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 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 ].

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). 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 (, 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. At higher pressures, ice-seven undergo a continuous transition into cubic ice-ten (ice X) where the 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 ].

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). 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 dielectric constant of ice-seven is about 150.

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 face centered positions of its body centered cubic structure 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].

Interactive Jmol structures are given.