Structural studies and polymorphism in amorphous solids and liquids at high pressure

Area of Science:

• Materials Science
• Condensed Matter Physics
• Physical Chemistry

Background:

• Amorphous materials exhibit structural changes upon compression, sometimes discontinuously, termed polyamorphism.
• Evidence for polyamorphic transitions (e.g., low- to high-density liquids) has been indirect, relying on thermodynamic properties.
• Understanding these transitions is crucial for materials exhibiting anomalous densification behavior.

Purpose of the Study:

• To review the phenomenon of polyamorphism in amorphous materials.
• To discuss experimental observations, simulation studies, and theoretical models of polyamorphism.
• To highlight recent advancements in understanding pressure-induced structural transformations.

Main Methods:

• Advanced X-ray and neutron scattering techniques.
• Molecular dynamics simulations.
• Thermodynamic analysis of two-state models.

Main Results:

• Direct observation of structural changes in polyamorphic systems under pressure.
• Validation of 'two state' or 'two species' models for anomalous liquid behavior.
• Identification of first-order transitions in supercooled liquids as critical phenomena.

Conclusions:

• Polyamorphism involves distinct low- and high-density amorphous states.
• Cooperative effects and thermodynamic properties drive these transitions.
• The review synthesizes current knowledge on polyamorphism across various systems.