[1] We present first-principles results for elastic moduli (bulk, K, and shear, G) and acoustic velocities (compressional, V P , shear, V S , and bulk V Φ ) of olivine (a) and wadsleyite (b) (Fe x ,Mg 1 À x ) 2 SiO 4 , at high pressure (P) and temperature (T) with varying iron content (0 ≤ x ≤ 0.125). Pressure and temperature derivatives of these properties are analyzed. We show that adding 12.5% of Fe in forsterite softens V P and V S by $3-6%, the same effect as raising temperature by $1000 K in dry olivine at 13.5 GPa-the same is true in wadsleyite. This study suggests that Fe is effective in producing seismic velocity heterogeneity at upper mantle and transition zone conditions and should be another key ingredient, in addition to temperature and water content variations, in interpreting seismic heterogeneities in the transition zone. The effect of Fe on density, elastic, and velocity contrasts across the a ! b transition is also addressed at relevant conditions. We show that simultaneous changes of composition, temperature, and pressure do not affect significantly the relative density contrasts. We also find that compressional and shear impedance contrasts result primarily from velocity discontinuities rather than density discontinuity.

[1] We study the influence of iron on the elasticity of (Mg 1−x , Fe x ) 2 SiO 4 olivine (0 ≤ x ≤ 0.125), a major constituent of the Earth's upper mantle. We calculate static elastic properties by first principles for this solid solution and investigate the effect of atomic arrangement, an artifact of supercell calculations, on all single crystal and poly-crystalline elastic moduli. From calculated wave propagation velocities we find the heterogeneity ratios of shear to compressional wave velocity, and bulk sound to shear wave velocity. Their values are, though limited to composition considerations, marginally consistent with seismic tomography.

Efficient thermoelectric materials are highly desirable, and the quest for finding them has intensified as they could be promising alternatives to fossil energy sources. Here we present a general first-principles approach to predict, in multicomponent systems, efficient thermoelectric compounds. The method combines a robust evolutionary algorithm, a Pareto multiobjective optimization, density functional theory and a Boltzmann semi-classical calculation of thermoelectric efficiency. To test the performance and reliability of our overall framework, we use the well-known system Bi2Te3-Sb2Te3.

We study the effects of cation inversion x (Mg $ Al, with x representing the fraction of Mg and Al exchanged) and magnetic substitution (Mn ! Mg) on the elastic properties of the MgAl 2 O 4 spinel system using density functional theory and Brillouin scattering techniques. Our computations show that cation inversion decreases the molar volume of spinel and produces a stiffening of C 11 and a softening of C 12. Simulations and experiments agree within 2%. Density functional theory also captures the qualitative effect of Mg $ Al on C 44 , that is, an initial softening for inversion degree at x , 0:125 and stiffening at x ¼ 1, with a disagreement of ,4%. The Zener anisotropy factor A decreases with increasing degree of inversion. All these trends are preserved at high pressures. The substitution of Mn for Mg produces and increases the molar volume of spinel, and it is accompanied by the softening of both C 11 and C 44 , and the stiffening of C 12 in good agreement with experimental results at ambient conditions. All these effects, which are qualitatively opposite to those of cation inversion, are enhanced at high pressures. The effect of Mn ! Mg on the elastic anisotropy of spinel is, however, qualitatively similar to that of cation inversion, i.e., it causes a decrease in the Zener factor A.