Study of initial stages of nucleation in minerals by low-frequency Raman and Fourier transform spectroscopies


We have investigated the influence of structural disorder in the amorphous solids, which came before the nucleation, on the intensity
of light scattering by the acoustic phonons. For the construction of non-fluctuational theory we need the data on the structural disorder in the nuclei until they reach the critical radius. It turned out that not only the processes of nucleation, but the phase transitions which are caused by these processes are also poorly studied. In the course of study of the amorphization arizing in quartz and feldspars it has been found that the phase transition to the triclinic structure and the amorphization proceed simultaneously. Thus, the transition to the triclinic structure may be viewed as an intermediate stage in amorphization. It appears that for these low-symmetric phase transitions there are still no analytical solutions in literature. According to the Landay theory of the phase transitions, the expansion of the free energy in terms of the degrees of order parameter which breaks the symmetry is made. The deformations which do not break the symmetry are not considered as the order parameters, as they are believed not to participate in the transition. Then it is postulated that the coefficient at quadratic member of the expansion is zero at the critical value of the external variable parameter, i.e., the cause of the soft mode occurrence is not explained. As a result, it was not possible to obtain a correct physical description of the phase transitions.
It is shown that the driving force for the structural phase transition induced by cation exchange is the internal pressure. Essentially new as
compared to the existing theory is the use of the symmetric along with the asymmetric components of the strain tensor as order parameters in the Gibbs potential, for they take part in the phase transition. It has been also found that the mechanism causing the occurrence of the soft acoustic mode is the linear-quadratic coupling of the symmetric with asymmetric components of the strain tensor which was earlier neglected.
Influence of transition from quasi-zero to two-dimensional geometries of nuclei on the low-frequency Raman spectra is analyzed.
It is shown that the coupling between electrons and acoustic phonons in the quasi-zero-dimensional nuclei occurs via an interaction, which arises from the perturbation of the electron wave function by motion of interfaces. It is found that scattering due to this mechanism dominates that from the deformation potential for nucleus sizes less than 10 nm.
Also, we have continued the investigation of the proper monoclinic-triclinic ferroelastic phase transition (FPT) in Sr-anorthite
(Sr,Ca)Al2Si2O8 which may be viewed as an initial stage of nucleation. We have tried to clarify the most general reason for elastic instability of the crystals. The surprising thing was that only the "kinematic" anharmonicity leads, if all other anharmonicities are neglected, to decreasing with pressure of the sound velocity corresponding to the soft mode. This anharmonic contribution occurs due to a nonlinear relation between the curvilinear space of q-coordinates of interatomic separation and x-space of the Cartesian atomic displacements, when the hydrostatic pressure is applied to a ferroelastic. We have also obtained the equation with whose help we can for the first time predict the phase transition pressure if the initial elastic moduli are known. In the course of study of the occurrence the crystal nuclei arizing in quartz and feldspars it has been found that the phase transition to the triclinic structure and the nucleation proceed simultaneously. Thus, the transition to the triclinic structure may be viewed as the beginning of the nucleation. It appears that for these low-symmetric phase transitions there are still no analytical solutions in literature. To understand this nucleation mechanism it is necessary to use a microscopic model, which is developed in the present work. We have also revealed some universal laws of the nucleation mechanisms. It turned out that berlinite, quartz and natrolite behave like anorthite at transition from different symmetry phases to the triclinic one. We have found that the values of the internal and the external pressures, at which the phase transition occurs, differ by almost an order of magnitude. This difference is obviously determined by the fact that the external pressure changes the elastic moduli as a result of the anharmonicity of the interatomic potentials, while the internal pressure, in the presence of cation exchange, acts directly on the chemical bonds and consequently changes the elastic constants more effectively.