Stability and compressibility of the high-pressure phases of Al 2O 3 up to 200 GPa: Implications for the electrical conductivity of the base of the lower mantle

Abstract

We have used a laser-heated diamond anvil cell to investigate the stability and compressibility of Cmcm CaIrO 3-type (post-perovskite structure) Al 2O 3 at pressures up to 200 GPa. A phase transformation from the Pbcn Rh 2O 3(II)-type to the CaIrO 3-type structure was observed at 130 GPa, which is consistent with previous theoretical studies. The observed CaIrO 3-type structure in Al 2O 3 is the same as that in MgSiO 3 post-perovskite, the main mineral of Earth’s lowermost mantle. We also calculated the Raman shifts of CaIrO 3-type Al 2O 3 and MgSiO 3 using density-functional perturbation theory. The similarity of the crystal structures and Raman spectra of CaIrO 3-type Al 2O 3 and MgSiO 3 suggests that the other physical properties of the two phases could be similar as well. Based on the high electrical conductivity of CaIrO 3-type Al 2O 3, we predicted a profile of electrical conductivity at the bottom of the lower mantle, which can explain Earth’s rotation period changes of a few milliseconds in Earth’s length of day on decadal timescales, if the exchange of angular momentum between the solid mantle and fluid core occurs by an electromagnetic coupling between the conducting core and mantle.