First-principles simulations of native point defects and ionic diffusion in high-pressure polymorphs of silica

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Several native point defects including vacancies, interstitials, and their complexes were studied in high-pressure polymorphs of silica (stishovite, CaCl2, α -PbO2, and pyrite types) up to 200 GPa within density-functional theory. The formation enthalpies of the individual defects strongly depend on atomic chemical potentials and the Fermi level. Their values were shown to increase by a factor of 2 over the entire pressure range studied with large differences in some cases between different phases. The Schottky defects are energetically most favorable at zero pressure whereas O-Frenkel pairs become systematically more favorable at pressures higher than 20 GPa. The activation enthalpies of ionic migrations obtained by the nudged-elastic-band method suggest that the interstitial mechanisms are favored over the vacancy hoping mechanisms. The geometric and electronic structures of defects and migrating ions vary largely among different types of defects. In particular, the O defects introduce localized electronic states. These structures remain qualitatively unchanged with compression showing similar trends among different polymorphs. © 2009 The American Physical Society.

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Physical Review B - Condensed Matter and Materials Physics

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