Convergent Raman features in high density amorphous ice, ice VII, and ice VIII under pressure

The high-pressure behavior of H(2)O ice at temperatures below 100 K has been investigated to 20 GPa by Raman spectroscopy. Between 10 and 80 K, high density amorphous (hda) ice formed from ice I(h) undergoes a phase transition near 5 GPa to ice VII' and a transition at 14 GPa to a different phase. At 14 GPa, a new low-frequency band appears at ~150 cm(-1) that steeply increases in frequency with pressure. The phase can be recovered to 0.1 MPa at temperatures below 80 K, and it transforms to low density amorphous ice (lda) upon heating. The phase is only present below 140 K and has spectral features that are identical to the phase formed from ice VIII at these pressures and temperatures. Similar changes have been observed in ice VII at these pressures but at room temperature. The results suggest structural changes having a common origin.