Polymorphs of Alumina Predicted by First Principles: Putting Pressure on the Ruby Pressure Scale

Fully optimized quantum mechanical calculations indicate that Al$_2$O$_3$ transforms from the corundum structure to the as yet unobserved Rh$_2$O$_3$ (II) structure at about 78 gigapascals, and it further transforms to Pbnm-perovskite structure at 223 gigapascals. The predicted x-ray spectrum of the Rh$_2$O$_3$ (II) structure is similar to that of the corundum structure, suggesting that the Rh$_2$O$_3$ (II) structure could go undetected in high-pressure x-ray measurements. It is therefore possible that the ruby (Cr$^{3+}$-doped corundum) fluorescence pressure scale is sensitive to the thermal history of the ruby chips in a given experiment.