First-Principles Investigations of Defects in Minerals

Abstract

The ideal crystal has an infinite 3‐dimensional repetition of identical units which may be atoms or molecules. But real crystals are limited in size and they have disorder in stacking which as called defects. Basically three types of defects exist in solids: 1) point defects, 2) line defects, and 3) surface defects. Common point defects are vacant lattice sites, interstitial atoms and impurities and these are known to influence strongly many solid‐state transport properties such as diffusion, electrical conduction, creep, etc. In thermal equilibrium point defects concentrations are determined by their formation enthalpies and their movement by their migration barriers. Line and surface defects are though absent from the ideal crystal in thermal equilibrium due to higher energy costs but they are invariably present in all real crystals. Line defects include edge‐, screw‐ and mixed‐dislocations and their presence is essential in explaining the mechanical strength and deformation of real crystals. Surface defects may arise at the boundary between two grains, or small crystals, within a larger crystal. A wide variety of grain boundaries can form in a polycrystal depending on factors such growth conditions and thermal treatment.In this talk we will present our first‐principles density functional theory based defect studies of SiO2 polymorphs (stishovite, CaCl2‐, α‐PbO2‐, and pyrite‐type), Mg2SiO4 polymorphs (forsterite, wadsleyite and ringwoodite) and MgO [1–3]. Briefly, several native point defects including vacancies, interstitials, and their complexes were studied in silica polymorphs upto 200 GPa. Their values 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 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. For Mg2SiO4 polymorphs native and protonic point defects were investigated upto 30 GPa. The Mg2+‐Frenkel defects in forsterite and MgO pseudo‐Schottky defects in wadsleyite and ringwoodite are energetically most favorable. Mg migration is easiest in forsterite and ringwoodite whereas Si migration is easiest in wadsleyite. Protons show substantially effect on structural transition pressures and PV equations‐of‐states. In our work on MgO, we showed that the point defect formation is easier in grain boundary interfacial regions than in bulk and pressure increasingly stabilizes interfacial vacancies relative to bulk thereby causing as enhancement in the vacancy concentrations. Symmetric tilt grain boundaries show structural phase transitions to asymmetric tilt grain boundaries under pressure.