Absence of proton tunneling during the hydrogen-bond symmetrization in δ − AlOOH

$\ensuremath{\delta}\ensuremath{-}\mathrm{AlOOH}$ 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-$\mathrm{H}\ensuremath{\cdots}\mathrm{O}$ 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 $\ensuremath{\delta}\ensuremath{-}\mathrm{AlOOH}$ 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 $10\ensuremath{-}11\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ and hydrogen bond symmetrization at $14.7\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. Calculated hydrogen potentials do not show any double-well character and there is no evidence for proton tunneling in our NMR data.