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.
Highlights
Hydrogen is an important chemical component in the Earth’s mantle, as even a small amount can strongly affect key properties of minerals, such as melting temperature, rheology, electrical conductivity, and atomic diffusion [1,2,3,4]
We investigate the phase transition, hydrogen bond symmetrization, and the possibility of proton tunneling in δ-AlOOH, combining density functional theory (DFT)-based calculations and high- and low-field highP nuclear magnetic resonance (NMR) spectroscopy
NMR experiments employing Lee-Goldburg decoupling pulses lead to line widths of ∼1.5 ppm [shown for 8 and 11 GPa in Fig. 4(a)] which permits the analysis of chemical shifts with ∼10 ppm [(Fig. 4(c)]
Summary
Hydrogen is an important chemical component in the Earth’s mantle, as even a small amount can strongly affect key properties of minerals, such as melting temperature, rheology, electrical conductivity, and atomic diffusion [1,2,3,4]. Over the past 20 years, many hydrous minerals, such as dense hydrous magnesium silicates [5], have been synthesized at high-pressure (P) and high-temperature (T ) conditions and investigated as potential candidates for hydrogen transport to the lower mantle. Most of these minerals decompose at P < 60 GPa, where phase H breaks down to MgSiO3 bridgmanite and a fluid component [6,7,8]. We investigate the phase transition, hydrogen bond symmetrization (a central unimodal proton distribution between the two respective oxygen atoms), and the possibility of proton tunneling in δ-AlOOH, combining density functional theory (DFT)-based calculations and high- and low-field highP nuclear magnetic resonance (NMR) spectroscopy. TRYBEL, MEIER, WANG, AND STEINLE-NEUMANN searching for characteristic features of a phase transition and using low-field NMR data at 5.6 GPa, we investigate indications of proton tunneling [25]
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