First-principles computation of mantle materials in crystalline and amorphous phases

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First-principles methods based on density functional theory are used extensively in the investigation of the behavior and properties of mantle materials over broad ranges of pressure, temperature, and composition that are relevant. A review of computational results reported during the last couple of decades shows that essentially all properties including structure, phase transition, equation of state, thermodynamics, elasticity, alloying, conductivity, defects, interfaces, diffusivity, viscosity, and melting have been calculated from first principles. Using MgO, the second most abundant oxide of Earth's mantle, as a primary example and considering many other mantle materials in their crystalline and amorphous phases, we have found that most properties are strongly pressure dependent, sometimes varying non-monotonically and anomalously, with the effects of temperature being systematically suppressed with compression. The overall agreement with the available experimental data is excellent; it is remarkable that the early-calculated results such as shear wave velocities of two key phases, MgO and MgSiO perovskite, were subsequently reproduced by experimentation covering almost the entire mantle pressure regime. As covered in some detail, the defect formation and migration enthalpies of key mantle materials increase with pressure. The predicted trend is that partial MgO Schottky defects are energetically most favorable in Mg-silicates but their formation enthalpies are high. So, the diffusion in the mantle is likely to be in the extrinsic regime. Preliminary results on MgO and forsterite hint that the grain boundaries can accommodate point defects (including impurities) and enhance diffusion rates at all pressures. The structures are highly distorted in the close vicinity of the defects and at the interface with excess space. Recent simulations of MgO-SiO binary and other silicate melts have found that the melt self-diffusion and viscosity vary by several orders of magnitudes with pressure, temperature, and composition. The predicted high compressibility and complex dynamical behavior can be associated with structural changes (involving non-bridging oxygen, oxygen tri-clusters, Si-O pentahedra, etc.) occurring on compression. We envision future prospect for massively parallel/distributed computing of unprecedented magnitude and scope in the study of relevant materials for Earth, super-Earth, and other planets. A renewed computational theme perhaps should be the first-principles simulations of large systems (with long runs) that are necessary to explore realistic (natural) compositions, polycrystalline phases, multi-component melts, crystal/melt interfaces, trace element partitioning, etc. 3 2

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Physics of the Earth and Planetary Interiors

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