Crystal structure and elasticity of Al-bearing phase H under high pressure

Abstract

Al has significant effect on properties of minerals. We reported crystal structure and elasticity of phase H, an important potential water reservoir in the mantle, which contains different Al using first principles simulations for understanding the effect of Al on the phase H. The crystal and elastic properties of Al end-member phase H (Al2O4H2) are very different from Mg end-member (MgSiO4H2) phase H and two aluminous phase H (Mg0.875Si0.875Al0.25O4H2 (12.5at%Al) and Mg0.75Si0.75Al0.5O4H2 (25at% Al)). However differences between Mg end-member phase H and aluminous phase H are slight except for the O-H bond length and octahedron volume. Al located at different crystal positions (original Mg or Si position) of aluminous phase H has different AlO6 octahedral volumes. For three Al-bearing phase H, bulk modulus (K), shear modulus (G), compressional wave velocity (Vp) and shear wave velocity (Vs) increase with increasing Al content. Under high pressure, density of phase H increases with increasing Al content. The Al content affects the symmetry of the phase H and then affects the density and elastic constants of phase H. The total ground energy of phase H also increases with increasing Al content. So an energy barrier for the formation of solid solution of phase H with δ-phase AlOOH is expected. However, if the phase H with δ-phase AlOOH solid solution does exit in the mantle, it may become an important component of the mantle or leads to a low velocity layer at the mantle.