The high-pressure phase of alumina and implications for Earth's D'' layer

Using ab initio simulations and high-pressure experiments in a diamond anvil cell, we show that alumina (Al ₂ O ₃ ) 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 ₂ O ₃ at zero Kelvin. The CaIrO ₃ -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 ₂ O ₃ , 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 perovskite/postperovskite transition of MgSiO ₃ is discussed.