Effect of chemistry on the stability and elasticity of the perovskite and post‐perovskite phases in the MgSiO₃‐FeSiO₃‐Al₂O₃ system and implications for the lowermost mantle

Using first‐principles calculations we predict the effects of composition on the perovskite to post‐perovskite phase transition. The transition is predicted at 107 GPa for pure MgSiO 3 . The addition of Al 2 O 3 slightly increases this transition pressure, and the addition of Fe 2+ considerably reduces it; the FeSiO 3 end‐member term is stable in the post‐perovskite modification with respect to perovskite at all pressures. We also determine the static equations of state, densities, elasticity and seismic wave velocities. At the transition V p increases slightly, and V s increases significantly, consistent with the seismic observations for D″. The addition of both Fe 2+ and Al 2 O 3 decrease the seismic wave velocities.