Abstract
The structural and dynamical properties of cubic O and O ice phases are studied using ab initio molecular dynamics combined with ultrasoft pseudopotentials. Phonon frequencies are extracted from the velocity autocorrelation functions; contributions from different normal modes in the phonon spectra are separated and easily identified. For the low-pressure phases, the agreement with the experimental data is reasonable and the isotope effects are well reproduced. High-pressure phases are also studied. The equations of state for cubic ice (ice ), and for the ice VII-VIII-X family, are calculated. It is found that the local-density approximation must be augmented with gradient corrections in order to obtain a proper description of the hydrogen bond. Finally, the hydrogen-bond symmetrization, which is responsible for the transition from ice VII-VIII to ice X, is studied and is predicted to occur at 49 GPa. The nature of the phase transition is found to be that of a mode-softening transition. The corresponding symmetrization is also studied in ice , but it is found to occur at a pressure of 7 GPa at which ice is unstable with respect to denser phases.
- Received 28 September 1992
DOI:https://doi.org/10.1103/PhysRevB.47.4863
©1993 American Physical Society