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

## TL;DR

This study uses ab initio molecular dynamics with quantum effects to investigate the pressure-induced phase transition in delta-AlOOH, revealing proton tunneling as a key mechanism for hydrogen bond symmetrization relevant to Earth's mantle conditions.

## Contribution

It demonstrates the role of nuclear quantum effects and proton tunneling in the phase transition of delta-AlOOH, providing new insights into hydrogen bond symmetrization under high pressure.

## Key findings

- Proton tunneling triggers the phase transition at around 10 GPa.
- Hydrogen bonds become symmetric and stable at approximately 30 GPa.
- H-O stretching modes soften and fade out near the transition pressure.

## Abstract

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