High-pressure elasticity of alumina studied by first principles

We investigate by first principles the elastic behavior of Al2O3-alumina un der pressure (up to 300 GPa) in the corundum and Rh2O3 (II) phase. The resu lts are in excellent agreement with available low pressure (<1 GPa) experim ental data. The anisotropy in elasticity for corundum decreases up to 50 GP a and then increases slowly with pressure whereas for the Rh2O3 (II) phase the anisotropy increases monotonically with compression. Strong shear wave anisotropy in the Rh2O3 (II) phase is found to be associated with the relat ively small c(55) modulus, and its softening at high pressures. Unlike coru ndum, the directions of the fastest and slowest wave propagation, and the m aximum polarization anisotropy of Rh2O3 (II) phase remain unchanged with pr essure. At the corundum to Rh2O3 (II) phase transition pressure (78 GPa at 0 K), the anisotropy increases by more than 100% but the density and wave v elocities increase only by 2%. The calculated (0 K) densities and wave velo cities at lower mantle pressures are slightly larger (by 5%) than the corre sponding seismic profiles. Our results suggest that the presence of free Al 2O3 in small amounts in the lower mantle may not be detected in seismic den sity and velocity profile. However, its anisotropy may produce a detectable signal, particularly, at pressure conditions typical of the D" region.