First-principles investigations of structural, elastic and electronic properties of hydrous fayalite

Abstract

The structure, elasticity and conductivity of hydrous fayalite (Fe₂SiO₄H_x (x= 0.25, 0.5, 0.75)) are discussed based on the First-principles. Fe₂SiO₄H_x (x= 0.25, 0.5, 0.75) models are established by hydrogen atoms occupying the interstice space of the fayalite (Fe₂SiO₄) unit cell. The optimized results show that hydrogen atoms form hydroxyl (OH⁻) with the surrounding oxygen atoms in the modeling course, which is consistent with the experimental conclusion that water exists in fayalite in the form of OH⁻. The calculated results of the elastic constants for Fe₂SiO₄H_x (x= 0.25, 0.5, 0.75) show that the elastic constants increase as the pressure increases and decrease as the water content increases at 0$\sim$30 GPa. Compared to Fe₂SiO₄, the ranges of compression wave velocity (V_p) of Fe₂SiO₄H_x (x= 0.25, 0.5, 0.75) are 1.02$\sim$3.52%, 3.06$\sim$4.30%, 3.63$\sim$6.93%; in the meanwhile, the ranges of shear wave velocity (V_s) are 0.01$\sim$7.01%, 0.40$\sim$8.85%, and 2.50$\sim$3.13%; this is in line with the wave velocity, which was observed to experimentally reduce to 2$\sim$5% in the low-velocity layer of the mantle. The decrease of wave velocity due to water may be the formation mechanism of low velocity layer. When analyzing the conductivity, the conductivity per relaxation time σ/τ gradually increases with the rise of temperature, while different pressures have little influence on σ/τ. At the same temperature, σ/τ increases with the water content of fayalite. The result theoretically explains why the upper mantle transition zone has the phenomenon of high conductivity.