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We have investigated the dynamic and static mechanisms
of the amorphization of the number of frame minerals induced by
external pressure and also by cation exchange. We have discovered that the
major contribution to the distortions of the tetrahedra in the α-quartz at
high pressure is their twisting [NN Ovsyuk and SV Goryainov. Phys. Rev. B 60, 14481 (1999)].
It is shown that just these twisting vibrational
modes lead to instability resulting in amorphization of the structure.
It has been found that the internal stress tensor,
generated by the cation exchange, is of a more complicated nature
than the tensor of the external stress [NN Ovsyuk and SV Goryainov. Europhys. Lett. 64, 351 (2003)].
This difference comes from a specific coupling of
the substituting cations with local, microscopic displacements of the
neighbouring atoms inside the unit cell. It becomes evident why in a
number of experiments a significant difference in the action of internal
and external pressures on the crystal structure is observed and, also, why
the internal pressure causes the greater anisotropy than the external one.
We have also revealed some universal laws of the
amorphization mechanisms. It turned out that berlinite, quartz and
natrolite behave like anorthite at transition from different symmetry
phases to the triclinic one. According to the Landay theory of the phase
transitions, 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., it is not explained in detail. It is assumed, that this
coefficient is coupled with the elastic constants. What actually happens
is that this coefficient is the product of the elastic constant matrix and
the kinematic coefficients matrix, which are responsible for the behaviour
of normal modes of a crystal. On the basis of the analysis made, using the
"ball-and-perfect springs" model, we come to a conclusion, that the basic
mechanism causing the proper phase transitions in some crystals is the
"kinematic" anharmonicity [NN Ovsyuk and SV Goryainov. Phys. Rev. B 66, 012102
(2002)]. This anharmonicity is coupled with the transition
from the natural curvilinear atomic q-coordinates (interatomic bonds and
angles between them) to the Cartesian coordinates of the atomic
displacements. The coincidence between our equation and experiment has
appeared to be unexpected, since this means, that predominantly
"kinematic" anharmonicity brings about a decrease of the sound velocity
with pressure, when all other anharmonicities are neglected. We believe
that it is possible to obtain similar equations for the phase transitions
in other crystals if we use combinations of the elastic moduli, expressing
the elastic stability conditions to be appropriate for the transitions in
these crystals as well as the mechanism of the softening of the shift
moduli considered in the present work. This mechanism is based on the fact
that each shift modulus linearly falls with pressure, which is in line
with the "kinematic" anharmonicity.
We have also observed the phase transition between low-density
amorphous (LDA) and high-density amorphous (HDA) zeolites using a high pressure Raman
spectroscopy [NN Ovsyuk and SV Goryainov. Appl. Phys. Lett. 89, 134103 (2006)].
It is found that this transition is apparently of the first order and occurs with a silicon
coordination rise. It is shown that the Raman spectra of the LDA-HDA phase transitions in zeolites
and in silicon are almost identical, suggesting a generality of amorphous-amorphous transformations
both in simple substances and in complex polyatomic materials with tetrahedral configurations.
Thus, the structure of the LDA phase obtained from crystalline minerals
at slowly increasing temperature or pressure has physical properties different from those of
a glass obtained by conventional methods, even upon
slowest melt cooling. For example, it can be more solid
and less chemically active, than the conventional glass
of the same composition. The LDA phase is also interesting
from the theoretical point of view because its
entropy is similar to that of the initial crystal. When a
glass is obtained not from a melt but from a crystal,
some structural elements can remain ordered. The transition
occurs at low temperatures (at which everything
is frozen in a conventional liquid or glass, and relaxation
may occur for several thousand years). Disordering
during solid-phase amorphization occurs in a certain
phonon mode and certain structural elements. As a
result, the entropy of an amorphous solid may exceed
the entropy of a crystal by a very small value.
Currently, we cannot make any quantitative estimates
of the enhancement of mechanical properties, because the
analytical expressions relating the mechanical strength of
glass (Young’s modulus) to the parameters characterizing
the local crystalline topology or the configuration energy
landscape (configuration entropy) of glass are absent in
the literature. However, we can state at the qualitative level
that, the larger the topological disorder (fluctuations of
bond angles and lengths), the smaller the elastic moduli at
the same density [NN Ovsyuk. Bulletin of the Russian Academy of Sciences: Physics 72, 1433 (2008)]. Large fluctuations lead to a decrease in
the average angular hardnesses. Generally, Young’s modulus
of glass can be several times smaller than that of crystal
at similar densities. A density deficit makes this difference
even more considerable.
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