title: Pressure-induced dissociation of water molecules in ice VII.
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.
authors.affiliation: Computational Astrophysics Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.
authors.forename: Toshiaki
authors.initials: T
authors.lastname: Iitaka
authors.affiliation: Center for Novel Material Science under Multi-Extreme Conditions, Graduate School of Material Science, University of Hyogo, Kamigori, Hyogo 678-1297, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Fukui
authors.affiliation: 1] Computational Astrophysics Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan [2] School of Materials Science and Engineering, Hefei University of Technology, Hefei, Anhui 230009, China.
authors.forename: Zhi
authors.initials: Z
authors.lastname: Li
authors.affiliation: National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan.
authors.forename: Nozomu
authors.initials: N
authors.lastname: Hiraoka
authors.affiliation: 1] Geodynamics Research Center, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan [2] Earth-Life Science Institute, Tokyo Institute of Technology, Tokyo 152-8550, Japan.
authors.forename: Tetsuo
authors.initials: T
authors.lastname: Irifune