Chemistry Reference
In-Depth Information
Fig. 6.17
Typical diffusion mechanism in solids
you would like to get out the train much faster than your neighbors. If you knew the
mechanism of fast diffusion, you could quickly move through densely packed
passengers.
One expects that some impurities occupy interstitial sites, leading to fast dif-
fusion. However, it is rather difficult to understand how some metallic impurities
could be incorporated into interstitial sites, when the atomic sizes of the impurities
are taken into account. Hydrogen atoms could be easily put on interstitial sites in a
crystal, but are not necessarily fast diffusers.
To clarify the origin of such anomalously fast diffusion, one must attempt a
direct observation of the fast diffusing impurities by an experimental method.
57
Fe
Mössbauer spectroscopy appears to be ideal, because it provides the atomistic
information on the lattice sites, the charge states, the jump frequency and the jump
vector, all of which are inevitably needed to construct an atomistic diffusion model.
There were, however, two big problems to overcome experimentally: (1) the reli-
ability of the available atomic jump theories in Mössbauer spectroscopy to deduce
the jump parameters mentioned above from experimental spectra, and (2) the lack
of Fe solubility which is generally reported in the fast diffusion systems. In order to
realize isolated
57
Fe impurities in the samples mentioned in the former paragraph,
the second problem appears to be impossible to solve as long as conventional
absorber experiment are used. This is because neither a simple alloying, nor a low
energy implantation of
57
Fe into a sample could provide an isolated
57
Fe atom
without forming clusters of
57
Fe, which would mask the fast diffusing components
in the Mössbauer spectrum.
To overcome the above mentioned difficulties in the studies of the fast diffusion
in metals, several research projects were designed and performed in 1980s for
about 10 years using the in-beam Mössbauer spectroscopy technique combining
