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Renata M. Wentzcovitch, Associate Fellow

First Principles Simulations of Materials Under Pressure

High pressure and temperature behavior of materials has long been the subject of extensive experimental and theoretical investigations in a wide range of scientific disciplines. One area where extreme pressures and temperatures are concerned is in geophysics and geochemistry of the EarthÕs deep interior. Information about the inner Earth is vital to understand the planet's thermal, chemical, and environmental evolution. Since direct samples can be available only from depths of few tens of kilometers, comparisons between seismological observations and the elastic properties of potentially relevant minerals and mineral assemblages are the only way to extract information regarding the composition and mineralogy of this region. However, robustness of this approach is greatly hindered by the lack of sufficient information about mineral properties. Most experimental studies are so far limited to ambient conditions or relatively low pressures and temperatures. In view of present experimental limitations, first-principles computer simulations have taken on increased significance in exploring properties of Earth's materials at geophysically relevant conditions.

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Oxides and silicates are generally considered as the building blocks of the Earth's mantle. These researchers have already investigated high pressure behavior of Mg- and Ca-bearing silicates and oxides of the lower mantle and recently extended to other phases like MgSiO3 ilmenite and Al2O3. Computations are performed using first-principles variable cell shape molecular dynamics method within the framework of density functional theory (with the pseudopotential and local density approximations). The predicted structural and elastic parameters are found to be in excellent agreement with available zero and low pressure experimental data. These results have allowed, for the first time, detailed comparisons with the seismological observations of the mantle regime in terms of density, and independent compressional (P) and shear (S) wave velocity profiles and anisotropy.

The work is now developing a more complete understanding of the structure, dynamics, and composition of the Earth's mantle. For this purpose, a detailed and improved knowledge about the physical properties of the relevant phases at combined conditions of pressures and temperatures is essential. Pressure and temperature dependencies of elastic moduli for many high pressure phases are still unmeasured. This project is working on a combined pressure-temperature investigation of the structural and elastic properties of the major mantle materials. Only when temperature contributions to the elastic properties are fully considered, can one resolve the existing controversy amongst several competing compositional models for Earth's upper and lower mantle regimes. Moreover, temperature-induced variations in mineral properties are important to study lateral variations in seismic wave velocities, and the possible phase transitions may provide explanation for the seismic reflectors observed in the lower mantle. Finally, thermal properties are fundamentally related to the dynamics of the mantle-thermal expansion is essentially responsible for the mantle convection.

Further work has studied thermoelastic properties of MgO, generally considered as the second most abundant phase in the lower mantle. Temperature breaks the symmetry of the lattice so determination of dynamics from first principles is a substantial computational challenge. This can be accomplished either by determining phonon frequencies or by molecular dynamics. Unlike static high pressure studies, finite temperature computations involve simulations of large supercells. The density-functional perturbation theory used in MgO provides an efficient method for determining full phonon dispersions. Calculated phonon frequencies for equilibrium and strained configurations are used to calculate thermodynamical potentials within the quasi-harmonic approximation and several derived quantities of geophysical interest without further approximations. Temperature dependencies of individual elastic constants at high pressures for MgO are obtained, for the first time, from first-principles simulations, and are expected to represent a major step in the high temperature studies of the Earth's forming phases. This approach is being applied to other important materials. However, in order to fully consider the anharmonic effects, finite temperature molecular dynamics of large supercells are needed.

This information is available in alternative formats upon request by individuals with disabilities. Please send email to alt-format@msi.umn.edu or call 612-624-0528.

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