Ball milling “ordinary” ice at low temperatures – a process that involves vigorously shaking a cryogenically-cooled container full of ice and steel balls – creates a previously unrecognized amorphous form with a density close to liquid water, researchers report. The finding suggests that water is more complex at low temperatures than previously recognized, which not only has implications for our understanding of water and its curious, unexplained anomalies, but also for our understanding of how water exists and functions throughout the universe. Frozen water can take many forms. There are 20 known common or crystalline phases of water ice and at least two families of amorphous form. Unlike common ice, whose molecules are regularly arranged in a hexagonal lattice, amorphous forms lack a highly ordered crystalline structure. Although almost all frozen water on 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, with low-density amorphous ice having a density of 0.94 g/cm3 and high-density amorphous ice forms, which start at 1.13 g/cm3. However, neither crystalline nor amorphous ices have a form with a density near that of liquid water (~1 g/cm3). This density gap is a cornerstone of our current understanding of water. Alexander Rosu-Finsen and colleagues now show that ball milling common ice at nearly -200 degrees Celsius (77 Kelvin) leads to a “medium-density” form of amorphous ice (MDA) with a density of 1.06 ± 0.06 g/cm3. Ball milling has been used to create other amorphous materials, like metallic alloys and inorganic compounds, but had not previously been applied to ice. Using a variety of experimental techniques and computational simulation, Rosu-Finsen et al. evaluated and characterized the nature of this new form of ice, revealing its distinct structure and unique mechanical properties. According to the authors, the findings open interesting new questions into the structural nature of MDA, including whether or not it represents the true glassy state of liquid water.
Medium-density amorphous ice
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