Water ice has many crystalline phases, along with a few amorphous structures. The complex structural diagram is important to understand because of the widespread importance of ice. University College London researcher Alexander Rosu-Finsen and his colleagues discovered a medium-density amorphous ice formed by ball milling hexagonal ice at low temperatures. Ball-milling is regularly used to make amorphous materials, but it had never been applied to ice.
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.
For the study, Dr. Rosu-Finsen and co-authors used a process called ball milling, vigorously shaking ordinary ice together with steel balls in a jar cooled to 77 K (minus 200 degrees Celsius).
They found that, rather than ending up with small bits of ordinary ice, the process yielded a novel amorphous form of ice that, unlike all other known ices, had the same density (1.06 g/cm3) as liquid water and whose state resembled water in solid form.
To understand the process at the molecular scale, they employed computational simulation.
By mimicking the ball-milling procedure via repeated random shearing of crystalline ice, they successfully created a computational model of the medium-density amorphous ice.
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