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Water structure and behavior

H2O

Ice-seven (ice VII) and ice ten (ice X)

 

Ice-seven (ice VII) is formed from liquid water above 3 GPa by lowering its temperature to ambient temperatures (see Phase Diagram). Its unit cell, which forms cubic crystal (Pn3m, 224; Laue class symmetry m-3m). Ice VII consists of two interpenetrating cubic ice lattices with hydrogen bonds passing through the center of the water hexamers and no connecting hydrogen-bonds between lattices. It has a density of about 1.65 g cm-3 (at 2.5 GPa and 25°C [8]), which is less than twice the cubic ice density as the intra-network O····O distances are longer to allow for the interpenetration. The cubic crystal (shown opposite) has cell dimensions 3.3501 Å (a, b, c, 90º, 90º, 90º; D2O, at 2.6 GPa and 22°C [361]) and contains two water molecules.

[ice vii substructure]

All molecules experience identical molecular environments. The hydrogen bonding is disordered and constantly changing as in hexagonal ice but ice-seven undergoes a proton disorder-order transition to ice-eight at about 5°C; ice-seven and ice-eight having identical structures apart from the proton ordering. Ice-seven is metastable indefinitely at 77 K.

Ice-seven has known triple points with ice-six and ice-eight (5C, 2.1 GPa), ice-eight and ice-ten (100 K, 62 GPa) and liquid water and ice-six (81.6C, 2.2 GPa). Interestingly, at high pressures (~ 2.3 GPa), liquid water can be made to freeze at over 100C (to give the more dense ice-seven). The dielectric constant of ice-seven is about 150.

Note that in this structural diagram the hydrogen bonding is ordered whereas in reality it is random (obeying the 'ice' rules: two hydrogen atoms near each oxygen, one hydrogen atom on each OO bond). As the H-O-H angle does not vary much from that of the isolated molecule, the hydrogen bonds are not straight (although shown so in the figures).

Interactive COW (Plug-in, ActiveX) and Chime (35 KB) structures are given.

As the pressure is raised, ice-seven appears to undergo a continuous transition into cubic ice-ten (ice X) where the ice protons are equispaced between the oxygen atoms [391]. The oxygen atoms are arranged in a body-centered cubic arrangement (8 neighbors) and the hydrogen atoms in a body-centered truncated cubic arrangement (12 neighbors).

Interactive COW (Plug-in, ActiveX) and Chime (48 KB) structures are given.

[Ice-ten cubic structure consisting of 8 unit cells of two water molecules plus extra atoms found within the basic cubic structure]

Please submit any comments and suggestions you may have.

Water: Home | Introduction | Icosahedral water clusters | Phase Diagram | Ice-1h | Ice-1c | Ice-two | Ice-three | Ice-four | Ice-five | Ice-six | Ice-eight | Ice-nine | Ice-eleven | Ice-twelve

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This page was last updated by Martin Chaplin
on 11 November, 2003

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