EQUATION OF STATE OF WATER AND MELTING CURVE OF ICE VII BASED ON SIMULTANEOUS MEASUREMENTS OF SOUND VELOCITY AND X-RAY DIFFRACTION OF ICE VII TO 19 GPa AND 873 K

We have measured the sound velocity of H₂O by Brillouin spectroscopy using membrane-style diamond anvil cell with resistance heating at elevated temperatures and pressures up to 873 K and 19 GPa. The unit cells of Ice VII and Au were determined by synchrotron X-ray diffraction, using Au as an in situ pressure gauge. All our samples were contained within and chemically insulated from the Re-gasket hole by a gold liner. Measurements of the sound velocity in liquid water have been extended to 8.0 GPa and 873K. We observed, generally, lower velocities (up to 10% at 723 K) than those given by previous studies [1, 2]. The melting of Ice VII was determined by monitoring the sound velocity drop and the disappearance of diffraction pattern of Ice VII upon melting. Our determination of the melting temperatures differ significantly from those given by previous studies [3, 4], with an observed discrepancy of 130 K at 8 GPa. Given the care taken in the present experiments to avoid potential contamination of the water sample due to reactions at high temperatures between the sample and the gaskets and/or pressure gauges, our new measurements likely represent the properties and phase diagram of pure water, which displays an extended stability field of the solid phase. Thus, our new measurements suggest that the melting curve of H₂O at high pressure needs to be reevaluated.

[1] Abramson & Brown (2003) GCA 68, 1827-1835. [2] Decremps et al. (2006) Ultrasonics 44, 1495-1498. [3] Datchi et al. (2000) Phys. Rev. B 61, 6535-6546. [4] Lin et al. (2004) J. Chem. Phys. 121, 8423-8427.