Static compression of H₂O-ice to 128 GPa (1.28 Mbar)

Abstract

The high-pressure behaviour of H₂O is of fundamental importance in both condensed matter and planetary physics^1,2. The hydrogen bonding in this system gives rise to a variety of phases at low pressures and temperatures (that is, <2 GPa and <300 K), including the recently discovered high-density amorphous phases³. Structural and equation-of-state^4,5 and spectroscopic^6–8 studies have been carried out in the 30–50 GPa range on the dense ices (ice VII and VIII), but no data are available on the properties of solid H₂O in the megabar pressure range (>100 GPa) where a variety of stable phases, including the metallic form, have been proposed⁹. Information on the properties of H₂O at these pressures has been limited to the results of shock-wave experiments, which probe the fluid phase at high pressures and temperatures¹⁰, and to theoretical statistical electron calculations^11–15. In this study we have compressed ice in a diamond-anvil cell to 128 GPa and measured the molar volume as a function of pressure by synchrotron X-ray diffraction techniques. The diffraction data are consistent with the body-centred cubic (b.c.c.) oxygen sublattice of ice VII persisting to the highest pressures of our measurements. The measured equation of state indicates that ice is less compressible at very high pressures than is suggested by recent experiments in the 30–50 GPa range, but more compressible than statistical electron and recent pair-potential models predict.