title: X-ray Raman Spectroscopy measurement for detecting pressure-induced
dissociation of water molecules in ice VII

authors; Toshiaki Iitaka^1, Hiroshi Fukui^2, Zhi Li^3,1, Nozomu Hiraoka^4, 
Tetsuo Irifune^5

1 Computational Astrophysics Laboratory, RIKEN, 2-1 Hirosawa, Wako,
Saitama 351-0198, Japan

2 Center for Novel Material Science under Multi-Extreme Conditions,
Graduate School of Material Science, University of Hyogo, Kamigori,
Hyogo 678-1297, Japan
	
3 School of Materials Science and Engineering, Hefei University of
Technology, Hefei 230009, Anhui, China	

4 National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan

5 Geodynamics Research Center, Ehime University, 2-5 Bunkyo-cho,
Matsuyama, Ehime 790-8577, Japan


Abstract:
The neutron diffraction pattern of D2O ice was recently measured at
pressures up to 52 GPa by Guthrie et al. (PNAS, 110, 10552 (2013)), who
proposed an octahedral interstitial model for ice at pressures above 13
GPa to account for the deviation of the 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.