First-principles prediction of a high-pressure hydrous phase of AlOOH

We have predicted a high-pressure hydrous phase of AlOOH stabilizing at similar to 170 GPa by first-principles density-functional calculations. The structure predicted has a cubic pyrite-type AlO2 framework with interstitial II atoms forming symmetric hydrogen bonds, whose symmetry is assigned to the space group Pa (3) over bar ( No. 205). The predicted delta-AlOOH to the pyrite-type phase sequence is analogous to a recent theoretical and experimental discovery of high-pressure phase evolution in InOOH and invokes the high-pressure phase relationship in SiO2, but the transition pressure is much greater in AlOOH than in InOOH. Relative enthalpies also indicate that the dissociation of this phase into a CaIrO3-type phase of Al2O3 plus ice X finally occurs at a further pressure of 300 GPa. The present results suggest that AlOOH has an unexpectedly wide stability range in pressure compared to common hydrous materials.