The effect of Fe on crystal structure and elasticity superhydous Phase H under high pressure by First-principles calculations

Abstract

Being one of the potentially important hydrous phases of the lower mantle, it is important to study the properties of phase H to understand the structure and composition of the mantle. The crystal structure, elastic modulus, and seismic wave velocity of phase H under different Fe concentrations (0, 12.5, 25, 100 at%) at 16–60 GPa were calculated by the first-principles simulation. The density of phase H linearly increases with increasing Fe concentration. The iron concentrations of 35.5–84.3 at% lead to densities matching the mantle density profile at different depths of the Earth. The effects of Fe on different elastic constants show varying tendencies. The K value increases with the Fe concentration, while the G value decreases. The values for Vp and Vs increase almost linearly with the rise in pressure. The Vp and Vs values decrease with increasing Fe content. The wave velocities of the pure-Mg phase H and Fe-bearing phase H are close to the typical velocity of the Earth at 500–1400 km depth. The FeOOH-AlOOH-MgSiH2O4-FeSiH2O4 system may be responsible for the observed seismic properties of LLSVP in the Earth’s lower mantle. The quantitative effect of Fe on the density, elastic moduli (K and G), and wave velocities (Vp and Vs) are listed as fitted equations. These results help to infer the Fe concentration and structure of the deep Earth.