Molecular Dynamics Simulations of Structural Transitions in Crystalline and Amorphous Inorganic Compounds

232 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2004.Using MD simulations I have (i) gained detailed insights into the transformation mechanisms of crystalline silica, (ii) discovered the structure of high-density C-III cristobalite (a phase that has been known to exist but was never indexed), (iii) revealed a ubiquitous tendency for oxygen ordering in various high-density crystalline phases of silica, and (iv) used the knowledge gained concerning structural transitions in crystalline silica to explain the nature of polyamorphism in the non-crystalline counterparts of this material. I observed both reversible and irreversible polyamorphic transitions in silica glass, depending on the thermo-mechanical condition at which the transitions take place. Macroscopically, reversible transitions are manifest in the anomalous thermo-mechanical behaviors of silica glass, such as an increase of the mechanical moduli upon expansion. Irreversible polyamorphic transitions occur under large compressive stresses provided adequate thermal activation for the necessary bond exchanges to take place. This leads to a permanent densification of silica glass. In addition to tetrahedral SiO 2, I also investigated vitreous B2O3, representative of network glasses that exhibit mixed coordination states under pressure. My MD simulations show that cation coordination change plays a crucial role in the polyamorphic transitions in B2O3 glass, whereas in silica it does not. Through this research, I gained a fundamental understanding of the atomistic origin of various anomalous behaviors in materials, such as negative thermal expansion, increase of elastic moduli with temperature, amorphization of crystals under compression, and polyamorphic transitions.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD