Phonon Dispersion of Ice under Pressure
Abstract
We report measurements of the phonon dispersion of ice Ih under hydrostatic pressure up to 0.5GPa, at 140K, using inelastic neutron scattering. They reveal a pronounced softening of various low-energy modes, in particular, those of the transverse acoustic phonon branch in the [100] direction and polarization in the hexagonal plane. We demonstrate with the aid of a lattice dynamical model that these anomalous features in the phonon dispersion are at the origin of the negative thermal expansion (NTE) coefficient in ice below 60K. Moreover, extrapolation to higher pressures shows that the mode frequencies responsible for the NTE approach zero at ∼2.5 GPa, which explains the known pressure-induced amorphization (PIA) in ice. These results give the first clear experimental evidence that PIA in ice is due to a lattice instability, i.e., mechanical melting.
- Publication:
-
Physical Review Letters
- Pub Date:
- November 2004
- DOI:
- Bibcode:
- 2004PhRvL..93v5901S
- Keywords:
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- 65.60.+a;
- 62.50.+p;
- 78.70.Nx;
- Thermal properties of amorphous solids and glasses: heat capacity thermal expansion etc.;
- High-pressure and shock wave effects in solids and liquids;
- Neutron inelastic scattering