Ab initio assessment of high-P,T postperovskite phase equilibria in the MgSiO3-Al2O3 pseudo binary system (Invited)

Details Ab initio assessment of high-P,T postperovskite phase equilibria in the MgSiO3-Al2O3 pseudo binary system (Invited) Ab initio assessment of high-P,T postperovskite phase equilibria in the MgSiO3-Al2O3 pseudo binary system (Invited)

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmdi21b..06t&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #DI21B-06

Physics

[8124] Tectonophysics / Earth'S Interior: Composition And State, [3924] Mineral Physics / High-Pressure Behavior, [7208] Seismology / Mantle

Scientific paper

First principles simulations based on the density functional theory have been applied with great success to investigate high-pressure crystallographic and thermodynamic properties of various classes of materials. It is definitely worthy to extend this powerful technique to high-P,T thermodynamics of multi-phase and multi-component systems, which are, in particular, of significant geophysical interest. Here, we try to develop the computation techniques for the multi component thermodynamics of Earth and planetary materials. Gibbs free energy at a pure state can be represented as a sum of the static and phonon contributions to free energy. The latter for solid phase is calculated combining the ab initio lattice dynamics method and quasiharmonic approximation as our previous works [1,2]. Additional chemical component yields the mixing contributions to free energy. Since atomic substitutions often obviously prefer some specific configurations due to the interdefect interactions, the mixing entropy may change from the ideal value depending on the number of configurations energetically allowed at a given T. Instead of simple ideal approximation, we investigate the non-ideal mixing properties based on the configurational density of state calculations combining the fundamental statistical mechanics. For the first example, we have investigated high-P,T post-perovskite phase equilibria in the MgSiO3-Al2O3 system [3], since it is of particular interest for the possible origin of the deep mantle D’’ seismic discontinuity. Although earlier studies reported a significant divariant loop suggesting no sharp transition expected to occur in this system, present calculations demonstrate that Al2O3 produces a narrow perovskite-post-perovskite coexisting P,T area (ΔP~ 1 GPa) for the pyrolitic Al concentration, which is sufficiently responsible to the seismic discontinuity. We also found that the perovskite structure spontaneously changes to the Rh2O3(II) structure with increasing the Al concentration involving very small displacements of the Mg-site cations. Results suggest the technique of multi-configuration sampling including phonon effects appears to work well for mineral compounds. References [1] T. Tsuchiya et al. Earth Planet. Sci. Lett., 224, 241 (2004) [2] J. Tsuchiya et al., Phys. Rev. B, 72, 020103(R) (2005) [3] J. Tsuchiya and T. Tsuchiya, Proc. Natl. Acad. Sci. USA, 105, 19160 (2008)

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