The polymorphism of ice : A review!

There are twenty or so three-dimensional crystalline phases [3500] (where the oxygen atoms are in fixed positions relative to each other, but the hydrogen atoms may or may not be disordered, and three amorphous (non-crystalline) phases (see [2145, 2349] for recent reviews of ice research). This large number is due to the open tetrahedrally arranged water molecular structure of hexagonal ice under normal atmospheric pressure and the large number of possible crystal structures that this ice can form as it is progressively crushed under high pressure.

All the crystalline phases of ice involve hydrogen-bonding water molecules with four neighboring water molecules (see left, and [1300] for a recent review). In most cases, the two hydrogen atoms are equivalent with hydrogen bonds of similar strength. The water molecules retain their symmetry obey the 'ice rules' j. For the most part, the ordering of the protons (in fixed positions with lower entropy) occurs at lower temperatures. In contrast, the pressure reduces the distances between second shell neighbors (lower volume and greater van der Waals effects). The H-O-H angle in the ice phases is expected to be a little less than tetrahedral (109.47°), at about 107°.

Natural snow and ice on Earth occur as hexagonal ice (ice Ih), as evidenced in the six-fold symmetry in ice crystals grown from water vapor (that is, snowflakes).

There are four different naturally occurring morphological forms of hexagonal ice; snow, firn (multi-year snow), freshwater ice, and sea ice [3584]. Hexagonal ice ([1969], ice Ih i see Phase Diagram), is in Space group P63/mmc, k 194; symmetry D6h, Laue class symmetry 6/mmm; analogous to β-tridymite silica or lonsdaleite. It possesses a relatively open low-density structure, where the packing efficiency is low (≈ 1/3) compared with simple cubic (≈ 1/2) or face-centered cubic (≈ 3/4) structures a (and in contrast to face-centered cubic close-packed solid hydrogen sulfide).

Cubic ice (Ice Ic) is a metastable b ice crystal that can be formed (a) by condensation of water vapor at ambient pressure but at less than -80 °C, see Phase Diagram), (b) below about -38 °C in tiny droplets (≈ 6 μm diameter) [1013], and (c) by reducing the pressure on high-pressure ices at 77 K. Cubic ice is usually found as transitional states between hexagonal and cubic ice [1236c] depending on the conditions of its formation and history [1236]. Indeed, the structure of ice that crystallizes initially from supercooled water is always stacking-disordered [1236e]. Ice crystals formed on homogeneous ice nucleation in deeply supercooled water nanodrops (r ≈ 10 nm) at ∼225 K were only 78 % cubic ice with 22% hexagonal ice; effectively 44% stacking-disordered ice [3032]. It does not seem easy to prepare cubic ice crystals with a greater proportion of cubic ice structure. However, the term 'cubic ice' has been historically used for these less-than-pure ice crystals. With increasing time, metastable cubic ice converts to hexagonal ice through the increasingly disordered stacking-disordered ice.

In contrast to ice Ih, however, water molecules have a staggered hydrogen-bonding arrangement to all of their neighbors, rather than to 3/4 of them. In contrast to the mirrored arrangement of the layers in hexagonal ice, the cubic ice crystal possesses identical layers placed on top of one another but with displacements. The result is that the density is almost the same as ice Ih.

Ice-four (ice IV) may be formed occasionally by the heating high-density amorphous ice at a slow (0.4 K ˣ min−1) rate from 145 K and at a constant pressure of 0.81 GPa [386] (a faster rate, for example, 15 K ˣ min−1, preferably produces ice-twelve). Ice-four is metastable within the ice-three, ice-five, and ice-six phase space (see Phase Diagram). It forms a rhombohedral crystal (Space group, 167; Laue class symmetry -3m1) with cell dimensions 7.60 Å (a, b, c; 70.1°, 70.1°, 70.1°, 16 molecules) [387]. All water molecules are hydrogen-bonded to four others in the crystal,, two as donors and two as acceptors. The structure is formed from a single catenated interpenetrating network of both puckered and flattish hexamers (these allow the penetration) consisting of more strongly hydrogen bound water.

The penetrating hydrogen bond is longer (2.921 Å), but these water molecules also possess three shorter hydrogen bonds (2.783 Å). The hydrogen bonds forming the flattish rings are also somewhat extended (2.876 Å). The networks are not entirely independent as three-quarters of the water molecules have one weaker hydrogen bond to the other network (2.806 Å, 143°); the fourth type of hydrogen bond present. Hence, molecules fall into two unequal classes experiencing different molecular environments.

Ice-twelve (ice XII) may be formed by heating high-density amorphous ice at a constant pressure of 0.81 GPa from 77 K to ≈ 183 K at a rate of ≥ 15 K min−1 and recovered at atmospheric pressure at 77 K [386]; a slower rate (≤ 0.4 K min−1) preferably producing ice-four). Ice-twelve is metastable within the ice-five and ice-six phase space (see Phase Diagram). It forms a tetragonal crystal (Space group, 122; Laue class symmetry 4/mmm).

In the crystal, all water molecules are hydrogen-bonded to four others, two as donor and two as acceptor. Ice-twelve contains a screw-type hydrogen-bonded arrangement (right-handed double helix) quite unlike that found in other crystalline forms of ice, with the smallest ring size consisting of seven molecules ([390] not five as reported [82]; two seven-membered rings can be seen top-left and bottom right in the opposite sub-structure). It has a density of 1.30 g cm−3 at 127 K and ambient pressure, somewhat greater than ice-five (1.23 g cm−3). The hydrogen-bonding is disordered and continually changing as in hexagonal ice.