- Ice VII
Ice VII is a cubic crystalline form of
ice formed when liquid water freezes at pressures above 2.2 GPa.cite web |url=http://www.iapws.org/relguide/meltsub.pdf | title = IAPWS, Release on the pressure along the melting and the sublimation curves of ordinary water substance, 1993 | accessdate = 2008-02-22 ] It can also be reached in the solid state by increasing the pressure onice VI at ambient temperature.G.P. Johari, A.Lavergne and E.Whalley, Journal of Chemical Physics Vol 61 p4292 (1974)] Like the majority of ice phases (including ice Ih), the hydrogen atom positions are disordered.V.F. Petrenko and R.W. Whitworth, The Physics of Ice, Oxford University Press, New York (2002).] In addition, the oxygen atoms are disordered over multiple sites.W.F. Kuhs, J.L. Finney, C. Vettier and D.V. Bliss, J. Chem. Phys. Vol. 81 (8) 3612-3623 (1984).] J.D. Jorgensen and T.G. Worlton, J. Chem. Phys. Vol 83 (1) 329-333 (1985).] R.J. Nelmes, J.S. Loveday, W.G. Marshall et al, Phys. Rev. Lett. Vol 81 (13) 2719-2722 (1998).] The structure of ice VII comprises ahydrogen bond framework in the form of two interpenetrating (but non-bonded) sub-lattices.Ice VII is the only disordered phase of ice that can be ordered by simple cooling, and it forms (ordered)
ice VIII below 273 K up to ~ 8 GPa. Above this pressure, the VII-VIII transition temperature drops rapidly, reaching 0 K at ~60 GPa.Ph. Pruzan, J.C. Chervin and B. Canny, J. Chem. Phys. Vol 99 (12) 9842-9846 (1993).] Thus, ice VII has the largest stability field of all of the molecular phases of ice. The cubic oxygen sub-lattices that form the backbone of the ice VII structure persist to pressures of at least 128 GPa;R.J. Hemley, A.P. Jephcoat, H.K. Mao et al, Nature, Vol. 330, 737-740 (1987).] this pressure is substantially higher than that at which water loses its molecular character entirely, formingice X .Ordinary water ice is known as ice Ih, (in the Bridgman nomenclature). Different types of ice, from ice II to ice XIV, have been created in the laboratory at different temperatures and pressures.
References
*cite web |url=http://www.lsbu.ac.uk/water/ice_vii.html |title=Ice-seven and ice-ten structures |accessdate=2008-01-02 |author=Chaplin, Martin |date=2007-10-26 |work=Water Structure and Science
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