Amorphous ice

Amorphous ice (non-crystalline (“vitreous”) ice) is an amorphous solid form of water. Common ice is a crystalline material where the molecules are regularly arranged in a hexagonal lattice whereas amorphous ice is distinguished by a lack of long-range order in its molecular arrangement. Amorphous ice is produced either by rapid cooling of liquid water (so the molecules do not have enough time to form a crystal lattice) or by compressing ordinary ice at low temperatures.

Although almost all water ice on Earth is the familiar crystalline ice I_h, amorphous ice dominates in the depths of interstellar medium, making this likely the most common structure for H₂O in the universe at large.

Just as there are many different crystalline forms of ice (currently 16 known), there are also different forms of amorphous ice, distinguished principally by their densities.

The production of amorphous ice hinges on the fast rate of cooling. Liquid water must be cooled to its glass transition temperature (about 136 K or −137 °C) in milliseconds to prevent the spontaneous nucleation of crystals. This is analogous to the production of ice cream from heterogeneous ingredients, which must also be frozen quickly to prevent the growth of crystals in the mixture.

Pressure is another important factor in the formation of amorphous ice, and changes in pressure may cause one form to convert into another.

Chemicals known as cryoprotectants can be added to water, to lower its freezing point (like an antifreeze) and increase viscosity, which inhibits formation of crystals. Vitrification without addition of cryoprotectants can be achieved by very rapid cooling. These techniques are used in biology for cryopreservation of cells and tissues.

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