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 200GPa 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 were energetically most favorable at zero pressure

whereas O-Frenkel pairs become systematically more favorable at pressures higher

than 20GPa. The activation enthalpies of ionic migrations obtained by the nudgedelastic-

band method suggested that the interstitial mechanisms were favoured 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.

First-Principles Simulations of Native Point Defects and Ionic Diffusion in High-

Pressure Polymorphs of Silica. A.K.Verma, B.B.Karki: Physical Review B, 2009,

79[21], 214115