High-pressure elasticity of alumina studied by first principles

We investigate by first principles the elastic behavior of Al₂O₃-alumina under pressure (up to 300 GPa) in the corundum and Rh₂O₃ (II) phase. The results are in excellent agreement with available low pressure (<1 GPa) experimental data. The anisotropy in elasticity for corundum decreases up to 50 GPa 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 relatively small c₅₅ modulus, and its softening at high pressures. Unlike corundum, the directions of the fastest and slowest wave propagation, and the maximum polarization anisotropy of Rh₂O₃ (II) phase remain unchanged with pressure. At the corundum to Rh₂O₃ (II) phase transition pressure (78 GPa at O K), the anisotropy increases by more than 100% but the density and wave velocities increase only by 2%. The calculated (O K) densities and wave velocities at lower mantle pressures are slightly larger (by 5%) than the corresponding seismic profiles. Our results suggest that the presence of free Al₂O₃ in small amount in the lower mantle may not be detected in seismic density and velocity profile. However, its anisotropy may produce a detectable signal, particularly, at pressure conditions typical of the D″ region.