Densification
in glassy networks has traditionally been described
in terms of short-range structures, such as how atoms are coordinated
and how the coordination polyhedron is linked in the second coordination
environment. While changes in medium-range structures beyond the second
coordination shells may play an important role, experimental verification
of the densification beyond short-range structures is among the remaining
challenges in the physical sciences. Here, a correlation NMR experiment
for prototypical borate glasses under compression up to 9 GPa offers
insights into the pressure-induced evolution of proximity among cations
on a medium-range scale. Whereas amorphous networks at ambient pressure
may favor the formation of medium-range clusters consisting primarily
of similar coordination species, such segregation between distinct
coordination environments tends to decrease with increasing pressure,
promoting a more homogeneous distribution of dissimilar structural
units. Together with an increase in the average coordination number,
densification of glass accompanies a preferential rearrangement toward
a random distribution, which may increase the configurational entropy.
The results highlight the direct link between the pressure-induced
increase in medium-range disorder and the densification of glasses
under extreme compression.
Deciphering the structural evolution in irreversibly
densified
oxide glasses is crucial for fabricating functional glasses with tunable
properties and elucidating the nature of pressure-induced anomalous
plastic deformation in glasses. High-resolution NMR spectroscopy quantifies
atomic-level structural information on densified glasses; however,
its application is limited to the low-pressure range due to technical
challenges. Here, we report the first high-resolution NMR spectra
of oxide glass compressed by diamond anvil cells at room temperature,
extending the pressure record of such studies from 24 to 65 GPa. The
results constrain the densification path through coordination transformation
of Al cations. Based on a statistical thermodynamic model, the stepwise
changes in the Al fractions of oxide glasses and the effects of network
polymerization on the densification paths are quantified. These results
extend the knowledge on densification of the previously unattainable
pressure conditions and contribute to understanding the origin of
mechanical strengthening of the glasses.
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