5590 J. Phys. Chem. B, Vol. 106, No. 22, 2002
Letters
which is important in geophysics, materials science, and
fundamental physics.30 Pressure-induced amorphization of crys-
talline silica occurs at 25-35 GPa and 300 K.31 Although this
amorphization is in a different p-T domain than that of ice I at
≈1.0 GPa and 77 K, it demonstrates the remarkable similarity
in physical properties of these tetrahedrally bonded com-
pounds.31,32 Teter et al.13 point out that “it is still unclear what
phases of silica are stable at high pressure, what types of
metastable phases are possible, and what structural similarities
may exist among them”. It is conceivable that the effect of
heating rate at a given pressure on the type of metastable phase
formed as outlined in this study for HDA is also important for
crystallization of pressure-amorphized silica and that its study
clarifies contradictory experimental results.
Figure 4. Schematic picture visualizing the requirement of different
temperature dependence of rate constants for the HDA f ice IV
transition (solid line) and the HDA f ice XII transition (broken). Te is
the temperature where the two rate constants have the same value.
Acknowledgment. We are grateful to Dr. O. Mishima for
information and to the “Forschungsfo¨rderungsfonds” of Austria
for financial support (Project No. 13930).
ice XII in Figure 1e. Thus, at low temperatures, the rate constant
for formation of ice IV (kHDAfice IV) must be much higher than
that of ice XII (kHDAfice XII), whereas at high temperatures the
opposite holds. This requires that kHDAfice XII increases much
more with temperature than kHDAfice IV and that the activation
energy for HDA f ice XII formation is much higher than that
for HDA f ice IV formation.
References and Notes
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This is visualized in Figure 4 in a schematic manner for
Arrhenius behavior. Below a certain temperature, Te, kHDAfice
IV is much larger than kHDAfice XII, and consequently, nearly
pure ice IV is formed. Above Te, the opposite holds, and nearly
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We finally discuss implications for the polymorphism of silica