Quantum versus classical protons in pure and salty ice under pressure

Abstract

It is generally accepted that nuclear quantum effects (NQEs) trigger the transition to the nonmolecular form of ice under increasing pressure. This picture is challenged in salty ice, where Raman scattering measurements up to 130 GPa of molecular ice VII containing NaCl or LiCl impurities show that the transition pressure to the symmetric phase ice X is shifted up by about 30 GPa, even at small salt concentrations. We address the question of how the inclusion of salt induces the drastic reduction of NQEs by selectively including NQEs in ab initio calculations of ice in the presence of distinct ionic impurities. We quantitatively show that this is mainly a consequence of the electric field generated by the ions. We propose a simple model that is able to capture the essence of this phenomenon, generalizing this picture to other charged defects and for any concentration. This result is potentially generalizable to most "dirty" ices in which the electric field due to the doping is much more significant than local lattice distortions. DOI: 10.1103/PhysRevB.93.024104 Ice under extreme conditions is present in many planets, both within our solar system [1] and beyond [2]. These ices are usually "dirty": Planetary real ices unavoidably contain impurities such as salt. 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.At high pressure, the two molecular phases of ice, proton disordered ice VII and proton ordered ice VIII [6,7], transform to ice X, the only known atomic phase of ice. Below the transition, the oxygens form a body-centered-cubic structure, and the hydrogen bonds are characterized by a prototypical double-well proton transfer potential. As the atoms get closer under the effect of increasing pressure, the intrinsic quantum nature of the protons produces more evident effects and favors the onset of quantum tunneling. Upon further reducing the distance between oxygens, the proton potential degenerates into a single-well potential [8], giving rise to a symmetric hydrogen bond, where the hydrogen is located midway between two neighboring O atoms. By including NQEs, the transition pressure P t is drastically reduced from about 90 GPa as classically predicted [9] to approximately 60 GPa [10][11][12][13][14][15][16], that is, the pressure for which the zero-point energy equals the barrier height [3,4]. However, the effect of a perturbation on a prototype of structural quantum effects in crystals is not a priori trivial from a fundamental point of view.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...

Probing ice VII crystallization from amorphous NaCl–D2O solutions at gigapascal pressures

“…above 5 GPa) but below the transition to ice X may be different from the behaviour observed in this work. As NaCl-ice samples obtained by annealing of a dilute amorphous precursor (NaClÁ52H 2 O) at much higher pressures (about 10 GPa) show effects compatible with a likely inclusion of salt in the ice lattice, 9 it is possible that higher pressures than the ones investigated in this study or lower salt concentrations are needed to keep the NaCl in the ice structure. This hypothesis is the object of a forthcoming study.…”

“…6 The presence or absence of ions has also been shown to control the occurrence of phase transitions and is linked to quantum effects in phases of water. 8,9 The structure of magnesium chloride hydrates, in particular the high-pressure hydrate MgClÁ2.10D 2 O at 2.5 gigapascal, also includes Mg 2+ in octahedral geometry 10 similar to what is observed for Li + . The difference in the behaviour of NaCl-solutions under compression raises the question of whether Na + ions can also be included in ice VII or not.…”

“…A recent experiment performed at much higher pressures using Raman spectroscopy, up into the megabar range, on ice phases nucleated from dilute NaCl solutions (NaClÁ52D 2 O) suggested that at least part of the NaCl was included in the ice lattice, as the hydrogen bond symmetrisation behaviour was different from the one expected for pure ice. 9,11 However, this hypothesis needs to be corroborated by a direct determination of the crystal structure of the solution produced under high pressure.…”

One-step electric-field driven methane and formaldehyde synthesis from liquid methanol

“…30 Moreover, the spontaneous presence of even stronger fields in the proximity of certain clean and apolar mineral surfaces 42 and in regions surrounding solvated ionic species 43–45 is well established.…”

Proton Conductivity in Phosphoric Acid: The Role of Quantum Effects

“…Recent ab initio molecular dynamics simulations revealed strong dependence of quantum effects in water on electric field and presence of ions [32,33]. Strong ionization of phosphoric acids can create significant electric fields which might affect the probability of proton tunneling in PAs.…”