Snowflakes are usually hexagonal, but under certain conditions, they can look like diamonds.

In a new study published on Wednesday in the journal Nature, Chinese scientists described the formation of ice crystallites in real-time at molecular resolution, solving the mystery of how water freezes into a diamond-like, face-centered cubic crystal structure.

The cubic ice is believed to be responsible for a particular type of halo around the Sun or Moon called Scheiner's halo. But whether cubic ice exists has long been controversial since detecting it in its pure phase was challenging in experiments.

The researchers, led by those from the Institute of Physics under the Chinese Academy of Sciences, used in-situ cryogenic transmission electron microscopy imaging and water vapor deposition on graphene to follow the ice crystals' formation from vapor at minus 170 degrees Celsius.

They found that most of the resulting ice was pure-phase cubic ice, demonstrating that the nucleation of cubic ice is preferential on low-temperature substrates. The proportion of ice with a hexagonal lattice structure increases over time.

A determining factor that affects its formation is a heterogeneous interface, easily found in snowfall, during which water molecules grow on the surface of tiny dust particles, noted the study.

The direct and real-space imaging in this study resolved the controversy concerning cubic ice, and deepened our understanding of ice formation behavior under different conditions, which may have broad applications in materials science, geology, and climate science, said the researchers.

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