Pressure Induced Phase Transformations in Silica
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ABSTRACT Silica, Si0 2 , is one of the most widely studied substance because of its complex and unusual properties. We have used a recently developed 2-body interaction force field [1] to study the structural phase transformations in silica under various pressure loading conditions. The specific transformations we studied are the a-quartz to stishovite, coesite to stishovite and fused glass to a dominantly six-coordinated dense glassy phase. Molecular dynamics simulations are performed under constant loading rates ranging from 0.1 GPa/ps to 1.0 GPa/ps, with final pressures upto 100 GPa and at temperatures of 300, 500, and 700 K. We observe the crystal to crystal transformations to occur reconstructively, whereas it occurs in a smooth and displacive manner from glass to a stishovite-like phase confirming earlier conjectures.[2] We studied the dependence of transition pressure on the loading rate and the temperature to elucidate the shock loading experiments. We also studied the unloading behavior of each transformation to assess the hysterisis effect.
INTRODUCTION Despite its simple chemical formula, SiO 2 , silica has some of the most complex, interesting and unusual properties of any material. It is ubiquitous in the earth's crust, and silicon and oxygen are abundant elements in the solar system. The manner in which silica responds to pressure and stresses also touches many fields of study, from geology to microelectronics. Understandanding the properties of silica has impact from making simple tools to glass fibers to the design, processing and manufacturing of micro- and nano-electronic devices. Due to the rich variety of its polymorphs
and amorphous states, it also serves as a model system for studies of high pressure structures and structural phase transformations. Over the last decade an increasing number of researchers from different fields of science attempted to investigate the various aspects of silica, such as structure, melting, phase behavior, and energetics using molecular mechanics, molecular dynamics and monte carlo methods. If silica is exposed to a shock wave, various low density silica polymorphs and its glass phase transform to high density stishovite along the Hugoniot isentrope. Several years ago Stolper and Ahrens[2] pointed out a difference in the a-quartz to stishovite vs. glass to stishovite transformation. .... This transformation takes place in a displacive manner rather than through reconsruction". In this study, we aim to shedding some light onto this sound conjecture by analyzing the microstructures of the model systems along the transformation path. The representation of atomic interactions is central to all classical computational approaches. We recently developed a simple 2-body interaction potential in which the long range electrostatic interactions are handled via a variable charge model and the short range valence and van der Waals interactions are represented by a morse potential [1]. The partial charges are determined by the charge equilibration method of Rappe and Goddard.[3]
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