Nucleation |
||
The scope of my interests now concerns the investigation
of the initial stages of nucleation in minerals by low-frequency
Raman and Fourier transform spectroscopies. Our preliminary results show
that these methods are highly effective for studying the structure
micro-inhomogeneities of the size from 1 to 10 nm. It should be noted,
that the initial stages of nucleation, are detected by Raman scattering,
when the infrared spectroscopy, and the X-ray diffraction, do not show any
change. We have investigated of the influence of the structural disorder
in the amorphous solids, which came before the nucleation, on the
intensity of light scattering by the acoustic phonons. We have observed
experimentally [NN Ovsyuk and VN Novikov. Phys. Rev. B 57, 14615 (1998)]
that in the low-frequency Raman light scattering
spectrum of the amorphous solids the boson peak situated in the acoustic
range is more sensitive to the structural order than the optical mode
presently used to determine the degree of disorder. We have shown that
this is because, unlike the coefficient of interaction with optical
vibrations, the coefficient of interaction between light and acoustic
vibrations contains an additional factor, the square of the reciprocal
correlation length of the vibrational excitations, i.e., the intensity of
light scattering by acoustic phonons has an additional dependence on the
degree of disorder. Thus, the results of our studies, have shown, that the
low-frequency Raman scattering, allow us, to observe modifications of the
correlation length, at early stages of crystallization. For our studies,
we took silicon and germanium, as well known models. The regularities
obtained, will be valid for minerals also.
The low-frequency Raman scattering appears to be a sensitive
method for the investigation of the initial stage of nucleation in multi- and monocomponent
systems [NN Ovsyuk. Europhys. Lett. 89, 26001 (2010)]. In the case of the
mono-component systems, when the crystalline nuclei do not have an acoustic mismatch
as compared to the amorphous matrix, we cannot observe
their surface vibrations, but we can make use of the fact that in nucleation there is a jump in
a structure correlation length. This results in a sharp decrease in contribution to the density of
the acoustic vibration states because of decreasing the concentration of structural correlations
on which acoustic phonons are localized. In addition, low-frequency Raman spectroscopy may be
useful in the research into the phase transformation in materials for phase-change memories.
|