Partially ordered state of ice XV

Abstract

Most ice polymorphs have order–disorder “pairs” in terms of hydrogen positions, which contributes to the rich variety of ice polymorphs; in fact, three recently discovered polymorphs— ices XIII, XIV, and XV—are ordered counter forms to already identified disordered phases. Despite the considerable effort to understand order–disorder transition in ice crystals, there is an inconsistency among the various experiments and calculations for ice XV, the ordered counter form of ice VI, i.e., neutron diffraction observations suggest antiferroelectrically ordered structures, which disagree with dielectric measurement and theoretical studies, implying ferroelectrically ordered structures. Here we investigate in-situ neutron diffraction measurements and density functional theory calculations to revisit the structure and stability of ice XV. We find that none of the completely ordered configurations are particular favored; instead, partially ordered states are established as a mixture of ordered domains in disordered ice VI. This scenario in which several kinds of ordered configuration coexist dispels the contradictions in previous studies. It means that the order–disorder pairs in ice polymorphs are not one-to-one correspondent pairs but rather have one-to-n correspondence, where there are n possible configurations at finite temperature.

Japan Association of Mineralogical Sciences

“…Graduate School of Science The University of Tokyo Figure 1 A single crystal of ice VI grown at 0.9 GPa and room temperature (diameter of the gasket hole is 0.3 mm). Ice XV, investigated by Komatsu et al (2016), is a hydrogen-bond ordered form of ice VI.…”

“…However, a recent study on ice XV, an ordered phase of ice VI (Fig. 1) that has been investigated by in situ high-pressure neutron diffraction and density functional theory calculations, showed that different kinds of configurations have quite similar free energies, and the observed powder neutron diffraction patterns also indicated a partially ordered state of ice XV (Komatsu et al 2016). Komatsu et al (2016) also mentioned that none of the completely ordered configurations is favored; instead, partially ordered states are established as a mixture of ordered domains at the temperatures investigated.…”

“…To be fair, as mentioned in the literature, this idea was already implied by Barclay Kamb in his written more than 40 years ago (Kamb 1973). Komatsu et al (2016) also stated that undiscovered different configurations are potentially detectable by neutron diffraction under an electric field and that exploring these will provide new insights into ice physics. The study by Komatsu and colleagues will provide an impetus for additional research for a 'new ice survey', not only for ice XV but also for all other ice polymorphs, which could have a significant impact on the concept of order-disorder phenomena.…”

New diversity form of ice polymorphism: Discovery of second hydrogen ordered phase of ice VI

“…The structural variety of ice arises from the geometric exibility of hydrogen bonds and hydrogen order-disorder phase transition 2 , and hydrogen ordering in ice structure also induces drastic changes in the dynamic/static properties of ice along with the phase transition, such as immobilization of molecular rotation 3 and ferro-or antiferroelectrically aligned molecular structures [4][5][6] . A prominent unsolved question 7 concerning the structural diversity induced by hydrogen ordering is whether a hydrogen-disordered phase of ice transforms into only one hydrogen-ordered phase, as inferred from the currently known phase diagram of ice, although the energies of its possible hydrogen con gurations are close because of the inherent geometric frustration of the ice lattice [8][9][10][11][12] . Recent experiments on a high-pressure hydrogen-disordered phase, ice VI, revealed an unknown hydrogen-ordered form (β-XV 13 ) besides the known ordered phase, ice XV.…”

Materials and Life Science Experimental Facility (MLF) at the Japan Proton Accelerator Research Complex II: Neutron Scattering Instruments

“…Therefore, the sample pressure can be changed even at low temperatures. This system is applied to the study of ice under low-T and high-P conditions [89,90]. PLANET is widely used in the fields of geoscience, high-pressure physics, and material science.…”