$\delta -\mathrm{AlOOH}$ is of significant crystallochemical interest due to a subtle structural transition near 10 GPa from a $P{2}_{1}nm$ to a $Pnnm$ structure, the nature and origin of hydrogen disorder, the symmetrization of the O-$\mathrm{H}\cdots \mathrm{O}$ hydrogen bond and their interplay. We perform a series of density functional theory-based simulations in combination with high-pressure nuclear magnetic resonance (NMR) experiments on $\delta -\mathrm{AlOOH}$ 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 NMR experiments at $10-11\mathrm{GPa}$ and hydrogen bond symmetrization at $14.7\mathrm{GPa}$. Calculated hydrogen potentials do not show any double-well character and there is no evidence for proton tunneling in our NMR data.

• Received 28 June 2021
• Revised 11 August 2021
• Accepted 26 August 2021

DOI:https://doi.org/10.1103/PhysRevB.104.104311

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Published by the American Physical Society