First-principles calculation of kinetic barriers and metastability for the corundum-to-Rh/sub 2/O/sub 3/(II) transition in Al/sub 2/O/sub 3/

We demonstrate that the rhombohedral corundum crystal and the orthorhombic Rh/sub 2/O/sub 3/ (II) crystal are structurally related by a diffusionless monoclinic common subgroup pathway, along which the intermediate structures have the space group P2/c symmetry. This transformation pathway is energetically favored because the majority of Al-O bonds are preserved during the transition. Only 1/3 of the four-coordinated anions and 1/2 of the six-coordinated cations break and reform a single bond. No other bonds are broken in the transformation. Our calculated forward (corundum-to-Rh/sub 2/O/sub 3/ (II)) enthalpy barrier (about 140 meV/atom) in Al/sub 2/O/sub 3/ is not pressure-sensitive at least up to 160 GPa, which is consistent with the observed metastability of the corundum phase around room temperature. In contrast, we predict a significantly decreasing backward (Rh/sub 2/O/sub 3/ (II)-to-corundum) enthalpy barrier on decompression, which suggests that the high-pressure Rh/sub 2/O/sub 3/ (II) phase of Al/sub 2/O/sub 3/ is unlikely to be recoverable at ambient conditions. In addition, we propose a general, common subgroup-based approach to compare elastic properties of two phases with different Bravais lattices. By treating both rhombohedral and orthorhombic phases with common monoclinic P2/c unit cells (non-primitive for the corundum phase), we show that the elastic properties of the high-pressure Rh/sub 2/O/sub 3/ (II) phase are very comparable to those of the ambient corundum phase