Prediction of a new phase transition in Al 2 O 3 at high pressures

We use density functional theory to investigate several high‐pressure polymorphs of Al 2 O 3 and predict a new stable polymorph at high pressures with post‐perovskite structure. The ambient pressure Rc corundum phase transforms to the Pbcn Rh 2 O 3 (II) structure at about 104 GPa. At about 156 GPa, the Rh 2 O 3 (II) structure transforms further to the Cmcm post‐perovskite structure. The Al 2 O 3 Pbnm perovskite structure is metastable at all pressures with respect to corundum, Rh 2 O 3 (II) and post‐perovskite. The metastable perovskite to post‐perovskite transition takes place at about 120 GPa, which is above the same transition in MgSiO 3 . Thus the presence of Al 2 O 3 in the lower mantle, dissolved in MgSiO 3 perovskite, will increase the transition pressure to post‐perovskite. Our calculations also show that Al 2 O 3 is most likely dissolved in the post‐perovskite phase of MgSiO 3 , increasing its stability at high pressures.