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Ice-seven (Ice VII)

Ice-seven (ice VII) [1226] is formed from liquid water above 3 GPa by lowering its temperature to ambient temperatures (see Phase Diagram). It may be considered the most stable phase as it exists over a wide range of temperature and pressure. It is a plausible constituent of giant planets and icy moons. It can be obtained at low temperature and ambient pressure by decompressing (D2O) ice-six below 95 K and is metastable over a wide range of pressure, transforming into LDA above 120 K [948]. It can also be formed using picosecond laser pulses (causing high pressure, ~10 GPa, shock waves) at 532 nm and 1064 nm wavelengths [2688].

 

ice vii substructure 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 O····O bond). This disorder gives rise to a zero-point entropy close to 3.414 J mol-1 K-1 [2153]. 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).

The Ice VII unit cell, which forms cubic crystal (Pn bar3 m, 224; Laue class symmetry m-3m) 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 8% longer (at 0.1 MPa) 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.

 

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.

 

Pressure volume data for ice VII at room temperature, from [1943]If the pressure is increased above about 14 GPa there are changes in the proton ordering [1428]. This is shown in the phase diagram and gives rise to a loss of the cubic symmetry [2126]. This has been thought due to some dissociation with some hydrogen ions occupying the octahedral cavities, eventually rising at greater than 30 GPa to plateau at one interstitial occupancy to every two water molecules [2027a]; a this is equivalent to two of each six positions (as shown below right) being occupied. These interstitial protons cannot form normal hydrogen bonds with the H2O oxygen atoms. Molecular dynamics and modeled X-ray Raman spectroscopy (XRS) spectra, however did not show such extensive interstitial occupancy [2376], so the exact structure remains somewhat unresolved. Dissociation is responsible for the peak in the electrical conductivity at about 10 GPa during compression of D2O from 5.5 GPa to 17 GPa between 400 K and 425 K and due to the increase in H3O+ and OH- charge carriers [2244]. At higher pressures, ice-seven undergo a continuous transition into cubic ice-ten (ice X) where most hydrogen atoms are situated midway between the oxygen atoms. The size of the ice crystal (two molar volumes per unit cell) continuously reduces over this large change in pressure with no obvious volume steps at the expected ice transitions [1943].

 

Partial structure of ice VII showing interstitial sitesShown opposite is a cartoon of ice-seven showing the octahedral interstitial cavities (shown as blue spheres a-f). The water molecules are shown with the positions of their four possible hydrogen atoms, although only two, or less, would be occupied obeying the 'ice rules'. Any interstitial hydrogen atoms (shown here labeled a-f) sit in the six octahedral cavities and are centered on the body-centered cubic square faces (as examples, octahedral cavity (a, shown gray) is made from the water molecules 1, 2, 3, 4, 9 and 10 and sits in the square face made from the water molecules 1, 2, 3 and 4 of the body-centered cube centered on water molecule 9; cavity (e) is formed from the water molecules 1, 4, 5, 8, 9 and 14 and in the square face made from the water molecules 1, 4, 5 and 8 of the body-centered cube.; and cavity (f) is formed from the water molecules 5, 6, 7, 8, 9 and 13 and in the square face made from the water molecules 5, 6, 7 and 8 of the body-centered cube).

 

In contrast to ice Ih, high density ices may incorporate some solutes into their crystal structure. Ice-seven may incorporate up to ~7.5 wt % NaCl into the octahedral cavities (face centered positions of its body centered cubic structure, described above right) at high pressure (for example, 4-21 GPa) [955]. Counter-intuitively, this incorporation causes a reduction in the unit cell dimensions and a greater than expected increase in density [955]. The inclusion of salt also raises the pressure required for conversion to ice X [2373 ].

 

Ice-seven has known triple points with ice-six and ice-eight (5 °C, 2.1 GPa), ice-eight and ice-ten (100 K, 62 GPa) and liquid water and ice-six (355 K, 2.216 GPa). Ice-seven pressure-volume data has been described [2337 ]. Interestingly, at high pressures (~ 2.3 GPa), liquid water can be made to freeze at over 100 °C (to give the more dense ice-seven). The relative permittivity (dielectric constant) of ice-seven is about 150.

 

 

Interactive Jmol structures are given.


Footnotes

a This explanation is disputed after re-examination using X-ray Raman spectroscopy [2027b]. [Back]

 

 

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This page was established in 2002 and last updated by Martin Chaplin on 16 August, 2016


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