Using ab initio simulations and high-pressure experiments in a diamond anvil cell, we show that alumina (Al2O3) adopts the CaIrO 3-type structure above 130 GPa. This finding substantially changes the picture of high-pressure behavior of alumina; in particular, we find that perovskite structure is never stable for Al 2O3 at zero Kelvin. The CaIrO3-type phase suggests a reinterpretation of previous shock-wave experiments and has important implications for the use of alumina as a window material in shock-wave experiments. In particular, the conditions of the stability of this phase correspond to those at which shock-wave experiments indicated an increase of the electrical conductivity. If this increase is caused by high ionic mobility in the CaIrO 3-type phase of Al 2O3, similar effect can be expected in the isostructural postperovskite phase of MgSiO 3 (which is the dominant mineral phase in the Earth's D؆ layer). The effect of the incorporation of Al on the perovskite͞postperovskite transition of MgSiO 3 is discussed.is an important ceramic material and a major chemical component of the Earth. High-pressure behavior of alumina is an issue of paramount importance for three reasons: (i) static compression experiments often rely on the ruby pressure scale (1), (ii) alumina is used as an optical window material in shock-wave experiments, and its properties (e.g., the electrical conductivity) will affect measurements, and (iii) alumina, although unlikely as an individual mineral phase in the Earth's mantle, is incorporated into mantle minerals and significantly affects their physical properties (e.g., refs. 2 and 3).According to theoretical (e.g., refs. 4 -6) and experimental (7-9) studies, at high pressures corundum transforms into the Rh 2 O 3 (II)-type structure. This well established transition, occurring at 80 -100 GPa (5-9), implies that the ruby pressure scale should not be used above Ϸ80 GPa in experiments that involve heating to temperatures where this phase transition will be kinetically feasible. Less clear evidence exists for a further transition. Lin et al. (9) reported that Rh 2 O 3 (II) structure is stable at least up to 130 GPa and 2,000 K, but above 130 GPa (at temperatures Ϸ1,500 K) shock-wave experiments of Weir et al. (10) indicated onset of an increase in the electrical conductivity that could be caused by a phase transition. Shock-wave Hugoniots of Al 2 O 3 compiled by Hama and Suito (11) also give evidence for a possible phase transition at 130 GPa with a small increase of density. Thomson et al. (6) predicted that an orthorhombic (Pbnm) perovskite structure becomes more stable than Rh 2 O 3 (II), but only above 200 GPa. The discovery of postperovskite CaIrO 3 -type polymorphs of Fe 2 O 3 (12) and MgSiO 3 (13,14) prompts the question of whether a similar phase exists for Al 2 O 3 .In our ab initio calculations, we consider four Al 2 O 3 polymorphs: corundum (space group R3 c), Rh 2 O 3 (II) (Pbcn), perovskite (Pbnm), and CaIrO 3 -type (Cmcm). Our calculations are based on the plane wave pseud...