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

Abstract

Several native point defects including vacancies, interstitials, and their complexes were studied in highpressure polymorphs of silica ͑stishovite, CaCl 2 , ␣-PbO 2 , and pyrite types͒ up to 200 GPa within densityfunctional 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-elasticband 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.

Ab initiostudy of Cu diffusion inα-cristobalite

“…The antibonding states in the conduction band are separated from the valence band by a band gap with width 5.7 eV. The predicted width of the band gap is much smaller than the experimental value of 8.9 eV [49], but is consistent with other ab initio calculations which have not included any non-local corrections for the exchange correlation energy [30,50,51].…”

“…On the other hand, many works based on DFT have been published about impurities and diffusion of lighter elements, such as oxygen [20], carbon [21], nitrogen [22], boron [23][24][25] and aluminum [26]. Also much attention has been paid to simulations of native defects in different SiO 2 modifications and their migration [27][28][29][30].…”

Hydrogen mobility in transition zone silicates

“…The presence of iron, additional defects, or experimental strain may explain the difference. Our computed activation enthalpies are larger than the ones obtained from static simulations (Verma and Karki 2009), by almost one order of magnitude in case of ringwoodite. Table 2 lists the diffusivity fits to our computed diffusion coefficients.…”

Anomalous elastic properties in stishovite

“…Such pressures can be encountered in meteorite impacts 15 and in the Earth's mantle. In fact, studying the mechanical properties of stishovite as a function of pressure is essential since it may be a significant constituent of the Earth's mantle, [16][17][18][19][20][21][22][23] and accurate knowledge of the elastic moduli and the pressure dependence of the silicate polymorphs which compose the mantle is essential for seismology. 4,24 This has led to many studies being devoted to the determination of the mechanical properties of stishovite, including the experimental determination of its elastic constants at different pressures.…”