First-principles study of high-pressure alumina polymorphs

We investigate by first principles the structural properties and relative stability of six alumina polymorphs, namely: corundum, Rh₂O₃ (II), Pbnm perovskite, R3c-perovskite, A-type rare-earth sesquioxide, and B-type rare-earth sesquioxide. The compressive behavior and pressure-induced changes in the local environment of different aluminum sites are investigated in detail for corundum, Rh₂O₃ (II) and the perovskite phases, which are the predicted stable phases within the pressure range of 0--450 GPa. This information is crucial for understanding the electronic states of a Cr⁺³ color center in alumina (ruby) and the resulting intra-d transitions, i.e., the ruby fluorescence. Implications of these pressure-induced phase transitions in Al₂O₃ to the ruby fluorescence pressure scale are discussed.