Kinetic boundaries and phase transformations of ice i at high pressure.

Raman spectroscopy in diamond anvil cells has been employed to study phase boundaries and transformation kinetics of H2O ice at high pressures up to 16 GPa and temperatures down to 15 K. Ice i formed at nearly isobaric cooling of liquid water transforms on compression to high-density amorphous (HDA) ice at 1.1-3 GPa at 15-100 K and then crystallizes in ice vii with the frozen-in disorder (ice vii') which remains stable up to 14.1 GPa at 80 K and 15.9 GPa at 100 K. Unexpectedly, on decompression of ice vii', it transforms to ice viii in its domain of metastability, and then it relaxes into low-density amorphous (LDA) ice on a subsequent pressure release and warming up. On compression of ice i at 150-170 K, ice ix is crystallized and no HDA ice is found; further compression of ice ix results in the sequential phase transitions to stable ices vi and viii. Cooling ice i to 210 K at 0.3 GPa transforms it to a stable ice ii. Our extensive investigations provide previously missing information on the phase diagram of water, especially on the kinetic paths that result in formation of phases which otherwise are not accessible; these results are keys for understanding the phase relations including the formation of metastable phases. Our observations inform on the ice modifications that can occur naturally in planetary environments and are not accessible for direct observations.