New form of crystalline ice may help learn about hydrogen bonds

Using neutron diffraction, scientists have characterized the crystalline structure of a newly named ice form, ice XIX.

Researchers described the exotic ice form in a new paper, published Thursday in the journal Nature Communications.

Almost all naturally occurring frozen water on planet Earth, whether ice or snow, exists in the hexagonal crystal form called ordinary ice -- or ice one. Common ice is characterized by its six-membered rings of oxygen atoms.

But as scientists have discovered over the last century, iced formed under various combinations of extreme pressures and temperatures develops different kinds of crystalline structures.

In the last 100 years, scientists have described a total of 18 different ice forms, each with different arrangements of oxygen and hydrogen atoms -- and differences in density.

The differing ice forms, including the new one, could help to understand hydrogen bonds -- which is relevant to a variety of scientific studies on Earth, as well as other planets -- researchers said.

"The density is different if the arrangement of oxygen atoms is different, something known as topology," study corresponding author Thomas Loerting told UPI in an email.

"Hexagonal ice is famous for its six-membered rings. The higher density of high-pressure ice phases can be reached either by compressing and distorting the hydrogen bonds or by changing the network topology," said Loerting, a researcher with the Institute of Physical Chemistry at the University of Innsbruck in Austria.

Even though ice forms are often characterized by the shape and size of their rings of oxygen atoms, as Loerting explained, ice's topological diversity is largely a reflection hydrogen's dynamism.

"The pattern of hydrogen atoms can either be random or it can follow a pattern. In ice VI the H-atoms are random, and it is called a disordered, frustrated ice," Loerting said.

"In ice XV and in ice XIX the H-atoms are aligned according to a pattern -- following symmetry. They are called ordered. Whether or not the H-atoms are ordered makes a huge difference," Loerting said.

Whether or not an ice form's hydrogen atoms are ordered or not has a significant effect on the ice's physical and electrical properties.

"H-disordered phases like common hexagonal ice can usually be deformed plastically -- this is the reason why glaciers flow," Loerting said. "H-ordered ices, by contrast, are extremely brittle and cannot be deformed plastically."

Scientists in search of new ice forms practice what's called crystallography.

Researchers precisely manipulate the ice formation process, ramping up the pressure or slowly heating ice frozen at extremely frigid temperatures.

After tweaking the ice formation process, scientists analyze the density of their frozen product, as well as the precise arrangement of the O and H atoms on a microscopic level.

About a decade ago, researchers at Innsbruck produced an ordered variant of ice VI, yielding what came to be named ice XV.

More recently, scientists tweaked the formation process of ice VI, cooling it rapidly to yield what scientists estimated was a new ordered variant of the parent ice -- a second ordered variant of the same parent ice.

To be certain, scientists needed to use neutron diffraction, but the imaging technique only works with ice formed by heavy water -- water featuring deuterium, a heavier hydrogen isotope. Frozen heavy water is known as deuterated ice.

Unfortunately, adding heavy hydrogen alters the ice formation process, complicating efforts to reproduce the new ice form.

Researchers realized they could minimize the disruption by mixing a tiny bit of normal water with heavy water before repeating their rapid-freezing process. The technique worked, and scientists were able to confirm the new arrangement of hydrogen atoms in what suggests is a newest ice form -- ice XIX.

"This is the first example in ice physics, in which a second ordered polymorph related to the same parental disordered phase could be realized in experiment," Loerting said.

As such, scientists will now be able to observe the transition between two ordered ice forms in a single experiment.

Since the 1980s, researchers at Innsbruck have discovered six novel ice forms, four crystalline forms and two amorphous forms.

Scientists spend time studying, imaging and discovering new ice forms because they can help researchers better understand hydrogen bonds.

"People from theory and molecular simulations have huge difficulties in modelling the hydrogen bond properly, and many models especially fail on reproducing the known ice polymorphs," Loerting said. "A new ice forms thus helps to improve functionals and potentials used to model and understand the hydrogen bond."

Understanding the density and properties of different ice phases is also essential to the study of celestial bodies and their icy cores and mantles -- bodies like the ice giants Uranus and Neptune or the icy moons Europa, Io, Ganymede and Titan.

On Earth, temperature and pressure doesn't vary all that much, but in other parts of the solar systems, unusual conditions can produce more exotic ice forms.

"There is a broad interest about ices from many fields, crystallographs, astrophysics, theoretical chemistry, environmental chemistry, etc. -- and this means the race for ice XX has already started," Loerting said.

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