Abstract
At ambient pressure, the hydrogen bond in materials such as ice, hydrates, and hydrous minerals that compose the Earth and icy planets generally takes an asymmetric O-H···O configuration. Pressure significantly affects this configuration, and it is predicted to become symmetric, such that the hydrogen is centered between the two oxygen atoms at high pressure. Changes of physical properties of minerals relevant to this symmetrization have been found; however, the atomic configuration around this symmetrization has remained elusive so far. Here we observed the pressure response of the hydrogen bonds in the aluminous hydrous minerals δ-AlOOH and δ-AlOOD by means of a neutron diffraction experiment. We find that the transition from P21nm to Pnnm at 9.0 GPa, accompanied by a change in the axial ratios of δ-AlOOH, corresponds to the disorder of hydrogen bond between two equivalent sites across the center of the O···O line. Symmetrization of the hydrogen bond is observed at 18.1 GPa, which is considerably higher than the disorder pressure. Moreover, there is a significant isotope effect on hydrogen bond geometry and transition pressure. This study indicates that disorder of the hydrogen bond as a precursor of symmetrization may also play an important role in determining the physical properties of minerals such as bulk modulus and seismic wave velocities in the Earth’s mantle.
Highlights
The existence of hydrogen in the Earth’s mantle is evidenced by the presence of high-pressure ice[1], hydrous minerals[2], and nominally anhydrous minerals with high water content[3] as inclusions in superdeep diamonds
This process is the so-called symmetrization of the hydrogen bond (H-bond)
The model with fully disordered hydrogen bond between two off-centered sites with half occupancy in each is refereed as a “disordered” phase in this paper following the previous studies on ice it is crystallography symmetric as well.) Considerable effort has been expended to investigate the pressure-induced symmetrization of the H-bond; regardless, most of the studies that have been conducted so far are based on indirect methods such as spectroscopy[6,7,8,9] and X-ray diffractions
Summary
The samples of δ-AlOOH and δ-AlOOD were synthesized at 18 GPa and 900 °C by using a Kawai-type multi-anvil press. The incident beam was introduced into the cell through the gasket, and the scattered neutrons were detected through the gasket by means of a detector fixed at 2θ = 89.5°. The cell was placed such that the compression axis was aligned coaxially with the beam, and the incident beam was introduced to the sample through the anvil. Test refinement of δ-AlOOH using deuterium and hydrogen at the hydrogen site resulted in no occupation of deuterium, showing that D-H isotope exchange did not occur between the sample and the deuterated pressure medium during the experiment. In the experiments using sintered diamond anvils, the diffraction peaks of diamond were observed; the lattice parameter of diamond was refined. For the synthesis of the difference Fourier maps, the data obtained at PLANET was used which could access to larger qmax to get good resolution. The comparison of the resolution of Fourier map using different qmax is shown in the Supplementary Fig. S5
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