Reassigning Hydrogen-Bond Centering in Dense Ice

Abstract

Hydrogen bonds in H2O ice change dramatically upon compression. Thereby a hydrogen-bonded molecular crystal, ice VII, is transformed to an atomic crystal, ice X. Car-Parrinello simulations reproduce the features of the x-ray diffraction spectra up to about 170 GPa but allow for analysis in real space. Starting from molecular ice VII with static orientational disorder, dynamical translational disordering occurs first via creation of ionic defects, which results in a systematic violation of the ice rules. As a second step, the transformation to an atomic solid and thus hydrogen-bond centering occurs around 110 GPa at 300 K and no novel phase is found up to at least 170 GPa.

Pressure-induced antifluorite-to-anticotunnite phase transition in lithium oxide

“…Ice also, in the past, has been predicted to exist in the antifluorite structure at sufficiently high pressure [3,4]. Since then this proposition has been called into question, but the actual high pressure structure remains to be seen experimentally, and is most currently not predicted to exist below 170 GPa [8]. Ice VII gradually becomes "symmetric" ice-X at the pressure range of 40-90 GPa, with a bcc oxygen sublattice, similar to that of ice VII but with hydrogen atoms occupying the central position between adjacent oxygen atoms.…”

“…However, a further transformation to an antifluorite phase in ice at some pressure above 150 GPa has been predicted [3,4], and experiments show changes in vibrational mode coupling [5] and single-crystal xray diffraction peak intensity [7] near 150 GPa. Recent studies argue that a new phase is either hexagonal or orthorhombic [6], but the existence and nature of this phase and the pressure at which it is reached are still uncertain [7,8]. In further similarity to ice, for which a high-pressure, high-temperature superionic phase has been predicted [9], ambient pressure Li 2 O becomes superionic at temperatures above 1350 K [10], prior to melting at 1705 K [11].…”

Chemistry of H2O and HF under Extreme Conditions

“…We observe a similar structural change in the H-H RDF in which the first peak lengthens from 1.63Å (close to the result for ambient conditions) to 1.85Å . These observations bear a strong resemblance to the ice VII to ice X transition in which the covalent O-H bond distance of ice becomes equivalent to the hydrogen bond distance as pressure is increased [29]. However, the superionic phase differs from ice X, in that the position of the first peak in g R OH ¡ is not half the distance of the first O-O peak [29].…”

“…These observations bear a strong resemblance to the ice VII to ice X transition in which the covalent O-H bond distance of ice becomes equivalent to the hydrogen bond distance as pressure is increased [29]. However, the superionic phase differs from ice X, in that the position of the first peak in g R OH ¡ is not half the distance of the first O-O peak [29]. We analyze the effect of the change in g R OH ¡ below in terms of the molecular speciation in the simulations.…”

The Reactivity of Energetic Materials at Extreme Conditions

“…These observations bear a strong resemblance to the ice VII to ice X transition in which the covalent O-H bond distance of ice becomes equivalent to the hydrogen bond distance as pressure is increased 66 . However, the superionic phase differs from ice X, in that the position of the first peak in g(R OH ) is not half the distance of the first O-O peak 66 .…”