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We conducted ab initio molecular dynamics simulations of the delta phase ofthe hydrous mineral AlOOH at ambient temperature and high pressure. Nuclearquantum effects were included through a Langevin dynamics in a bath of quantumharmonic oscillators.  We confirm that under increasing pressure delta-AlOOH undergoes a phasetransition from a $P2_1nm$ structure with asymmetric and disordered O-H bondsto a stiffer $Pnnm$ phase with symmetric hydrogen bonds, which should be stablewithin the pressure and temperature ranges typical for the Earth's mantle.  The transition is initially triggered by proton tunneling, which makes themean proton position to coincide with the midpoint of the O-O distance, atpressures as low as 10 GPa. However, only at much larger pressures, around 30GPa as previously found by other calculations, the Pnnm phase with symmetrichydrogen bonds is stable from the classical point of view.  The transition is also characterized through the analysis of the H-Ostretching modes, which soften considerably and fade out around 10 GPa in theP2_1nm structure, when thermal and nuclear quantum effects are taken intoaccount in the simulations. At variance, the harmonic picture is not adequateto describe the highly anharmonic effective potential that is seen by theprotons at the transition.  Finally, we propose that the picture of a dynamical transition to thehigh-symmetry and proton-centered Pnnm phase, which is brought about by theonset of proton tunneling, could be confirmed by quasi-elastic neutronscattering and vibrational spectroscopy under pressure.

Thermal and nuclear quantum effects in the hydrogen bond dynamical symmetrization phase transition of delta-AlOOH