Proton Ordering of Cubic Ice Ic: Spectroscopy and Computer Simulations.

Philipp Geiger,Markus Macher,Christoph Dellago,Georg Kresse,Thomas Loerting,Jürgen Bernard,Cesare Franchini,Josef N Stern

doi:10.1021/jp500324x

Philipp Geiger, Markus Macher + Show 6 more

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https://doi.org/10.1021/jp500324x

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Abstract

Several proton-disordered crystalline ice structures are known to proton order at sufficiently low temperatures, provided that the right preparation procedure is used. For cubic ice, ice Ic, however, no proton ordering has been observed so far. Here, we subject ice Ic to an experimental protocol similar to that used to proton order hexagonal ice. In situ FT-IR spectroscopy carried out during this procedure reveals that the librational band of the spectrum narrows and acquires a structure that is observed neither in proton-disordered ice Ic nor in ice XI, the proton-ordered variant of hexagonal ice. On the basis of vibrational spectra computed for ice Ic and four of its proton-ordered variants using classical molecular dynamics and ab initio simulations, we conclude that the features of our experimental spectra are due to partial proton ordering, providing the first evidence of proton ordering in cubic ice. We further find that the proton-ordered structure with the lowest energy is ferroelectric, while the structure with the second lowest energy is weakly ferroelectric. Both structures fit the experimental spectral similarly well such that no unique assignment of proton order is possible based on our results.

Highlights

Despite its simple molecular structure, water has a remarkably complex phase diagram
On the basis of vibrational spectra computed for ice Ic and four of its proton-ordered variants using classical molecular dynamics and ab initio simulations, we conclude that the features of our experimental spectra are due to partial proton ordering, providing the first evidence of proton ordering in cubic ice
While in some crystalline ice structures, including ice Ih as well as cubic ice, ice Ic, only the oxygen atoms form a regular lattice and the protons are disordered, in other ice phases, such as ice II, the protons are arranged in a regular way

Summary

Introduction

Despite its simple molecular structure, water has a remarkably complex phase diagram. While in some crystalline ice structures, including ice Ih as well as cubic ice, ice Ic, only the oxygen atoms form a regular lattice and the protons are disordered, in other ice phases, such as ice II, the protons are arranged in a regular way. There exist pairs of ice structures, such as ice Ih and its protonically ordered counterpart ice XI, which have nearly identical oxygen sublattices but differ in their proton order. Salzmann and collaborators have identified experimentally[2,3] several previously unknown ice phases, completing the pairings V/ XIII, XII/XIV, and VI/XV, which differ only in proton ordering

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