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

We have predicted a high-pressure hydrous phase of AlOOH stabilizing at {~}170 GPa by first-principles density-functional calculations. The structure predicted has a cubic pyrite-type AlO₂ framework with interstitial H atoms forming symmetric hydrogen bonds, whose symmetry is assigned to the space group Pa3̄ (No. 205). The predicted {δ}-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.