Structural and elastic anisotropy of crystals at high pressures and temperatures from quantum mechanical methods: The case of Mg2SiO4 forsterite

Abstract

Articles you may be interested inOn combining temperature and pressure effects on structural properties of crystals with standard ab initio techniques J. Chem. Phys. 141, 124115 (2014) We report accurate ab initio theoretical predictions of the elastic, seismic, and structural anisotropy of the orthorhombic Mg 2 SiO 4 forsterite crystal at high pressures (up to 20 GPa) and temperatures (up to its melting point, 2163 K), which constitute earth's upper mantle conditions. Single-crystal elastic stiffness constants are evaluated up to 20 GPa and their first-and second-order pressure derivatives reported. Christoffel's equation is solved at several pressures: directional seismic wave velocities and related properties (azimuthal and polarization seismic anisotropies) discussed. Thermal structural and average elastic properties, as computed within the quasi-harmonic approximation of the lattice potential, are predicted at high pressures and temperatures: directional thermal expansion coefficients, firstand second-order pressure derivatives of the isothermal bulk modulus, and P-V -T equation-of-state. The effect on computed properties of five different functionals, belonging to three different classes of approximations, of the density functional theory is explicitly investigated. C 2015 AIP Publishing LLC. [http://dx

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“…Indeed, PBE0 gives an excellent description of the a lattice parameter (with a deviation smaller than 0.04%), only slightly overestimating the c one (by 1.2%), reliably describes the optical anisotropy of the system (with a of 0.011 with respect to the experimental value of 0.013) and provides an excellent description of the elasticity of the system, with a bulk modulus of 90.4 GPa with respect to the experimental value of 89.4 GPa; an overestimation by about 1% is perfectly compatible with the reduction of the bulk modulus that would be induced by the neglected zeropoint motion and that could be evaluated by quasi-harmonic calculations. [55][56][57][58][59] As expected, hybrid functionals describe a significantly larger gap E g (5.8-6.2 eV) than local-density and generalized-gradient approximations. All functional give a good description of the atomic positions within the cell.…”

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