Stability limits and structure of glasses, liquids, and crystals from computer simulation.

摘要

In the principal branch of this research, Molecular Dynamics computer simulation (MD) method and Lattice Dynamic calculations (LD) are employed to study the crystalline-to-amorphous transition in silicate perovskites. The stability limits for various silicate (CaSiO{dollar}sb3,{dollar} MgSiO{dollar}sb3,{dollar} BaSiO{dollar}sb3,){dollar} and non-silicate (CaTiO{dollar}sb3,{dollar} Ca(Si-Ti)O{dollar}sb3rbrack{dollar} perovskite phases under change of pressure, in this case on decompression from high pressure stable state, are established. These phases are found to undergo crystal to glass transition when decompressed from their high pressure stable phases. Structural changes involve conversion of the six coordinated silicon to tetrahedrally coordinated chains. Lattice vibrational characteristics are calculated for the purpose of understanding the nature of this phase transition and identifying the mechanism associated with the amorphization. In the CaSiO{dollar}sb3{dollar} model system amorphization is found to be driven by an optical mode softening mechanism. The glassy product which forms on decompression, as the system expands beyond the volume of where the instability occurs, is almost identical in equation of state to the glass formed by the conventional means of quenching of the ambient pressure liquid.; In a separate study, MD is conducted on SiO{dollar}sb2{dollar} liquid to investigate its phase behavior for comparison with the anomalous behavior of water. Rigid-ion SiO{dollar}sb2{dollar} liquid, which is known to behave in a more exaggerated and anomalous fashion than real SiO{dollar}sb2,{dollar} was found to have very similar phenomenology to that of water despite the very different nature of the pair potentials. For instance, SiO{dollar}sb2{dollar} shows a pressure dependent density maximum similar in character to that found in water, and like ST2 water, it too does not display a re-entrant spinodal. Studies on SiO{dollar}sb2{dollar} glass include the pressure-induced structural changes, observing a glass-glass polyamorphic transformation like that seen recently in amorphous water. The existence of distinct glassy states of different densities is accompanied by transition from four coordinated to a high density amorphous phase of six coordinated silicon.; Finally, MD simulation method is employed to study silica glass with small compositional variations such as those obtained by adding sodium aluminate to change the charge structure while retaining the network. The idea is to seek choices of materials that shift the high frequency IR absorption peak to a lower value, while leaving the other physical properties unchanged. This is of interest in fiber optics applications to modify the wavelength of maximum transmittance and thereby to reduce the optical power losses of silica glass at a particular laser frequencies. Our results indicate that these modifications seem to effectively shift the absorption edge of the fiber glass towards lower frequencies.