Atomistic simulation of the effect of temperature and pressure on point defect formation in MgSiO₃perovskite and the stability of CaSiO₃perovskite

Abstract

We present an atomistic simulation method for calculating the defect formation free energy and defect volume using lattice dynamics. The periodic simulation methods have been extended to allow charged systems with subtraction of defect-defect interactions to be studied routinely. This allows constant pressure minimization to be used rather than the more traditional constant volume method, allowing direct calculation of defect formation volumes and defect free energies or enthalpies rather than lattice energy.We have applied this method to the calculation of enthalpies and volumes of vacancy and Schottky formation as a function of pressure and to the solution of Ca in the lower mantle mineral MgSiO 3 perovskite. The results indicate that as the pressure increases the defect volume of vacancy formation decreases and above approximately 50 GPa this decrease actually leads to a reduction in the enthalpy of formation rather than the expected increase as the pressure is increased further. The total Schottky enthalpy however continues to increase as a function of pressure although the rate of change of energy decreases with increasing pressure.The solution of Ca with MgSiO 3 perovskite is shown to be very unfavourable and indicates that Ca will form its own CaSiO 3 perovskite phase within the conditions expected within the lower mantle. This result is important when considering the amount and location of trace elements such as Al within the mantle that have been shown to be preferentially located within CaSiO 3 perovskite rather than MgSiO 3 perovskite.

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

“…␣-PbO 2 to pyrite phase transition. Our results are comparable with previous calculations 17, 32 and experimental 33,34 data for silicates at high pressure.…”

Solid solution behaviour of CaSiO3 and MgSiO3 perovskites

“…CaSiO 3 perovskite has also instigated several theoretical calculations (Stixrude et al 1996;Chizmeshya et al 1996;Warren et al 1998;Magyari-Kope et al 2002a, b;Akber-Knutson et al 2002;Jung and Oganov 2005;Caracas et al 2005), and is found to be Pnam and I4/mcm at pressures above 14.2 GPa at 0 K. For a review of these calculations, see Jung and Oganov (2005). A variety of physical properties such as high temperature, high pressure properties, in particular elasticity (Adams and Oganov 2006;Li et al 2006), trace element incorporation (Corgne et al 2003), melting behaviour (Wang 1999) and solubility with MgSiO 3 (Kesson et al 1995;Watson et al 2000;Vitos et al 2006) have been investigated using theoretical calculations.…”

“…This difference in ionic size was calculated according to a scheme based on bond strengths, coordination number and single crystal measurements. Watson et al (2000) employed an atomistic simulation method based on the Born model of solids in which the individual interactions are defined by interatomic potentials. They calculated the Ca partitioning within the lower mantle through the solution energy of Ca in MgSiO 3 .…”

Lattice Dynamics from Force-Fields as a Technique for Mineral Physics

“…Their work showed that solution energies show a parabolic dependence on ionic radii and in the case of heterovalent substitutions, the solution energy of a trivalent cation was dependent on the nature of the charge compensating species. Watson et al (2000) investigated the defect chemistry of MgSiO 3 perovskite at high pressure and its ability to form solid solutions with CaSiO 3 . At the time, experimental observations gave conflicting results.…”