Dynamical Instabilities of Ice X

Abstract

Water ice X is stable in the 120–400 GPa pressure range, as obtained from lattice dynamical calculations performed using the density-functional theory. Below 120 GPa, it is characterized by one unstable flat phonon band, which generates the disordered ice X structure. Above 400 GPa, ice X has an unstable phonon mode in $M$, which leads to the $Pbcm$ orthorhombic structure obtained in previous molecular-dynamics calculations [M. Benoit, M. Bernasconi, P. Focher, and M. Parrinello, Phys. Rev. Lett. 76, 2934 (1996)]. Therefore, based on lattice dynamics, we propose that the high-pressure low-temperature phase-transition sequence in ${\mathrm{H}}_2\mathrm{O}$ ice is ice VIII–disordered ice X–ordered ice X–ice $Pbcm$.