Absence of proton tunneling during the hydrogen bond symmetrization in $\delta$-AlOOH

TL;DR

This study combines simulations and experiments to investigate hydrogen bond behavior in δ-AlOOH under high pressure, revealing a structural phase transition and bond symmetrization without proton tunneling.

Contribution

It provides the first combined theoretical and experimental analysis showing no proton tunneling during hydrogen bond symmetrization in δ-AlOOH.

Findings

Phase transition at 10-11 GPa confirmed by NMR and simulations.

Hydrogen bond symmetrization occurs at 14.7 GPa.

Hydrogen potentials lack double-well character, indicating no proton tunneling.

Abstract

$\delta$-AlOOH is of significant crystallochemical interest due to a subtle structural transition near 10 GPa from a $P2_1nm$ to a $Pnnm$ structure, the nature and origin of hydrogen disorder, the symmetrization of the O-H$\cdots$O hydrogen bond and their interplay. We perform a series of density functional theory based simulations in combination with high-pressure nuclear magnetic resonance experiments on $\delta$-AlOOH up to 40 GPa with the goal to better characterize the hydrogen potential and therefore the nature of hydrogen disorder. Simulations predict a phase transition in agreement with our nuclear magnetic resonance experiments at $10-11$ GPa and hydrogen bond symmetrization at $14.7$ GPa. Calculated hydrogen potentials do not show any double-well character and there is no evidence for proton tunneling in our nuclear magnetic resonance data.