Anomalies and phase transitions of dense ice

We propose that the anomalies in ice VII and ice VIII can be explained by the formation of ferroelectric (FE) and anti-ferroelectric (AFE) domains. In the FE domains the dipoles point along the same direction along the 4-fold axis, while in AFE they point alternatively up and down. The splitting of the X-ray diffraction peaks and the appearance of new Raman modes are the result of different a/c ratios in the two FE and AFE parts of the structure [2,3]. Then, based on lattice dynamical calculations, we show that the transition between ices VII and VIII and ice X goes through a disordered phase X in the 60- 115 GPa pressure range. In this regime the H atoms are allowed to freely bounce back and forth between their two O neighbors. Ice X is stable up to about 420 GPa. The first post-ice-X structure occurs due to a phonon softening in ice X. The resulting structure has orthorhombic symmetry [4,5,6]. We show that the softening is also visible in the development of an elastic anomaly after 160 GPa.