[1] By means of static ab-initio computations we investigate the influence of Al on the recently discovered perovskite to post-perovskite phase transition in MgSiO 3 . We examine three substitution mechanisms for Al in the two structures: MgSi ! AlAl; SiSiO ! AlAl5; and Si ! AlH. The substitutions introducing oxygen vacancies (highly unfavorable, energetically) and water (favorable) both lower the 0 Kelvin transition pressure, whereas charge coupled substitution increases it relative to 105 GPa for pure MgSiO 3 . From the transition pressures for 0, 6.25, and 100 mol% charge coupled Al 2 O 3 incorporation and simple solution theories, we estimate the phase diagram of Al-bearing MgSiO 3 at low Al concentrations. Assuming the Clapeyron slope is independent of Al concentration, we find the perovskite-to-post-perovskite transition region to span 127-140 GPa, at 6.25 mol% Al 2 O 3 . When the upper pressure limit is bounded by the core-mantle boundary, the phase coexistence region has width 150 km.

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[1] The equation of state and structure of CaSiO 3 perovskite are determined at high pressures using the density-functional-based Variationally Induced Breathing (VIB) method, modified with a small, parameterized covalent correction to the short-range Si-O interaction. Our thermally corrected room temperature equation of state parameters (V 0 = 45.90 ± 0.02 Å , K 0 = 228 ± 2 GPa, K 0 = 4.3 ± 0.1) are in very good agreement with near-hydrostatic compression data as well as recent calculations based on first-principles methods. We examined the tetragonal, orthorhombic, and triclinic structures, and compared their stabilities relative to the cubic structure at lower mantle pressures. We found that the orthorhombic perovskite structure of CaSiO 3 is the thermodynamically stable phase at all mantle pressures. The structural and energy differences between the orthorhombic and cubic lattices under static conditions are small, and increase only slightly with pressure. Consequently, the cubic structure may be thermodynamically stable at lower mantle temperatures.