Molecular dynamics simulation of vacancy-solute binding free energy in periclase
- PDF / 2,269,949 Bytes
- 7 Pages / 595.276 x 790.866 pts Page_size
- 104 Downloads / 160 Views
ORIGINAL PAPER
Molecular dynamics simulation of vacancy‑solute binding free energy in periclase Adriaan A. Riet1 · James A. Van Orman2 · Daniel J. Lacks1 Received: 23 May 2020 / Accepted: 3 August 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The free energy of binding between a cation vacancy and charge-balancing tetravalent cation in MgO is calculated from molecular dynamics simulations, at temperatures of 1200–2400 K and pressures of 0–25 GPa. The entropy of binding, obtained from the temperature dependence of the binding free energy, is found to be independent of pressure and to have a value of 13(5) J/mol K (2σ). The binding volume, obtained from the pressure dependence of the binding energy, is independent of temperature and has a value of 1.7(3) cm3/mol (2σ). These results are in excellent agreement with experimental estimates for several different trivalent cations, showing that the binding entropy and volume are not strongly dependent on the identity or charge of the cation. The binding energy is predicted to increase with depth along a mantle adiabat, leading to lower concentrations of unbound vacancies, and correspondingly slower diffusion rates for unbound cations. Keywords Periclase · Molecular dynamics · Diffusion
Introduction Periclase (MgO) is considered to be the second most abundant phase in Earth’s lower mantle, and may be the most abundant mineral in the ultra-low velocity zones just above the core (Wicks et al. 2010; Garnero et al. 1998). Cation diffusion in periclase has been shown to occur primarily through an extrinsic defect mechanism (Vočadlo et al. 1995), and to potentially be fast enough to permit cation diffusion on a length scale of 1–10 km at core-mantle boundary conditions (Van Orman et al. 2003). MgO is also a useful model material for examining the influence of pressure and temperature on defect and diffusion properties, as it has a high melting temperature and does not undergo a pressureinduced phase transition until 400 GPa. In MgO, solutes with valence greater than two are accompanied by vacancies on cation sites that are created to balance the excess charge of the solute. Because the solute and vacancy have opposite effective charges, they experience * Daniel J. Lacks [email protected] 1
Department of Chemical and Biomolecular Engineering, Case Western Reserve University, Cleveland, OH, USA
Department of Earth, Environmental, and Planetary Sciences, Case Western Reserve University, Cleveland, OH, USA
2
a coulombic attraction that tends to bind them as pairs on neighboring cation sites (e.g. Sempolinski and Kingery 1980; Carroll et al. 1988; Van Orman et al. 2009; Crispin and Van Orman 2010). Similar solute-vacancy binding occurs in many other crystalline materials, including halides (Bassani and Fumi 1954), olivine (Jollands et al. 2018) and metals (Yamamoto and Doyama 1973; Klemradt et al. 1990; Hoshino et al. 1996; Ohnuma et al. 2009). The formation of solute-vacancy pairs has strong effects on diffusion and other properties that
Data Loading...
