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Lab discovers first pentagonal ice crystals by chance

点击量:   时间:2019-03-09 08:11:00

By Colin Barras (Image: Kenneth Libbrecht) They say that no two snowflakes are the same, but like all natural ice crystals they are only variations on the same six-sided theme. Now physicists have found the first evidence of pentagonal ice crystals. See a short image gallery showing what the research team found The unusual rod-like ice crystals were formed on a copper surface in an ultrahigh vacuum at temperatures of -173 °C. Under those conditions, a tiny quantity of water begins to adsorb onto the copper, spontaneously forming into freestanding rods. “Since these chains were first observed a couple of years ago it was assumed they must be built from hexagons,” says chemist Angelos Michaelides at University College London. He is a member of the team that studied the rods under a microscope and noticed that the top of each rod has a regular zigzag arrangement of protrusions. These are too small to be the usual hexagon arrangements of water molecules – known as hexamers – that underlie natural ice crystals. The team calculated that the pattern was in fact caused by projecting oxygen and hydrogen atoms from water molecules, forced upwards rather than lying flat as usual. Simulations show that this could only be down to a pentagonal arrangement of water molecules rather than hexagons. Liquid water can occasionally take on a pentagonal form in “caged compounds” – lattices of one kind of molecule that trap and contain a second kind of molecule – but it was thought ice was always hexagonal. “That was a big surprise,” says Michaelides. But “we are as confident as we can be that this is the first time that a pentagon-based ice structure at a surface has been observed.” The first water molecules that cling to the surface stick to high points formed by the copper atoms. The shape they take on forces the next water molecules into the pentagonal form as they attach to the surface. The new result suggests even more exotic ice crystal shapes are possible, says Michaelides. “There is no hard and fast rule that metal-supported ice structures must be built from pentagons,” Michaelides says. “Heptagons and other interesting structures may be possible.” However, it is unlikely that anything other than hexagonal snowflakes will ever be found in nature. “Macroscopic ice pentagon structures [would] require growth in more than one dimension, and we know we cannot fully fill even 2D space with pentagons,” says Michaelides. “This is a fascinating piece of work,” says Martin McCoustra at Heriot Watt University in Edinburgh, UK, who was not involved in the study. “There is no doubt that the more we learn of water and how it behaves, the more complex this simple yet universally important solvent appears to get,” he says. See a short image gallery showing what the research team found Journal reference: Nature Materials (DOI: 10.1038/nmat2403) See our feature on the weirdness of water: Water: The quantum elixir More on these topics: