Transition from the Rh2O3(II)-to-CaIrO3 structure and the high-pressure-temperature phase diagram of alumina

The high-pressure behavior of alumina has been investigated by first-principles computations throughout the range of calibration of the ruby pressure scale. It is found that at 0K the transformation from corundum to Rh2O3(II)-type alumina at 87-113GPa is followed by a second one to the CaIrO3 structure at 150-172GPa. Quasiharmonic free-energy calculations show that both transformations display negative Clapeyron slope, which is a consequence of the decrease in polyhedral connectivity along the structural sequence. Like the first transformation, the second one should also have significant effects on the ruby pressure scale, especially if ruby is cycled across the phase boundaries.