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, ${\mathrm{Rh}}_2{\mathrm{O}}_3$ (II), $\mathrm{Pbnm}$ perovskite, $R3\mathrm{}¯c$-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, ${\mathrm{Rh}}_2{\mathrm{O}}_3$ (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 ${\mathrm{Cr}}^{+3\mathrm{}}$ color center in alumina (ruby) and the resulting intra-$d$ transitions, i.e., the ruby fluorescence. Implications of these pressure-induced phase transitions in ${\mathrm{Al}}_2{\mathrm{O}}_3$ to the ruby fluorescence pressure scale are discussed.