Structure of liquid water under high pressure up to 17 GPa

The structure of liquid water was studied along the melting curve up to 17.1 GPa and 850 K by in situ x-ray diffraction. Because an oxygen atom has a much larger x-ray scattering power than that of a hydrogen atom, pressure dependence of local molecular arrangements was revealed straightforwardly. At low pressures, the local structure changed toward a simple liquidlike structure through an increase in the coordination number of water molecules. Once densely packed structure was achieved around 4 GPa, the volume was reduced through the decrease in the intermolecular distance on further compression. Classical molecular-dynamics simulations well reproduced the experimental results although the degree of agreement depended on pressure. Limitations of the pair-potential model were discussed.