Structure of water under high temperature and pressure

Katayama, Yoshinori; Ikeda, Takashi; Hattori, Takanori   ; Saito, Hiroyuki; Aoki, Katsutoshi; Fukui, Hiroshi*; Tange, Yoshinori*; Funakoshi, Kenichi*

Liquid water shows unique properties and they are related to the network structure formed by hydrogen bonds between water molecules. To study pressure dependence of structure of water, we measured X-ray diffraction of water up to 20 GPa using multi-anvil type presses in SPring-8. The coordination number of water molecules increased rapidly up to 4 GPa while maintaining the intermolecular distance. First-principles molecular dynamics simulations were also performed for high-density water. The obtained oxygen-oxygen radial distribution functions were in excellent agreement with the experimental results. Results of the simulations revealed that temperature was more important factor for the structural change. We have performed additional measurements at high temperatures. The calculated oxygen-oxygen radial distribution functions were again consistent with our experimental results. These studies clarified a pressure-temperature region where the significant structural change occurred.