WILDING, Martin, WILSON, Mark and MCMILLAN, Paul F. (2006). Structural studies and polymorphism in amorphous solids and liquids at high pressure. Chemical Society Reviews, 35 (10), 964-986.
Abstract
When amorphous materials are compressed their structures are expected to change in response to
densification. In some cases, the changes in amorphous structure can be discontinuous and they
can even have the character of first-order phase transitions. This is a phenomenon referred to as
polyamorphism. Most evidence for polyamorphic transitions between low and high density
liquids or analogous transformations between amorphous forms of the same substance to date has
been indirect and based on the changes in thermodynamic and other structure-related properties
with pressure. Recent studies using advanced X-ray and neutron scattering methods combined
with molecular dynamics simulations are now revealing the details of structural changes in
polyamorphic systems as a function of pressure. Various ‘‘two state’’ or ‘‘two species’’ models are
used to understand the anomalous densification behaviour of liquids with melting curve maxima
or regions of negative melting slope. Thermodynamic analysis of the two state model leads to the
possibility of low- to high-density liquid transitions caused by differences in bulk thermodynamic
properties between different amorphous forms and on the degree of cooperativity between
low- and high-density structural configurations. The potential occurrence of first-order transitions
between supercooled liquids is identified as a critical-like phenomenon. In this tutorial review we
discuss the background to polyamorphism, incorporating the experimental observations,
simulation studies and the two-state models. We also describe work carried on several systems
that are considered to be polyamorphic.
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