Mechanisms of Al 3+ incorporation in MgSiO 3 post-perovskite at high pressures

Abstract

Aluminum is the fifth most abundant element in the Earth's mantle, yet its effect on the physical properties of the newly found MgSiO 3 post-perovskite ( PPv), the major mineral of the Earth's Dʺ layer, is not fully known. In this paper, large-scale ab initio simulations based on density functional theory (DFT) within the generalized gradient approximation (GGA) have been carried out in order to investigate the substitution mechanism of Al 3+ into PPv at high pressures. We have examined three types of Al substitution in PPv: 6.25 mol% Al substitution via a charge-coupled mechanism (CCM), 6.25 mol% Al substitution via oxygen-vacancy mechanism (OVM), and an oxygen-vacancy Si-free end member Mg 2Al 2O 5. For both the CCM and OVM, five models, where the Al atoms were put in different positions, were simulated at various pressures in the range 10–150 GPa. Our calculations show that the most favorable mechanism is a charge-coupled substitution where Al 3+ replaces the next-nearest-neighbor cation pairs in the PPv structure. The calculated zero-pressure bulk modulus of Al-bearing PPv is 3.15% lower than that of the Al-free PPv. In agreement with previous works, we find that the incorporation of Al 2O 3 slightly increases the post-perovskite phase transition pressure, with the Al partition coefficient K = 2.67 at 120 GPa and 3000 K.