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Ice XI is the hydrogen-ordered form of Ih, the ordinary form of ice. Different phases of ice, from ice II to ice XVI, have been created in the laboratory at different temperatures and pressures. The total internal energy of ice XI is about one sixth lower than ice Ih, so in principle it should naturally form when ice Ih is cooled to below 72K. The low temperature required to achieve this transition is correlated with the relatively low energy difference between the two structures. Water molecules in ice Ih are surrounded by four semi-randomly directed hydrogen bonds. Such arrangements should change to the more ordered arrangement of hydrogen bonds found in ice XI at low temperatures, so long as localized proton hopping is sufficiently enabled; a process that becomes easier with increasing pressure. Correspondingly, ice XI is believed to have a triple point with hexagonal ice and gaseous water at (~72 K, ~0 Pa). ==Properties== Ice XI has an orthorhombic structure with space group Cmc21 containing eight molecules per unit cell. Its lattice parameters are a=4.465(3) Å, b=7.859(4) Å, and c=7.292(2) Å at 5 K. 〔A. J. Leadbetter, R. C. Ward, J. W. Clark, P. A. Tucker, T. Matsuo and S. Suga, The equilibrium low-structure of ice, J. Chem. Phys. 82 (1985) 424–428.〕 There are actually 16 crystallographically inequivalent hydrogen-ordered configurations of ice with an orthorhombic structure of eight atoms per unit cell, but electronic structure calculations show Cmc21 to be the most stable. Another possible configuration, with space group Pna21 is also of interest, as it is an antiferroelectric crystal, which Davidson and Morokuma incorrectly suggested as the most stable structure in 1984. In practice, ice XI is most easily prepared from a dilute (10 mM) KOH solution kept just below 72 K for about a week (for D2O a temperature just below 76 K will suffice). The hydroxide ions create defects in the hexagonal ice, allowing protons to jump more freely between the oxygen atoms (and so this structure of ice XI breaks the 'ice rules'). More specifically, each hydroxide ion creates a Bjerrum L defect and an ionized vertex. Both the defect and the ion can move throughout the lattice and 'assist' with proton reordering. The positive K+ ion may also play a role because it is found that KOH works better than other alkali hydroxides. The exact details of these ordering mechanisms are still poorly understood and under question because experimentally the mobility of the hydroxide and K+ ions appear to be very low around 72K.〔Chris Knight and Sherwin J. Singer, ''Theoretical study of a hydroxide ion within the ice-Ih lattice'', Physics and Chemistry of Ice (Proceedings of the 11th International Conference on the Physics and Chemistry of Ice), ed., Werner F. Kuhs (Royal Soc. of Chemistry, 2007), p. 339.〕〔Chris Knight and Sherwin J. Singer, ''Tackling the problem of hydrogen bond order and disorder'', Physics and Chemistry of Ice (Proceedings of the 11th International Conference on the Physics and Chemistry of Ice), ed., Werner F. Kuhs (Royal Soc. of Chemistry, 2007), p. 329.〕 The current belief is that KOH acts only to assist with the hydrogen reordering and is not required for the lower-energy stability of ice XI. However, calculations by Toshiaki Iitaka in 2010 call this into question. Iitaka argues that the KOH ions compensate for the large net electric dipole moment of the crystal lattice along the c-axis. The aforementioned electronic structure calculations are done assuming an infinite lattice and ignore the effects of macroscopic electric fields created by surface charges. Because such fields are present in any finite size crystal, in non-doped ice XI, domains of alternating dipole moment should form as in conventional ferroelectrics.〔 It has also been suggested that the ice Ih => ice XI transition is enabled by the tunneling of protons. Although ice XI is thought to be a more stable conformation than ice Ih, the transformation is very slow. According to one report, in Antarctic conditions it is estimated to take at least 100,000 years to form without the assistance of catalysts. Ice XI was sought and found in Antarctic ice that was about 100 years old in 1998.〔H. Fukazawa, S. Mae, S. Ikeda and O. Watanabe, Proton ordering in Antarctic ice observed by Raman and neutron scattering, Chem. Phys. Lett. 294 (1998) 554–558〕 A further study in 2004 was not able to reproduce this finding, however, after studying Antarctic ice which was around 3000 years old. The 1998 Antarctic study also claimed that the transformation temperature (ice XI => ice Ih) is , which is far higher than the temperature of the expected triple point mentioned above (72 K, ~0 Pa). Ice XI was also found in experiments using pure water at very low temperature (~10 K) and low pressure – conditions thought to be present in the upper atmosphere.〔K. Furić, V. Volovšek, Water ice at low temperatures and pressures; new Raman results, J. Mol. Structure 2010; 976 174–180.〕 Recently, small domains of ice XI were found to form in pure water; its phase transition back to ice Ih occurred at 72 K while under hydrostatic pressure conditions of up to 70 MPa. Ice Ih that has been transformed to ice XI and then back to ice Ih, on raising the temperature, retains some hydrogen-ordered domains and more easily transforms back to ice XI again.〔M. Arakawa, H. Kagi and H. Fukazawa, Annealing effects on hydrogen ordering in KOD doped ice observed using neutron diffraction, J. Mol" ''Structure'' 2010; 972 111–114.〕 A neutron powder diffraction study found that small hydrogen-ordered domains can exist up to 111 K. There are distinct differences in the Raman spectra between ices Ih and XI, with ice XI showing much stronger peaks in the translational (~230 cm-1), librational (~630 cm-1) and in-phase asymmetric stretch (~3200 cm-1) regions.〔K. Abe, Y. Ootake and T. Shigenari, ''Raman scattering study of proton ordered ice XI single crystal'', in Physics and Chemistry of Ice, ed. W. Kuhs (Royal Society of Chemistry, Cambridge, 2007) pp 101–108〕〔K. Abe and T. Shigenari, Raman spectra of proton ordered phase XI of ICE I. Translational vibrations below 350 cm-1, J. Chem. Phys. 134 (2011) 104506.〕 Ice Ic also has a proton-ordered form. The total internal energy of ice XIc was predicted as similar as ice XIh 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ice XI」の詳細全文を読む スポンサード リンク
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