title: Proton Ordering of Cubic Ice Ic: Spectroscopy and Computer Simulations.
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.
authors.affiliation: Faculty of Physics, University of Vienna , Boltzmanngasse 5, 1090 Vienna, Austria.
authors.forename: Philipp
authors.initials: P
authors.lastname: Geiger
authors.affiliation: Faculty of Physics, University of Vienna , Boltzmanngasse 5, 1090 Vienna, Austria.
authors.forename: Christoph
authors.initials: C
authors.lastname: Dellago
authors.affiliation: Faculty of Physics, University of Vienna , Boltzmanngasse 5, 1090 Vienna, Austria.
authors.forename: Markus
authors.initials: M
authors.lastname: Macher
authors.affiliation: Faculty of Physics, University of Vienna , Boltzmanngasse 5, 1090 Vienna, Austria.
authors.forename: Cesare
authors.initials: C
authors.lastname: Franchini
authors.affiliation: Faculty of Physics, University of Vienna , Boltzmanngasse 5, 1090 Vienna, Austria.
authors.forename: Georg
authors.initials: G
authors.lastname: Kresse
authors.affiliation: Institute of Physical Chemistry, University of Innsbruck , Innrain 52a, 6020 Innsbruck, Austria.
authors.forename: Jürgen
authors.initials: J
authors.lastname: Bernard
authors.affiliation: Institute of Physical Chemistry, University of Innsbruck , Innrain 52a, 6020 Innsbruck, Austria.
authors.forename: Josef N
authors.initials: JN
authors.lastname: Stern
authors.affiliation: Institute of Physical Chemistry, University of Innsbruck , Innrain 52a, 6020 Innsbruck, Austria.
authors.forename: Thomas
authors.initials: T
authors.lastname: Loerting