Mechanisms of Al3+ incorporation in MgSiO3 post-perovskite at high pressures

Aluminum is the fifth most abundant element in the Earth's mantle, yet its effect on the physical properties of the newly found MgSiO3 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 Al3+ 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 Mg2Al2O5. 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 Al3+ 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 Al2O3 slightly increases the post-perovskite phase transition pressure, with the Al partition coefficient K = 2.67 at 120 GPa and 3000 K.