Impact of CaCl2 -induced chemical pressure on the phase transition of H2O at high pressure

Understanding the phase behavior and structural properties of salt water at high pressures is essential for understanding the dynamics and physical characteristics of icy planets. In this study, we employed high-pressure experimental and ab initio simulation techniques to investigate the impact of CaCl₂ on the structure of ice VII. Our findings reveal that 1.8 mol% CaCl₂ can be incorporated into the ice VII structure above 10 GPa. This CaCl₂-bearing ice VII (Cb VII) exhibits a lower O-H stretching frequency in the Raman spectra as well as a reduced volume of the unit cell compared to pure ice VII. In contrast to doping ice VII with other salts such as LiCl and NaCl that leads to an increase of the ice VII to ice X transition pressure occurring at 100--150 GPa, CaCl₂ doping stands out by reducing the transition pressure. It shifts the transition to a pressure of 52 GPa, which is significantly lower than the transition pressure of 80 GPa in the pure H₂O ice system. This notable distinction highlights the unique influence of CaCl₂ on the phase behavior of water under high pressure, and we attribute these effects to the phenomenon of chemical pressure induced by CaCl₂ within the ice VII structure. Our study suggests that the presence of a modified ice VII phase, contaminated with salt and referred to as Cb VII, may influence the composition, structure, and evolution of planets.