Pressure Evolution of the High-Frequency Sound Velocity in Liquid Water

The high-frequency sound velocity v_∞ of liquid water has been determined to densities of 1.37 g/cm³ by inelastic x-ray scattering. In comparison to the hydrodynamic sound velocity v₀, the increase of v_∞ with density is substantially less pronounced, indicating that, at high density, the hydrogen-bond network is decreasingly relevant to the physical properties of liquid water. Furthermore, we observe an anomaly in v_∞ at densities around 1.12 g/cm³, contrasting the smooth density evolution of v₀.