Between the tips of two diamonds, researchers squeezed a microscopic speck of water until internal pressures reached millions of times that at Earth’s surface, according to a study set to appear in Physical Review Letters. When scientists fired lasers at the trapped sample, heating it past 2,000 degrees Celsius, the water did not boil away. Instead, according to the experimental data, its oxygen atoms locked into an exotic solid configuration known as hexagonal close-packed, or hcp, ice.
Replicating Planetary Interiors With Diamond-Anvil Cells
To reach these extreme states, Alexis Forestier of France’s Alternative Energies and Atomic Energy Commission and his team used diamond-anvil cells to trap water samples roughly 12 micrometers wide. According to the study, researchers surrounded the water with laser-absorbing boron-doped diamond and probed it using an X-ray beam less than a micrometer across at the European Synchrotron Radiation Facility.
As the team pushed the pressure to more than two million times that at Earth’s surface, the hcp structural pattern steadily took over the X-ray signal. The hcp configuration took over completely when conditions reached 219 gigapascals and 2,630 kelvins (roughly 2,357°C or 4,274°F). This phase represents what scientists define as superionic water, a state initially predicted more than 30 years ago but only examined directly via nanosecond X-ray diffraction in recent years.
Did you know? Superionic water features oxygen atoms locked in a solid crystal lattice while hydrogen nuclei—protons—flow freely through the structure like a liquid, giving the material unique conductive properties.
How Atomic Shifts Drive Superionic Behavior
The transition between familiar superionic structures and the newly observed hcp ice involves a subtle atomic shuffle. According to the research, the oxygen atoms shift similarly to stacked balls settling into a new arrangement. This microscopic reorganization governs which phase remains most stable under extreme heat and pressure.
Previous findings from a Nature Communications study involving shock-compressed superionic water noted mixed face-centered cubic (fcc) and hcp structures above roughly 150 gigapascals and around 2,500 kelvins. Arianna Gleason, a SLAC physicist and co-author on that earlier research, noted that the unique structure of superionic water likely gives rise to its conductive properties.
Data from the new hcp phase revealed unusual thermal expansion near 1,700 kelvins (1,427°C or 2,600°F), pointing to entry into the superionic state. Because most of this expansion happened in just one crystallographic direction, researchers suggest hydrogen may flow more easily through the ice along some paths than others.
Implications for the Magnetic Fields of Uranus and Neptune
The discovery of hcp superionic ice provides new data for modeling planetary interiors, particularly regarding ice giants like Uranus and Neptune. Data from the Voyager 2 spacecraft previously revealed that Uranus and Neptune have magnetic fields unlike Earth’s relatively orderly dipole. Their fields are strongly tilted and strikingly non-axisymmetric.
Superionic water stands among the candidate materials that planetary researchers have long suggested could produce those peculiar fields via electrical currents flowing inside a relatively thin conducting zone within the planets. Should hcp superionic ice be present inside the planets, its unique traits could diverge from the conventionally presumed fcc structure and alter existing simulations concerning the transport of mass and electrical current through these worlds.
Future research aims to map the exact stability boundaries of hcp ice and measure electrical behavior as hydrogen moves through the crystal lattice under planetary-core conditions.
Frequently Asked Questions
What is superionic ice?
Superionic ice is an exotic phase of water where oxygen atoms form a rigid solid lattice while hydrogen nuclei flow freely through the structure like a liquid.
How do scientists create superionic ice in the laboratory?
According to researchers, scientists compress microscopic water samples between two diamond tips at pressures millions of times greater than Earth’s surface and heat them with lasers past 2,000 degrees Celsius.
Why does hcp superionic ice matter for planetary science?
The newly observed hexagonal close-packed ice structure exhibits directional hydrogen flow and thermal expansion properties that may help explain the unusual, tilted magnetic fields observed on Uranus and Neptune.
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