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.

Figure 1

Phase diagram of water with points where measurements were conducted. Inset shows the low-pressure region.

Figure 2

(Color online) Molecular structure factor, $S\left(Q\right)$, at various pressures and temperatures. Thick solid lines (black) indicate $S\left(Q\right)$ obtained by the present x-ray diffraction experiments. Dashed (violet) line indicates $S\left(Q\right)$ at ambient conditions reported in the literature (Ref. 8). Solid gray (green) lines indicate results of classical MD simulations, obtained by Fourier transformation of the simulated O-O pair-correlation function. Dotted (black) line indicates window function used for the Fourier transformation of $S\left(Q\right)$ to obtain pair-correlation function.

Figure 3

(Color online) Pair-correlation function, $g\left(r\right)$, at various pressures and temperatures. Thick solid lines (black) indicate experimental results. Solid gray (green) lines indicate O-O pair-correlation functions obtained by classical MD simulations. Dotted (red) lines indicate theoretical O-O pair-correlation functions after broadening by the experimental resolution function. See text for details.

Figure 4

(a) Pressure dependence of the position of the first peak, $r_1$, in $g\left(r\right)$. Solid line indicates O-O distance in ice VII at room temperature (Ref. 33). (b) Pressure dependence of the coordination number, $N_1$, of water molecules calculated by two different methods. Closed squares indicate twice the integrated value of radial distribution function, RDF, up to the maximum of the first peak in RDF. Closed circles indicate value obtained by integration of RDF up to $1.35r_1$, an estimate of the minimum position between the first and the second peaks in RDF.