoid any tex-
tural effect of the powder. The samples do not require any vacuum.
However, measurements are generally performed at low temperature
(i.e., in the presence of liquid nitrogen). The samples must therefore not