New form of ice with density close to liquid water raises new questions

BY a process that involves vigorously shaking a cryogenically cooled container full of ordinary ice and steel balls, called ball milling, scientists at the University of London created a previously unknown amorphous form with a density close to liquid water. The discovery suggests that water is more complex at low temperatures than previously recognised. The finding was reported in the latest issue of Science.

Frozen water can take many forms. There are 20 known common or crystalline phases of water ice and at least two families of the amorphous form. Molecules of common ice are arranged regularly in a hexagonal lattice. The amorphous forms, however, lack any ordered crystalline structure. Scientists believe that while almost all frozen water on the earth exists as crystalline ice, amorphous ice is likely the most common structure for water in the universe at large.

In general, amorphous ices are distinguished by their densities: low-density amorphous ice with a density of 0.94 g/cm ³ and high-density amorphous ice forms that start at 1.13 g/cm ³. However, hitherto known forms of crystalline and amorphous ices have densities near that of liquid water (~1 g/cm ³). This density gap is the cornerstone of the current understanding of water.

Alexander Rosu-Finsen and colleagues showed that ball milling common ice at nearly −200 ^oC (77 kelvin) leads to a “medium-density” form of amorphous ice (MDA) with a density of 1.06 ± 0.06 g/cm ³.

Ball milling has never before been applied to ice. Using various experimental techniques and computational simulation, Rosu-Finsen’s team evaluated and characterised the nature of this new form of ice, revealing its distinct structure and unique mechanical properties. The findings, the scientists say, open interesting new questions into the structural nature of MDA, including whether it represents the true glassy state of liquid water.