Stability and compressibility of the high-pressure phases of Al2O3 up to 200 GPa: Implications for the electrical conductivity of the base of the lower mantle

We have used a laser-heated diamond anvil cell to investigate the stability and compressibility of Cmcm CaIrO3-type (post-perovskite structure) Al2O3 at pressures up to 200 GPa. A phase transformation from the Pbcn Rh2O3(II)-type to the CaIrO3-type structure was observed at 130 GPa, which is consistent with previous theoretical studies. The observed CaIrO3-type structure in Al2O3 is the same as that in MgSiO3 post-perovskite, the main mineral of Earth’s lowermost mantle. We also calculated the Raman shifts of CaIrO3-type Al2O3 and MgSiO3 using density-functional perturbation theory. The similarity of the crystal structures and Raman spectra of CaIrO3-type Al2O3 and MgSiO3 suggests that the other physical properties of the two phases could be similar as well. Based on the high electrical conductivity of CaIrO3-type Al2O3, 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.