POLYMORPHS OF ALUMINA PREDICTED BY FIRST PRINCIPLES - PUTTING PRESSURE ON THE RUBY PRESSURE SCALE

Fully optimized quantum mechanical calculations indicate that Al2O3 tr ansforms from the corundum structure to the as yet unobserved Rh2O3 (I I) structure at about 78 gigapascals, and it further transforms to Pbn m-perovskite structure at 223 gigapascals, The predicted x-ray spectru m of the Rh2O3 (II) structure is similar to that of the corundum struc ture, suggesting that the Rh2O3 (II) structure could go undetected in high-pressure x-ray measurements. It is therefore possible that the ru by (Cr3+-doped corundum) fluorescence pressure scale is sensitive to t he thermal history of the ruby chips in a given experiment.