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Scientists have created a previously unconfirmed form of ice under pressures exceeding 2 million times Earth’s atmospheric pressure and temperatures above 2,000 °C. The discovery adds another phase to water’s already unusual behavior and could help scientists better understand the interiors of ice giant planets.
The newly observed phase is called hexagonal close-packed, or hcp, ice. It consists of oxygen atoms arranged in a tightly packed crystal structure while hydrogen nuclei can move through the lattice, giving the material characteristics of superionic ice, a strange state that combines properties of solids and liquids. The findings were reported in a study led by physicist Alexis Forestier.
Water has more than 20 known solid phases, with different arrangements emerging as temperature and pressure change. Under ordinary conditions, heating water eventually turns it into vapor. But at pressures of hundreds of gigapascals, the enormous force prevents the molecules from expanding normally, allowing exotic forms of ice to exist even at temperatures that would ordinarily vaporize water.
The researchers compressed tiny samples of water between diamond tips to pressures reaching 230 gigapascals, equivalent to about 2.3 million times atmospheric pressure at sea level. They simultaneously heated the samples with lasers to temperatures as high as 2,630 kelvins, or about 2,357 °C.
To determine how the atoms were arranged, the team used a narrow synchrotron X-ray beam to examine changes in the ice’s crystal structure. The measurements revealed the hcp arrangement, which had been predicted theoretically but had not previously been unambiguously identified in experiments.
The experiments also showed a transition between hcp and another superionic form known as face-centered cubic, or fcc, ice. At 155 gigapascals and 2,000 kelvins, both structures appeared in the measurements. At 197 gigapascals and 2,250 kelvins, the hcp structure became more prominent. At 219 gigapascals and 2,630 kelvins, it dominated the sample.
The discovery could have implications far beyond laboratory physics. Superionic ice is thought to exist deep within Uranus and Neptune, where its ability to conduct electricity may influence how magnetic fields are generated inside the planets.
However, scientists do not yet know precisely how hcp ice behaves electrically or mechanically. Further experiments and theoretical work will be needed to determine its stability and properties under planetary conditions.
