Thermodynamic Stability of Ice II and Its Hydrogen-Disordered Counterpart: Role of Zero-Point Energy

Abstract

We investigate why no hydrogen-disordered form of ice II has been found in nature despite the fact that most of hydrogen-ordered ices have hydrogen-disordered counterparts. The thermodynamic stability of a set of hydrogen-ordered ice II variants relative to ice II is evaluated theoretically. It is found that ice II is more stable than the disordered variants so generated as to satisfy the simple ice rule due to the lower zero-point energy as well as the pair interaction energy. The residual entropy of the disordered ice II phase gradually compensates the unfavorable free energy with increasing temperature. The crossover, however, occurs at a high temperature well above the melting point of ice III. Consequently, the hydrogen-disordered phase does not exist in nature. The thermodynamic stability of partially hydrogen-disordered ices is also scrutinized by examining the free-energy components of several variants obtained by systematic inversion of OH directions in ice II. The potential energy of one variant is lower than that of the ice II structure, but its Gibbs free energy is slightly higher than that of ice II due to the zero-point energy. The slight difference in the thermodynamic stability leaves the possibility of the partial hydrogen-disorder in real ice II.

Doping-induced disappearance of ice II from water’s phase diagram

“…Recent computational studies have shown that the phase transition from ice II to the hypothetical hydrogen-disordered ice IId would only take place at temperatures well within -3 -the stability region of the liquid highlighting a strong resilience of ice II towards hydrogen disordering. 13,14 Here we test the response of water's phase diagram with respect to a specific disturbance, the addition of small amounts of ammonium fluoride (NH4F). While NH4F is well-known to mix with the 'ordinary' ice Ih across a large composition range 15,16 and has been shown to incorporate into clathrate hydrates, 17,18 the mixing of NH4F with high-pressure phases of ice has not been previously investigated.…”

2D IR spectroscopy of high-pressure phases of ice

“…40,41 Ice II is a fully hydrogen-ordered form of ice with R3 space group symmetry. [42][43][44] It is the only phase of ice for which the hydrogendisordered counterpart is unknown; 45 upon heating, it transforms to either ice Ih, III, V, or VI depending on the pressure. Ice II has a hexagonal unit cell comprising 36 water molecules and four distinct types of hydrogen bonds of equal multiplicity.…”

Molecular-Level Interpretation of Vibrational Spectra of Ordered Ice Phases

“…With more sophisticated methods for modeling water, computational investigations can better aid in interpreting the one and two dimensional IR and Raman spectra beyond the limits of experiment. Recent computational investigations into the ice phases have focused on spectroscopic interpretation, 30,51,52 energetics of phases 53 and competition between proton order and disorder, 59,60 the ferroelectricity (or lack thereof) of ice XI, 61 phase transitions and volume isotope effects, 49,[62][63][64] phonon/normal mode calculations, 15,52,[65][66][67] and the pressure dependence of proton hopping. 68 It has recently been demonstrated that a rigorous representation of the water properties can be derived from many-body molecular dynamics (MB-MD) simulations performed with the MB-pol potential energy function.…”