Polyamorphism and Structural Transitions during Glass Formation

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0072258KiefferThere is increasing evidence that the glassy and liquid states are structurally and thermodynamically distinct, although both amorphous. This implies that the glass transition is a "polyamorphic" transformation. Furthermore, a tendency of the substance to undergo structural transitions in the glassy state could affect materials properties such as non-linear optical responses and mechanical behaviors. This project comprises the investigation of the structural evolution in glass-forming materials as a function of pressure and temperature. The extent to which structural relaxation during glass formation involves polyamorphic structural transitions, as opposed to a viscous slowing within an invariant structure, will be determined. Structural characterization in supercooled melts will be accomplished using a combination of Brillouin and Raman light scattering and molecular dynamics simulations, which will allow one to identify the mechanical response of the structure and the nature of structural building blocks. Brillouin light scattering will be used to determine the high-frequency complex mechanical modulus of glass-forming melts at the molecular scale. The real component, or storage modulus, provides information on the structural integrity and network connectivity, while the imaginary component corresponds to the energy dissipated in aperiodic motions of small structural constituents (e.g., atomic hopping). Simultaneous Raman scattering will allow one to establish a direct correlation between this visco-elastic behavior and the symmetries and abundance of molecular building blocks. Molecular dynamic simulations will be used to reconcile the experimentally determined Raman spectra and visco-elastic properties with a detailed geometric description of the molecular structure. The manifestation of polyamorphism will be revealed by examining a series of glass-forming systems, chosen to systematically sample a wide range of glass-forming attributes, such as the valences, bonding types of the network elements, and melt fragility. Systems will include germanates, phosphates, tellurites, and chalcogenides. The relationship between the nature of the network former and the mechanisms of transitions between different non-crystalline polymorphs will be established by studying the effects of simultaneous pressure and temperature on the transition processes using heated diamond anvil cells.The objective of this project is to clarify the concept and manifestations of polyamorphism. With this clarification, our understanding of important issues in glass science will be advanced, since a tendency of a material to undergo structural transitions in the glassy state could affect its properties, such as non-linear optical responses and mechanical behaviors, which are important for high tech applications. Expanding the range of applicability of glasses will enable new technologies, including photonics, optical telecommunication and computing, drug and radiation delivery, bio-medical implants, sensors, energy storage and generation, nuclear waste containment, and light-weight metallic alloys.