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Scientists have identified the 19th form of water ice. The exotic four-sided crystals of this rare ice variety, now nicknamed ice XIX, form at ultra-low temperatures and ultra-high pressures.
It only exists in lab experiments, but researchers say it reveals more about others ice shapes, which can be found deep in the Earth’s mantle and on very cold planets and moons.
“To name a new form of ice, you have to elucidate exactly what the crystal structure is,” said lead researcher Thomas Loerting, professor of physical chemistry at the University of Innsbruck in Austria. This means figuring out the simplest repeating structure of the crystal, where all the atoms are located in that structure, and what the symmetry of the crystal structure is, Loerting said.
“Only if all of this is known are you allowed to name your ice … Ice XIX is now the name of the new phase of ice discovered in our work,” he told Live Science in an email.
An article by Loerting and his colleagues describing the new form of ice appeared on February 18 in the newspaper Nature communications, alongside a study by Japanese researchers who verified the discovery.
Related: Snowflake gallery: no two are alike, of course
A new ice cream
Almost everyone is familiar with the beautiful variety of six-sided snowflakes, which reflects the hexagonal arrangement of oxygen atoms in the water ice crystals that make them up.
But the regular six-sided ice crystals – Ice I – are actually just one of its many forms, called polymorphs. And until recently, 18 different polymorphs of water ice had been officially identified – although only six-sided ice is common on Earth. Although ice cream may seem simple, it is a complicated thing. For example, only the oxygen the atoms in the water molecules of six-sided ice crystals form a hexagonal shape, while their hydrogen atoms are randomly oriented around them. This makes ice I a “messy” or “frustrated” ice cream in ice cream terminology. One of the properties of these messy ice creams is that they can warp under pressure: “This is the reason why glaciers sink,” Loerting said.
On the other hand, the hydrogen atoms in many of the other ice polymorphs also have their own crystal patterns, and they are therefore called “hydrogen-ordered” or “H-ordered”. Unlike messy ices, H-order ices are very brittle and will shatter rather than warp, he said.
In these terms, the 19th newly identified ice form is H-order ice; in fact, it is an H-order form of a disordered ice, called ice VI, which has a random pattern of hydrogen atoms. And Ice VI also has another H-order polymorph, Ice XV, in which the hydrogen atoms are aligned in an entirely different pattern.
“Ice VI, Ice XV and Ice XIX are all very similar in terms of density. [because] they share the same type of network of oxygen atoms, ”Loerting said. But they differ in terms of the positions of the hydrogen atoms. “This is the first time that such a relationship between ice polymorphs has been discovered, and it could allow experiments to study the transitions between one shape and another,” he says.
Crystal structure
Loerting’s team first made Ice XIX in their lab experiments three years ago, by slowing down the cooling process of Ice XV to around minus 170 degrees Celsius (minus 274 degrees Fahrenheit) and increasing the pressure drastically to about 2 gigapascals. But the details of its crystal structure eluded them until they could study it with a process called neutron diffraction, which can reveal the atomic structure of a material by bouncing a stream of neutrons off it. and examining the resulting diffraction pattern.
Under normal circumstances, neutron diffraction requires replacing the water in a sample with heavy water containing additional neutrons. But pure heavy water was impractical for the XIX ice experiments because it freezes much slower, Loerting said. The breakthrough consisted in dosing the heavy water with a fraction of ordinary light water, producing water that froze quickly but still allowed neutron diffraction.
Loerting explained that the structure of water ice is a key to the nature of hydrogen bonding, which is poorly understood. It’s also important for understanding celestial bodies, such as the ice giants Uranus and Neptune and the frozen moons of Jupiter (including Europa, Io, and Ganymede), where other ice polymorphs are found.
“It is of great interest in astrophysics to know the density and properties of ice phases, to be able to understand the behavior of the ice coats or the ice cores of these celestial bodies,” he said.
And there are a lot more ice polymorphs out there. The discovery of XIX ice makes six ice polymorphs discovered at the University of Innsbruck since the 1980s, and Loerting hopes his team will discover the next one as well. “The Ice XX race started yesterday, and I’m hoping my research group will be the one to release it,” he said.
“Originally posted on Live Science.
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