CEA physicists find a third superionic ice phase in a diamond anvil cell
CEA physicists formed hexagonal close-packed superionic ice from ultrapure water at 130-200 GPa and above 1800 K. The team says the finding gives a rationale for adding the phase to models of Uranus and Neptune, though confirming it exists inside either planet is the next step.
The Scientist · Science desk
Key figures: the team applied pressures up to 229 GPa, starting from 80 GPa. CEA researchers saw the hcp superionic phase from 130 GPa, across 130-200 GPa. Forestier says it becomes more stable above 150-200 GPa.
| Figure | Value | Note | Claim |
|---|---|---|---|
| Highest pressure applied to ultrapure water | 229 GPa | applied from 80 GPa | 7 |
| hcp phase seen, low end of range | 130 GPa | range 130–200 GPa, per CEA researchers | 9 |
| hcp more stable above, per Forestier | 150 GPa | Forestier: above 150–200 GPa | 3 |
What happened
- The team, led by Alexis Forestier and Gunnar Weck, added two cup-shaped boron-doped diamond discs to the anvil surfaces and laser-heated a 12 micron sample.
- They took the water from 300 to 2630 K at pressures between 80 and 229 GPa.
- A sub-micron X-ray beam at the ID27 beamline in Grenoble produced high-quality diffraction spectra despite the sample's small size.
- The phase is the third known superionic form of water, after the body-centred cubic phase first observed in 2016.
Why it matters
- Who has to decide decision Teams modelling Uranus and Neptune have to decide whether to add a third superionic phase to their interiors before its boundaries are mapped.
- What it limits constraint Superionic behaviour in the new phase is so far inferred from thermal expansion, which limits how confidently its conductivity can be fed into magnetic field models.
- What it limits constraint The sample was ultrapure water, while the ice hypothesised to generate the planets' fields is described as water, ammonia and methane, so the match to the planets is partial.
The structural result is the firmer half. According to the CEA researchers, the diffraction spectra showed the hcp phase emerging alongside the fcc phase across 130 to 200 GPa, at temperatures above 1800 K [9]. That makes the heating design relevant. The set-up heats the sample uniformly, which avoids the large temperature gradients that complicate measurements in conventional diamond anvil cells [6]. A two-phase pattern is easier to trust when the sample is not hot in one place and cool in another. The work is reported in Physical Review Letters [11].
The superionic half is a claim about hydrogen. The team describes liquid-like hydrogen diffusing through a solid lattice of oxygen atoms [4]. In theory, protons in this kind of ice move so readily through empty sites in the oxygen lattice that conductivity may exceed 100 Siemens per centimetre, almost the level of a metal [12]. That conductivity is what the planetary argument needs. The magnetic fields of Uranus and Neptune are skewed and off-centre with respect to the planets' axes of rotation, and scientists have hypothesised that conductive superionic ice in the outer third of each planet generates them, instead of molten iron in the core as on Earth [1].
Forestier framed the result with care. "The experimental observation of this new hcp superionic phase now provides a rationale for incorporating its mechanical properties and ionic conductivity into these models of planetary interiors," he said [2]. He added that the phase can coexist with the fcc phase and becomes more stable above 150 to 200 GPa, so it probably exists deeper within the inner shell of the ice giants [3].
In our view, calling this experimental support for the planetary models claims too much. It supplies a candidate phase to them. The thing this doesn't tell you is how much of either planet sits inside the 130 to 200 GPa window where the phase appears; the report does not include planetary pressure and temperature profiles [9].
What to watch
- Brillouin spectroscopy results: anomalies in the speed of sound would be the more direct superionic signature Weck says the team wants.
- The CEA team's map of stable regions, phase transition lines and hcp melting behaviour, which it says will help show whether the phase is present in Uranus and Neptune.
- Whether published interior models of Uranus and Neptune start including the hcp phase's conductivity and mechanical properties.
Clarity's read
What the record supports and how the coverage leans. The claims behind it follow.
Reality
- Evidence62
- Adoption
- Not scored
- Hype gap+10
- Incentives
- Not scored
- Confidence50
Claim ledger
Ranked by verification strength, evidence, and original report placement.
- [1]
The magnetic fields of Uranus and Neptune are skewed and off-centre with respect to the planets' axis of rotation, and scientists have hypothesised that they arise from conductive superionic warm dense ice (water, ammonia and methane) in the planets' outer third, instead of from molten iron in the core as on Earth.
- [2]
"The experimental observation of this new hcp superionic phase now provides a rationale for incorporating its mechanical properties and ionic conductivity into these models of planetary interiors," Forestier said.
- [3]
Forestier said the hcp phase can coexist with the fcc superionic phase and become more stable at higher pressures (above 150-200 GPa), and so probably exists deeper within the inner shell of the ice giants.
- [4]
By heating ultrapure water while compressing it to extreme pressures, CEA physicists formed an ice phase with a hexagonal close-packed structure and liquid-like hydrogen diffusing through a solid lattice of oxygen atoms.
- [5]
A CEA team led by physicists Alexis Forestier and Gunnar Weck added two high-thermal-conductivity cup-shaped discs of boron-doped diamond to the diamond anvil surfaces and laser-heated a 12 micron-sized sample.
- [6]
The set-up let the team heat the sample uniformly, avoiding the large temperature gradients that complicate measurements in conventional diamond anvil cells.
- [7]
The team heated ultrapure water from 300 to 2630 K while applying pressures between 80 and 229 GPa.
- [8]
The Extremely Brilliant Source's sub-micron beam, part of the ID27 beamline at the European Synchrotron Research Facility in Grenoble, gave high-quality X-ray diffraction spectra despite the small sample size.
- [9]
According to the CEA researchers, the X-ray spectra showed a new hcp phase of superionic ice emerging alongside a previously observed fcc phase across a pressure range of 130-200 GPa and temperatures above 1800 K.
- [10]
The discovery brings the total number of superionic phases of water to three, the other being the body-centred cubic phase first observed in 2016.
- [11]
The results are reported in Physical Review Letters.
- [12]
Theory predicts that in superionic ice hydrogen ions diffuse so readily through empty sites in water's oxygen lattice that ionic conductivity may exceed 100 Siemens per centimetre, almost as conductive as metals.
- [13]
The researchers' next steps are to define the pressure-temperature regions where the phases are stable, map the transition lines between molecular and superionic phases, and understand how the hcp phase behaves as it melts, which will help confirm whether these phases are stable and present within Uranus and Neptune.
- [14]
"We also want to obtain more direct experimental signatures of superionic behaviour, beyond the thermal expansion we observed," Weck said.
- [15]
Weck said Brillouin spectroscopy measurements could reveal anomalies in the speed of sound.
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Sources
1 independent publisher whose own reporting we read for this story.
- physicsworld.comNew type of superionic water ice observed for the first time
1 article · October 16, 2026
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