Effect of salt on the H-bond symmetrization in ice

Abstract

The richness of the phase diagram of water reduces drastically at very high pressures where only two molecular phases, protondisordered ice VII and proton-ordered ice VIII, are known. Both phases transform to the centered hydrogen bond atomic phase ice X above about 60 GPa, i.e., at pressures experienced in the interior of large ice bodies in the universe, such as Saturn and Neptune, where nonmolecular ice is thought to be the most abundant phase of water. In this work, we investigate, by Raman spectroscopy up to megabar pressures and ab initio simulations, how the transformation of ice VII in ice X is affected by the presence of salt inclusions in the ice lattice. Considerable amounts of salt can be included in ice VII structure under pressure via rock-ice interaction at depth and processes occurring during planetary accretion. Our study reveals that the presence of salt hinders proton order and hydrogen bond symmetrization, and pushes ice VII to ice X transformation to higher and higher pressures as the concentration of salt is increased.salty ices | extreme conditions | ice bodies | H-bond symmetrization | proton quantum effects A ll models of the interior of ice bodies in the universe rely on our knowledge of the behavior of a few simple molecules under high pressure and temperature (1-22), water being the most intriguing of them, due to its abundance and its connection to life existence.New data delivered by various space missions, such as the Voyager, Galileo, and Cassini-Huygens missions, have greatly improved our understanding of the icy bodies within the solar system (23-25), and recent discoveries of extrasolar planets with significant water ice content have highlighted the importance of high-pressure H 2 O-rich phases in planetary physics beyond the solar system (26).Water displays an unusually rich pressure-temperature phase diagram, which mainly derives from the open geometry of the water molecule, which favors tetrahedral arrangements, and from the possibility of configurational disorder of the protons in the lattice. However, at high pressure (above about 2 GPa), the system experiences a large densification as a result of the interpenetration of low-pressure structures, and only two molecular phases are known, ice VIII and VII (3, 4). Ice VII is composed of two interpenetrating but not interconnected tetrahedral hydrogen-bonded networks of water molecules of normal cubic ice, Ic, with a body-centered-cubic (bcc) oxygen structure. The orientation of the water molecules is disordered, resulting in a paraelectric phase. In the antiferroelectric ice VIII phase, the water molecules and the associated dipole moments on the two sublattices possess long-range order and point in opposite directions, promoting a slight tetragonal distortion of the cubic unit cell along the staggered polarization (3, 4). In these two phases, the hydrogen bonds are characterized by a pronounced double-well proton transfer potential. Upon reducing the distance between donor and acceptor oxygens, the proton potential degenerate...

Quantum versus classical protons in pure and salty ice under pressure

“…In salty ices, according to QTB-AIMD, the VII to X transition occurs at approximately 90 GPa for both LiCl and NaCl ices, again in very good agreement with experiments [5]. Moreover, P t is also independent of the actual position of Na + , whether interstitial or substitutional.…”

“…1 shows the 200-1000 cm −1 region of the Raman spectra recorded at different pressures up to 130 GPa. The oxygen-oxygen T 2g vibrational mode, which is indicative of the cupritelike structure of phase X, clearly appears at 87 ± 5 GPa, similarly to what is observed in LiCl ice [5]. Thus, the presence of small quantities of salt impurities (here, 1 NaCl for 53 H 2 O), which is likely in natural ices, shifts P t by about 30 GPa, roughly the same pressure shift as observed when quantum effects are neglected in pure ice [3,4].…”

“…In pure ice, nuclear quantum effects (NQEs) play a crucial role by drastically decreasing the VII-X phase transition pressure [3,4]. However, recent experiments question the role of NQEs in LiCl-doped ice [5]. Whether this is specific to LiCl ice and the mechanism by which the quantum behavior of the protons can be hindered is still an issue.…”

“…Here, we report high resolution Raman scattering experiments on NaCl ice up to Mbar pressures. These challenging conditions have been achieved by the use of a diamond anvil cell equipped with extra low-fluorescence synthetic diamonds and by fast quenching of a 5 μm droplet of 2% NaCl water solution in order to produce ice VII-doped ice following the procedure described in the Supplemental Material [5,17,18]. The upper panel of Fig.…”

“…Recent studies [5] on LiCl-water solutions at different salt to water ratios pointed out that the properties of ice change drastically when LiCl salt is homogeneously included into ice VII and that the transition pressure to the phase X strongly depends on the presence of ionic impurities. Here, we report high resolution Raman scattering experiments on NaCl ice up to Mbar pressures.…”