FIRST-PRINCIPLES STUDY OF HIGH-PRESSURE ALUMINA POLYMORPHS

We investigate by first principles the structural properties and relat ive stability of six alumina polymorphs, namely: corundum, Rh2O3 (II), Pbnm perovskite, R (3) over bar c-perovskite, A-type rare-earth sesqu ioxide, and B-type rare-earth sesquioxide. The compressive behavior an d pressure-induced changes in the local environment of different alumi num sites are investigated in detail for corundum, Rh2O3 (II) and the perovskite phases, which are the predicted stable phases within the pr essure range of 0-450 GPa. This information is crucial for understandi ng the electronic states of a Cr+3 color center in alumina (ruby) and the resulting intra-d transitions, i.e., the ruby fluorescence. Implic ations of these pressure-induced phase transitions in Al2O3 to the rub y fluorescence pressure scale are discussed. [S0163-1829(98)09517-4].