Application of empirical ionic models to SiO2 liquid: Potential model approximations and integration of SiO2 polymorph data |
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| Affiliation: | 1. Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining 810008, China;2. Department of Chemistry, Faculty of Science, Fukuoka University, 8-19-1 Nanakuma, Jonan, Fukuoka 814-0180, Japan;3. Key Laboratory of Salt Lake Resources Chemistry of Qinghai Province, Xining 810008, China;1. Departamento de Ingeniería Textil y Papelera, Universitat Politècnica de València, Escuela Politécnica Superior de Alcoy, Plaza Ferrándiz y Carbonell s/n, 03801 Alcoy, Spain;2. Departamento de Mecánica de los Medios Continuos y Teoría de Estructuras, Universitat Politècnica de València, Escuela Politécnica Superior de Alcoy, Plaza Ferrándiz y Carbonell s/n, 03801 Alcoy, Spain |
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| Abstract: | Structural and thermodynamic properties of crystalline SiO2 and SiO2 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 〈Si-O-Si〉 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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