Physicists have discovered a new strange tetragonal phase of water ice

(ORDO NEWS) — A new crystalline form of water ice has been discovered in fleeting transitions between phases at high pressures.

It is called Ice-VIIt and occurs in the process of sliding a substance between two already known cubic forms of molecular arrangement. Although it is unlikely that Ice-VIIt will naturally appear on the surface of the Earth, it can tell a lot about how water behaves on huge alien worlds.

We may think that this is a common occurrence, but water is actually quite a strange liquid compared to other liquids known to us. The arrangement of molecules within the frozen form of water – ice – can vary significantly depending on environmental conditions.

We know of at least 19 solid phases of ice, some of which occur naturally and some have only been observed in the laboratory.

The ice that you see in a freezer or that falls from the sky in the form of snowflakes or hailstones is the most common natural ice on Earth. It’s called Ice-I, and its oxygen atoms are arranged in a hexagonal lattice. However, this structure is geometrically unstable: hydrogen atoms dangle randomly in it.

When physicists cool Ice-I at different temperatures and apply different pressures to it, the hydrogen and oxygen atoms in it can periodically take on different shapes, sometimes even ordering more neatly. These various forms of water ice are not always stable, but we can examine them in the laboratory to reveal their curious molecular structures.

Two of these cubic phases are Ice-VII, in which the hydrogen is disordered, and Ice-X, which is symmetrical. They can be obtained by subjecting ice to high pressure, which is tens to hundreds of thousands higher than Earth’s atmospheric pressure at sea level, and Ice-VII at even lower pressure than Ice-X.

To study the transitions between the phases of ice, a team of physicists led by Zach Grande at the University of Nevada at Las Vegas conducted experiments with ice under high pressure, using a new technique to measure the properties of ice under pressure.

The researchers squeezed a sample of water in a diamond anvil, causing it to freeze into a jumble of crystals. Lasers were then used to heat the sample, causing it to melt, and then freeze it again, turning it into what the researchers called a powdered collection of crystals.

By gradually increasing the pressure in the anvil with periodic laser bursts, the researchers created ice-VII and observed the transition to ice-X. Between them, thanks to a new measurement technique, they also observed a new intermediate phase, Ice-VIIt.

In this phase, the Ice-VII cubic lattice is stretched along one of the vectors so that the structure acquires a rectangular shape with a cubic area, and then transforms into a symmetrical, fully ordered Ice-X cubic structure. This arrangement is known as tetragonal.

The team also showed that Ice-X can form at much lower pressures than previously thought. Ice-VII is formed at a pressure of about 3 gigapascals, that is, at a pressure of 30,000 atmospheres. According to the team’s observations, the transition to Ice-VIIt occurs at a pressure of about 5.1 gigapascals.

In previous reports, the transition pressure for Ice-X has been in the range of 40 to 120 gigapascals. However, Grande and his team observed that the transition between Ice-VIIt and Ice-X occurred at about 30.9 gigapascals.

This, the team says, should help resolve the Ice-X transition pressure debate.

“Zak’s work demonstrated that the transition to the ionic state occurs at much, much lower pressure than previously thought,” said physicist Ashkan Salamat of the University of Nevada at Las Vegas.

“This is the missing piece and the most accurate measurement of water under these conditions.”

This, the team says, could have important implications for studying the interior conditions of other worlds. Water-rich planets outside the solar system could have Ice-VIIt in abundance, they say, even raising the possibility of having conditions suitable for the origin of life.


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