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Modeling Oxides, Ceramics, Zeolites and Glasses

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Silica Force Fields

We have developed new interatomic potentials for the descriptions of the various forms of silica.1 We used Morse potentials for the short range interactions and the electrostatic interactions are taken as pure coulombic, i.e. long ranged. The charges of the ions are taken to be configuration dependent. They are determined by using the Charge Equilibration method of Rappe and Goddard. Using these new interatomic potentials we performed structure optimizations and molecular dynamics simulations of all silica polymorphs. We compare and contrast our results with the potentials developed earlier by other researchers. To test the transferability of this interaction potentials we have studied pressure induced phase transformations such as quartz to stishovite and coesite to stishovite under various loading rates and temperatures. 2

Simulation of silica polymorphs and glasses

In the past few years, considerable effort has gone into developing force fields to use in molecular dynamics (MD) simulations of inorganic glasses. These simulations are used to obtain molecular level insights into the structure of glasses. However, there has been no published systematic study detailing the impact of various MD approaches used to generate the glass structure upon the calculated structure parameters. In this paper, we investigate the effect of the initial structure (randomly generated or melt obtained from crystal), annealing cycle (initial soak temperature and time as well as cooling rate), system size (ranging from 600 to 3240 atoms), type of dynamics (NPT vs. NVT) and force field upon a variety calculated structural parameters of vitreous silica. These parameters include density, radial distribution function, bond angle distribution, and ring size distribution. The reproducibility of these calculated parameters is also studied. 3

Simulation Phase transitions in Silica

Silica, SiO2, is one of the most widely studied substance, and it has some complex and unusual properties. We have used recently developed 2-body interaction force field to study the structural phase transformations in silica under various pressure loading conditions. The specific transformations we studied are alpha-quartz to stishovite, coesite to stishovite and fused glass to stishovite-like dense, a dominantly six-coordinated glassy phase. Molecular dynamics simulations are performed under constant loading rates ranging from 0.1 GPa/ps to 1.0 GPa/ps, pressures upto 100 GPa and at temperatures 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. To elucidate the shock loading experiments, we studied the dependence of transition pressure on the loading rate and the temperature. To assess the hysterisis effect we also studied the unloading behavior of each transformation. 2

Molecular structures from alpha quartz simulation: before phase transformation and after the phase transformation.

Force Fields and Simulation of Aluminophosphates

Aluminophosphate zeolite is an artificial material which shows strange hydrophilicity. We have been investigating the reason by quantum mechanics, and found that the hydrophilicity of this zeolite depended on the local geometric deformation and, speculated that the site-specific hydrophilicity might be explained by differences in the stiffness of the local deformation determined by the location with the pore structure. In order to test these ideas, we construct a reliable force field based on the new MS-Q one developed by Demiralp, Cagin, and Goddard.1 Our force field well reproduces the experimental structure of VPI-5. 4

Extensions and Current Work

Recent efforts focused on extending these studies to other oxides5, zeolites6, and clays.7

References

  1. Silica Force Fields Ersan Demiralp, T. Cagin, W. A. Goddard, III.
  2. Phase Transformations in Silica T. Cagin, Ersan Demiralp, W. A. Goddard, III.
  3. Simulation of Silica Glasses Ersan Demiralp, T. Cagin, W. A. Goddard, III.
  4. Modeling VPI-5 Osamu Kitao, Ersan Demiralp, T. Cagin, W. A. Goddard, III.
  5. in progress, Ersan Demiralp, T. Cagin, W. A. Goddard, III.
  6. in progress, Ersan Demiralp, Jan Sefcik, Osamu Kitao, T. Cagin, and W. A. Goddard, III
  7. in progress, Ersan Demiralp, Jan Sefcik, T. Cagin and W. A. Goddard, III.

Collaborators

Ersan Demiralp, Norman T. Huff, Jan Sefcik, Osamu Kitao and W. A. Goddard, III.

Last modified: December 1997.

tahir@www.wag.caltech.edu