Pressure-induced dissociation of water molecules in ice VII.

Toshiaki Iitaka,Nozomu Hiraoka,Hiroshi Fukui,Tetsuo Irifune,Zhi Li

doi:10.1038/srep12551

Abstract

The neutron diffraction pattern of D2O ice was recently measured at pressures up to 52 GPa by Guthrie et al., who proposed an octahedral interstitial model for ice at pressures above 13 GPa to account for the deviation of the observed crystal structure from that of ice VII. In this article, the octahedral interstitial model was re-examined in terms of the interstitial occupancy and X-ray Raman spectroscopy (XRS) spectra. The interstitial occupancy calculated using first-principles molecular dynamics simulations was negligibly small compared to that of the interstitial model. The oxygen K-edge spectra calculated for the interstitial model exhibited two additional low-energy peaks originating from water molecules and hydroxides that are interacting with interstitial protons, respectively, whereas these low-energy peaks were not observed in the experimentally measured spectra. These results suggest that the interstitial model cannot explain the XRS spectra of ice VII at pressures above 13 GPa and that more precise structure measurements and analyses are necessary to reveal the nature of the pressure-induced transition.

Highlights

Scrutinising the nature of the transition at Pc, especially the change of the hydrogen position predicted by the octahedral interstitial model[6], would be interesting
The theoretical oxygen K-edge XAS spectrum, which can be directly compared to the X-ray Raman spectroscopy (XRS) spectrum measured in the dipole limit, was calculated for the ice VII model and the octahedral interstitial model; these spectra were subsequently compared with the experimental XRS spectra
The molecular dynamics simulations were performed for 100 ps with a time step of 0.1 fs using an NVT ensemble at temperatures of 300 K and 800 K controlled by a Nose-Hoover thermostat[11]

Summary

Theoretical and Experimental

The occupancy of interstitial hydrogen was evaluated using the trajectories generated by first-principles molecular dynamics simulations. The calculations were performed with the GGA-PBE16 exchange-correlation functional, periodic boundary conditions with Brillouin zone integration with 3 × 3 × 3 Monkhorst-Pack k-points, and RmtKmax = 3, where Rmt is the smallest atomic sphere radius and Kmax is the magnitude of the largest K vector in the expansion of the Kohn–Sham equation. These parameters are sufficient for systems with hydrogen atoms. XRS is more frequently used in high-pressure experiments for light elements than XAS because XRS uses hard X-rays that can penetrate the sample cell

Results and Discussion

Author Contributions

Additional Information