Investigation of new models pressure-induced amorphization for two distinct topological classes of minerals |
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For verification of the new amorphization model [M.H. Cohen et al., Journal of Non-Cryst. Solids 307, 602
(2002)] which is considered in our work, it is necessary to
know, whether amorphization results in a change of topology of minerals or
it remains the same. It is not generally accepted - within the extensive
literature - that there are two distinct classes of materials which could
be labelled as pressure amorphized. To the first class, belong a
conventional glass, whose atomic structure is not topologically equivalent
to any crystalline structure. To the second class, belong the materials
with random displacements of the nuclei which do not destroy the
crystalline topology. The authors of this new model suppose that since the
above distinction has not been clearly made, a general physical picture of
the microscopic processes underlying amorphization under pressure has not
been available yet. For these two cases, the effective Hamiltonians,
describing the elastic deformation energy in terms of atomic displacements
will be constructed in a different manner and hence amorphization models
should be different.For gaining a complete understanding of to which on of
these two classes belong amorphization substances, we have investigated
the dynamic and the static mechanisms of amorphization of a number
of frame minerals subject to the external pressure and, also, to the
cation exchange.To reveal universal laws the amorphization mechanisms we
investigated phase transitions of some frame minerals in the triclinic
structure as an intermediate stage of amorphization. We have shown that
transition in the amorphous phase of anorthite, berlinite, quartz,
natrolite and edingtonite is described by the common equation, obtained in
the process of considering the microscopic model of the phase transition.
According to our model in the above-mentioned minerals, one or more
branches of the phonon spectrum, connected to the module C44 softens and
flattens with increasing pressure. In this case, the calculation of the
lattice dynamics shows the following picture of amorphization pressure:
The random displacements in the amorphous phase are associated with the
incipient instability of at least one nearly flat phonon branch. These
random displacements are large enough to eliminate the diffraction picture
through reduction of the Debye-Waller factor, but not so large as to
distroy the crystal structure topology. In this case, the amorphization
arises from a random displacive or orientational transition instead of
from a reconstructive transition and the memory of the original topology
of the low-pressure crystal structure being retained through the remaining
stable branches. For verification whether the memory of the original
topology of the crystal structure in the minerals in question is retained
or not, the amorphization of natrolite Na2[Al2Si3O10] 2H2O (Khibiny, Kola
Peninsula, Russia) end edingtonite Ba[Al2Si3O10] 4H2O (Bohlet, Sweden) was
investigated at high pressures up to 11 GPa, using the Raman spectroscopy.
It turned out that after release of pressure amorphization, these minerals
recover their Raman spectra, suggesting that flat branches of the phonon
spectrum hold in the memory about the original topology of the crystal
structure, and therefore the new model is suitable for describing the
microscopic processes underlying the amorphization of these minerals.
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