Researchers Discover Rare Proton Pattern In Ice

Water and ice cover most of our planet, but we know very little about them. Now, however, we have a better understanding of how atoms behave in ice.

AsianScientist (Apr. 8, 2016) – Sure, you drink it everyday—but water, in its many forms, still holds many mysteries. Now, researchers in Japan have explained in detail the theory behind their experiments that show how protons inside ice behave. Their findings have been published in Physical Review B.

Everything from a cloud of smoke to a solid rock takes that specific form because of the collective behavior of the atoms that make up that object. However, how do atoms choose how to behave? What choices are available to them?

“We understand almost everything about how a single quantum particle behaves. But, put a group of quantum particles together and anything can happen,” explained team leader Professor Nic Shannon from the Theory of Quantum Matter Unit at Okinawa Institute of Science and Technology Graduate University.

“Surprisingly, we still don’t really know what happens in something as simple as ice.”

A water molecule, H2O, is formed when an oxygen atom forms covalent bonds with two hydrogen atoms. In ice, these water molecules are connected by weaker hydrogen bonds.

Water ice is unique, because the oxygen atoms are ordered in hexagonally-shaped crystals similar to honeycombs, but the hydrogen atoms do not follow a regular pattern. Instead, they respect the so-called ‘ice rules’: each bond can contain only one atom and each oxygen atom has two hydrogen atoms adjacent to it.

But there are virtually infinite ways for the protons to satisfy this rule, even in a small piece of ice. So are atoms in ice ordered or disordered?

The research team has answered this question by offering a theoretical explanation of the results from an experiment carried out in England, in which neutrons were scattered from crystals of frozen heavy water (D2O). When neutrons are scattered from the ordered atoms in a crystal, experiments show a regular pattern of spots.

Meanwhile, scattering from completely disordered atoms is uniform, and featureless. But for protons in ice, neither of these things happen, and experiments instead show patterns which look like bowties, and like the letter ‘M.’

The ‘bowties,’ technically called ‘pinch points,’ are particularly interesting, because they show that protons are not completely disordered. They are locally ordered, yet globally disordered.

Pinch points are indicated with black circles. Credit: OIST
Pinch points are indicated with black circles. Credit: OIST

This pattern is very rare in nature; it happens only in ice, a type of magnet called spin ice and a class of materials called proton-bonded ferroelectrics.

“The presence of pinch points is telling us that protons can be described mathematically by a type of theory called a gauge theory, which is common to all the fundamental forces of nature. This is one of nature’s best tricks,and is the basis of the Standard Model of elementary particles,” explained Shannon.

“The more that we learn about water, the more we realize that it is one the strangest and most beautiful things in the Universe.”

The article can be found at: Benton et al. (2016) Classical and Quantum Theories of Proton Disorder in Hexagonal Water Ice.


Source: Okinawa Institute of Science and Technology Graduate University; Photo: Shutterstock.
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