HR: 1340h
AN: MR23A-0191    [Abstracts]
TI: Post-Perovskite Phase in Al$_2$O$_3$ and Implications for the Lower Mantle
AU: * Caracas, R
EM: r.caracas@gl.ciw.edu
AF: Carnegie Institution of Washington, Geophysical Laboratory, 5251 Broad Branch Rd., N.W., Washington, DC 20015 United States
AU: Cohen, R E
EM: r.cohen@gl.ciw.edu
AF: Carnegie Institution of Washington, Geophysical Laboratory, 5251 Broad Branch Rd., N.W., Washington, DC 20015 United States
AB: We use the local density approximation of the density functional theory to investigate several high-pressure polymorphs of Al$_2$O$_3$ and we predict a new stable polymorph at high pressures with post-perovskite structure. We find that the ambient pressure R$\overline{3}$c corundum phase transforms to the Pbcn Rh$_2$O$_3$(II) structure at about 105 GPa, (with $\Delta$V=-2.3%, $\Delta\rho$=0.1 g/cm$^3$ shifts), in agreement with previous first-principles studies. At about 156 GPa, the Rh$_2$O$_3$(II) structure transforms to the Cmcm post-perovskite structure (with $\Delta$V=-2.9%, $\Delta\rho$=0.16 g/cm$^3$ shifts). The Al$_2$O$_3$ Pbnm perovskite structure is metastable at all pressures with respect to corundum, Rh$_2$O$_3$(II) and/or post-perovskite. The metastable perovskite to post-perovskite phase transition takes place at about 122 GPa while the Rh$_2$O$_3$(II) to perovskite at about 208 GPa. We obtain similar results using the generalized gradient approximation. Our results indicate that the incorporation of Al$_2$O$_3$ in MgSiO$_3$ will contribute to the stability of the post-perovskite phase of MgSiO$_3$ with respect to perovskite. In the 90-150 GPa pressure range, the Al$_2$O$_3$ perovskite and post-perovskite structures have specific volumes about 0.37-0.22 and 0.19-0.14 $\AA^3$/molecule smaller than the corresponding structures of MgSiO$_3$, respectively, suggesting that broad Al$_2$O$_3$-MgSiO$_3$ solid solutions are possible at the lower mantle conditions and more likely for the post-perovskite than perovskite phase. We also find that Al$_2$O$_3$ will slightly reduce the elastic constants of post-perovskite MgSiO$_3$. To our knowledge, this is the first study of Al$_2$O$_3$ at high pressure where the new post-perovskite phase is considered and theoretically predicted.
DE: 3620 Crystal chemistry
DE: 3672 Planetary mineralogy and petrology (5410)
DE: 3909 Elasticity and anelasticity
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting