Application of empirical ionic models to SiO 2 liquid: Potential model approximations and integration of SiO 2 polymorph data

Details Application of empirical ionic models to SiO 2 liquid: Potential model approximations and integration of SiO 2 polymorph data Application of empirical ionic models to SiO 2 liquid: Potential model approximations and integration of SiO 2 polymorph data

May 1987

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987gecoa..51.1209e&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 51, Issue 5, pp.1209-1218

Physics

11

Scientific paper

Structural and thermodynamic properties of crystalline SiO 2 and SiO 2 liquid have been examined with Monte Carlo (MC), molecular dynamics (MD), and energy minimization (EM) calculations using several ionic potential models obtained from the literature. The MC and MD methods calculate the same structural and thermodynamic properties for liquids when the same potential model is used. The Ewald (1921) method of calculating coulomb interactions reproduced most successfully the structure of liquid silica. Approximating the coulomb interaction by eliminating the inverse lattice sum results in predicted bond distances that are too short and an average angle of approximately 180°. Introduction of a cut-off in the potential energy function produces irregular tetrahedra and inconsistencies in predicted Si-O coordination in silica liquid. The system internal energies show that liquid structures derived from random starting configurations can be metastable relative to structures calculated from crystalline starting configurations. The static lattice properties of the polymorphs alpha-quartz, coesite, and stishovite were used to evaluate further the accuracy of different sets of repulsive parameters for the full Ewald ionic model. Most of the models studied reproduced poorly the measured structures and elastic constants of the polymorphs. The major weakness of the ionic model is the unreasonably large Si-O bond strength (120 × 10 -12 ergs/bond) when formal ionic charges are used. Fractional charge models with a small Si-O bond strength (30 × 10 -12 ergs/bond) improve the agreement with experimental data. However, further improvement of the ionic model should include reducing the Si-O bond strength to values in better agreement with published estimates (7 × 10 - 12 to 13 × 10 -12 ergs/bond). By using additional information to constrain the parameterization of the ionic model, such as estimated bond strengths and static properties of the silica polymorphs, a model more representative of the interparticle interactions may be obtained.

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