HIGH-PRESSURE ROTATION PHASE OF ICE VI. Gabuda S.P., Institute of Inorganic Chemistry, Novosibirsk, Russia

The proton magnetic resonance (PMR) was used to study the molecular mobility in high pressure phases of ice. The routine continuous line NMR spectrometer was equipped with a special high-pressure vessel of beryllium bronze for registration of PMR spectra of samples being under the pressures up to 2 GPa. During the study it was found that the narrow PMR line of liquid water remains almost unchanged when the pressure exceeds the value of ice VI crystallisation (0.8 GPa at room temperature). The broad NMR line of solid water (ice) arises only when the hydrostatic pressure is 1.3 GPa or more. The half-widths and mean square widths (or second moment M) of this line are pressure independent from 1.3 to 1.5 MPa. But the numerical value M=9.0(0.5 G² of recorded spectra are much less, than M ( 40 G², computed for the ice VI structure. It should be noted that M value of NMR spectrum of ice Ih is 36 G², that is close to the expected rigid lattice ice VI value.

The observed behaviour of PMR spectra of water under the high pressure correlates very closely with the behaviour of PMR spectra of H₂S at decrease of the temperature. Reasoning from the correspondence between the low temperature behaviour of molecular crystals and of those at high pressures one can conclude that at approximately 0.8 GPa water crystallises into the superplastic phase, and at 1.3 GPa it undergoes a transformation into the "rotation" phase. The real rigid high pressure crystal lattice of ice VI probably is formed only when the pressure is more, than 1.5 GPa (at room temperature).