High Pressure Elasticity of MgSiO 3 Perovskite, MgO and SiO 2
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PEROVSKITE,
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BIJAYA B. KARKI Department of Physics and Astronomy, University of Edinburgh, Edinburgh, EH9 3JZ, Scotland Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA
ABSTRACT Full elastic constant tensors (cij) of three minerals namely, MgSiO 3 perovskite, MgO and Si0 2 , are obtained as a function of pressure up to 140 GPa using first-principles computer simulations based on the local density and pseudopotential approximations. The zero pressure values and initial pressure dependence of athermal elastic constants derived from stress-strain relations are in excellent agreement with available experimental data. We find that elastic moduli, wave velocities and anisotropy of the minerals are strongly pressure dependent, particularly, in the vicinity of the structural transformations. In the view of the present experimental limitations at realistic conditions of the inner Earth, our results for high pressure elasticity are expected to be of substantial geophysical significance. Comparisons based on compressional and shear wave velocities support the prevailing hypothesis of Mg-rich silicate perovskite dominated composition for the lower mantle.
INTRODUCTION High pressure behavior of the elastic properties of materials is important for several reasons. Elasticity determines the mechanical deformations and stability of solids under external stresses. The propagation of sound (elastic) waves in a medium and the elastic anisotropy can be studied from knowledge of the elastic constants. In addition, the elastic constants give substantial insight into the nature of binding forces in crystals. Above all, the high pressure elasticity of potentially relevant minerals is of substantial geophysical importance for the Earth's interior. Because of the absence of samples from the deep interior, comparisons between seismological observations and the elastic properties of candidate minerals are the only way to extract information regarding the composition and mineralogy of the Earth. In this paper, we present the first-principles results on the high pressure elasticity of three important minerals, namely MgSi0 3 orthorhombic perovskite, MgO and Si0 2 , and discuss their implications from the point of view of physics and geophysics. It is generally accepted that the lower mantle is composed of an assemblage of silicates and oxides dominated by Mgsilicate perovskite. Most of the experimental studies of these minerals are so far limited to
219 Mat. Res. Soc. Symp. Proc. Vol. 499 1998 Materials Research Society
ambient conditions or relatively low pressures and temperatures. In view of present experimental limitations, first-principles computer simulations have proven an attractive alternative in exploring the properties of the Earth's materials at the geophysically relevant conditions.
METHOD The computations are based on the density functional theory (DFT) using the local density and pseudopotential approximations both of which have already been used in a wide v
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