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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. [ABSTRACT FROM AUTHOR]

The phase transition from ice VII to ice X under extreme pressures is an example where quantum protondelocalization coexists with classical thermal fluctuations. We investigate this transition, including quantumeffects on the nuclear motion through adapted Langevin dynamics. This approach, which allows us to followthe semiclassical trajectories of protons, provides excellent agreement with experimental vibrational spectraindicating a transition pressure of about 65 GPa. Furthermore, we map the full dynamical problem onto apressure-dependent, one-dimensional mean-field potential for the proton. By solving exactly the corresponding Schödinger equation, we disentangle tunneling and quantum delocalization from classical thermal effects andidentify the transition through the topological changes of the proton ground state and its susceptibility. Theprocess is dominated by quantum effects even at ambient temperature and can be considered to be a paradigmaticcase of a quantum-driven phase transition. [ABSTRACT FROM AUTHOR]