Pressure effect of self‐diffusion in periclase (MgO) by molecular dynamics

Abstract

[1] Pressure effect of self-diffusion in periclase (MgO) was investigated by molecular dynamics (MD), and viscosity variation of lower mantle periclase was predicted by employing the Nabarro-Herring model. Self-diffusion coefficients were derived directly from recorded trajectories of atoms. The MD calculations were performed at 2400-6000 K and 0-140 GPa. Migration energy (H m * ) and formation energy (H f * ) of ordinary pressure (0 GPa) are consistent with experimental data. Observed pressure dependence of self-diffusion coefficients was turned from negative with increasing pressure. The behavior of self-diffusion coefficients predicts softening of lower mantle periclase with increasing depth. We predict 1.3 times reduction of the viscosity (0.3 K/km geothermal gradient) or 2.0 times (0.6 K/km geothermal gradient).Citation: Ito, Y., and M. Toriumi (2007), Pressure effect of self-diffusion in periclase (MgO) by molecular dynamics, J. Geophys.

The influence of ferric iron and hydrogen on Fe–Mg interdiffusion in ferropericlase ((Mg,Fe)O) in the lower mantle

“…The activation energy estimated by Sempolinski and Kingery (1980) from ionic conductivity in MgO and theoretical studies (Ammann et al 2011;Ita and Cohen 1997;Ito and Toriumi 2007) agrees reasonably well with this study. Also the activation volume of vacancy migration obtained in the numerical simulations (Ammann et al 2011;Ita and Cohen 1997;Ito and Toriumi 2007) agrees well with this study. These values are slightly lower than 3.0 cm 3 / mol in MgO determined from Mg self-diffusion assuming an extrinsic vacancy concentration independent of pressure and temperature (Van Orman et al 2003).…”

“…Cation diffusion in (Mg,Fe)O was investigated both experimentally through the measurements of Mg selfdiffusion in MgO (periclase) and Fe-Mg interdiffusion in (Mg,Fe)O (e.g., (Blank and Pask 1969;Demouchy et al 2007;Holzapfel et al 2003;Mackwell et al 2005;Rigby and Cutler 1965;Van Orman et al 2003;Yamazaki and Irifune 2003) and theoretically by first principle and/ or molecular dynamic simulations (e.g., Ammann et al 2011;Ita and Cohen 1997;Ito and Toriumi 2007). Those experimental and theoretical studies suggest that cation diffusion in (Mg,Fe)O occurs by a vacancy mechanism in the extrinsic regime where vacancies V VI ″ at octahedrally coordinated cation sites are predominantly created chemically by impurities such as trivalent cations including Fe 3+ (for review, see de Koker and Stixrude 2010; Van Orman and Crispin 2010).…”

Frenkel Defects and Interstitial Atoms in Periclase (MgO) at High Temperature by Molecular Dynamics Simulation

“…This estimated value was larger than that calculated by ito and Toriumi [3], 2.1 × 10 −11 m 2 /s. This suggested that diffusion process readily occurred because both Frenkel defects and interstitial atoms exited in the crystal.…”

“…[3]. Because in these experimental studies and the past Md study, the structure included the Schottky defects as initial state, the self-diffusion processes were easy to occur.…”

“…ito and Toriumi [3] prepared pair vacancies of magnesium and oxygen ions (Schottky defects) in Md system of periclase. in the present study, we simulated the perfect Md crystal with no vacancies (no Schottky defects) as the initial state to reveal the appearance of Frenkel defects and interstitial atoms.…”