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Superionic Ice

How Water Can Be Solid and Liquid-Like at the Same Time

Big Question: How can pressure keep part of water structurally solid while temperature makes another part mobile enough to behave like an ionic liquid?

Quick Answer

At extreme pressure, oxygen atoms in water can be forced into a tightly packed crystalline arrangement. At the same time, very high temperature can break the usual fixed molecular arrangement of H₂O and allow hydrogen nuclei to diffuse rapidly through the oxygen framework.

The result is neither ordinary solid ice nor ordinary liquid water. The oxygen lattice gives the material long-range crystalline order, while mobile hydrogen ions make it highly ionically conductive.

Shock-compression experiments at Lawrence Livermore National Laboratory supplied the first experimental evidence for superionic water, and later X-ray diffraction experiments directly revealed the crystalline oxygen lattice.

Lawrence Livermore National Laboratory — First Experimental Evidence for Superionic Ice →

What You Will Learn

• Why water has many solid phases.
• Why pressure changes which crystal structure is stable.
• Why high temperature does not always force complete melting.
• How oxygen can remain ordered while hydrogen becomes mobile.
• Why mobile ions make the material electrically conductive.
• How shock compression creates planetary-interior conditions.
• How X-ray diffraction identifies a crystal lattice.
• Why several superionic structures can exist.
• What ice XVIII means.
• Why superionic water may exist inside Uranus and Neptune.
• How it could affect planetary magnetic-field models.
• Why “solid and liquid at once” is useful shorthand but not a literal mixture of ordinary ice and ordinary water.

Part 1 — Ordinary Ice Is Only One Water Crystal

The familiar ice in a freezer is usually hexagonal ice Ih. Its water molecules occupy an open hydrogen-bonded crystal structure.

Increase pressure and that open structure becomes energetically expensive. Water adopts denser arrangements instead.

Scientists have identified many crystalline ice phases because the balance among molecular orientation, hydrogen bonding, density and pressure changes across the phase diagram.

Part 2 — Pressure Can Stabilise a Solid at High Temperature

Whether matter is solid or liquid depends on both temperature and pressure.

High temperature favours disorder and mobility. High pressure favours structures that pack matter efficiently.

Under giant-planet conditions, the pressure is so enormous that a dense oxygen lattice can remain stable even while hydrogen becomes highly mobile.

Part 3 — The H₂O Molecule Stops Being the Best Picture

At everyday conditions, drawing separate bent H₂O molecules is useful.

At extreme pressure and temperature, protons can transfer between neighbouring oxygens rapidly. The material is better described as a dense oxygen framework with mobile hydrogen ions rather than as permanently intact individual water molecules.

same chemical elements, different collective organisation.

Part 4 — Why “Superionic”?

An ion conductor carries electric current through the motion of charged atoms or atomic groups rather than mainly through free electrons.

In superionic water, hydrogen nuclei move rapidly through the lattice and provide very high ionic conductivity.

This makes the material behave electrically more like a molten salt or fast-ion conductor than like ordinary freezer ice.

Part 5 — First Experimental Evidence Came From Shock Compression

Computer simulations predicted superionic water decades before laboratories could test it directly.

LLNL researchers used dynamic compression to create pressures and temperatures comparable with planetary interiors. Measurements showed extremely high protonic conductivity and an unusually high melting temperature—two major predicted signatures of the superionic phase.

At roughly 200 GPa, LLNL reported melting temperatures near 5000 K—conditions vastly beyond ordinary water experiments.

Part 6 — X-Rays Revealed the Oxygen Lattice

Electrical conductivity alone could suggest unusual ion motion, but a solid phase also requires structural evidence.

Later laser-driven experiments compressed and heated water while firing X-ray pulses through it. The scattered X-rays produced diffraction patterns consistent with a crystalline oxygen lattice.

LLNL — X-Ray Diffraction Reveals the Atomic Structure of Superionic Ice →

Part 7 — Ice XVIII

One high-pressure superionic phase observed in dynamic experiments has a face-centred cubic oxygen lattice and is commonly called ice XVIII.

The name does not mean there are only eighteen possible forms of ice forever. Ice nomenclature expands as distinct crystal structures and hydrogen-ordering states are characterised.

Part 8 — There Is More Than One Superionic Structure

Later static high-pressure experiments reported both body-centred and face-centred cubic oxygen lattices across different pressure–temperature regions.

A 2021 Nature Physics study mapped the stability fields of two superionic ice phases using laser-heated diamond-anvil cells, X-ray diffraction and optical spectroscopy.

Nature Physics — Structure and Properties of Two Superionic Ice Phases →

Part 9 — Solid and Liquid-Like Are Describing Different Components

Saying superionic ice is “solid and liquid at the same time” can be useful, but it should be unpacked.

• The oxygen positions show long-range crystalline order.
• The hydrogen ions diffuse rapidly through that structure.

It is not a glass containing ordinary liquid-water puddles. It is one phase with two very different dynamical behaviours inside it.

Part 10 — Why It Can Be Dark

Superionic water under extreme conditions can absorb visible light strongly and may appear dark or black in experiments and models.

This does not mean “black ice” is a new pigment. Optical response changes because pressure, temperature and electronic structure alter how the material interacts with electromagnetic radiation.

Part 11 — Uranus and Neptune May Contain Enormous Amounts

Uranus and Neptune are often called ice giants because planetary models include large inventories of substances such as water, ammonia and methane beneath their outer atmospheres.

Interior pressures and temperatures overlap regions where superionic water is predicted and experimentally observed. Nature Physics experiments suggest face-centred cubic superionic water could be stable across important giant-planet conditions.

This does not mean their interiors are made of pure laboratory H₂O. Real planetary mixtures contain several elements and phases.

Part 12 — Why Magnetic Fields Enter the Story

Planetary magnetic fields require moving electrically conducting material arranged in a dynamo.

Uranus and Neptune have unusually tilted and non-dipolar magnetic fields. Highly conductive ionic layers, perhaps surrounding or overlying more rigid superionic regions, may help constrain where their dynamos operate.

LLNL notes that superionic interiors support “thin-dynamo” style models, but this remains a planetary-model inference rather than a direct sample from Neptune.

Part 13 — How a Laboratory Makes Planetary Matter for Nanoseconds

1. A tiny water sample is prepared between high-strength materials.
2. Powerful laser pulses drive shocks through the sample.
3. Pressure rises to hundreds of gigapascals.
4. Temperature rises to thousands of kelvin.
5. Water enters an extreme high-pressure phase.
6. An X-ray pulse probes the sample while the state exists.
7. Scattered X-rays record lattice spacing and symmetry.
8. Electrical or optical measurements test conductivity and phase behaviour.
9. Results are compared with quantum simulations and thermodynamic models.

How Do We Know?

• Shock experiments measure pressure, temperature and conductivity.
• X-ray diffraction reveals long-range oxygen order.
• Optical spectroscopy distinguishes superionic and fluid regimes.
• Diamond-anvil experiments map static high-pressure phases.
• First-principles simulations predict hydrogen diffusion and lattice stability.
• Independent experimental approaches overlap in key regions of the phase diagram.

Observation vs Inference

• Observation: extreme-condition water displays very high ionic conductivity.
• Observation: X-ray diffraction shows a crystalline oxygen lattice in the relevant regime.
• Inference: hydrogen ions are highly mobile through that lattice.
• Planetary inference: such phases may occupy large regions inside Uranus and Neptune.
• Boundary: the exact composition, phase proportions and dynamo geometry of those planets remain model-dependent.

Common Misconceptions and Better Models

Misconception	Better model
Ice cannot exist at thousands of degrees.	Very high pressure can stabilise dense solid structures at extreme temperature.
Superionic ice is half ordinary ice and half ordinary water.	It is one phase with ordered oxygen and mobile hydrogen.
The H₂O molecules remain permanently intact.	Hydrogen becomes highly mobile and molecular identity is not fixed in the everyday sense.
It conducts electricity like copper.	Its high conductivity is mainly ionic rather than ordinary metallic electron conduction.
Scientists found chunks of it on Earth.	It is created transiently under extreme laboratory pressure and temperature.
Superionic ice proves exactly what Uranus and Neptune contain.	It constrains plausible planetary-interior models but does not directly sample those planets.

Checkpoint Questions

1. Why does pressure change water’s solid phases?
2. What remains solid-like in superionic ice?
3. What becomes liquid-like?
4. Why is the phase electrically conductive?
5. Why is X-ray diffraction important?
6. What is ice XVIII?
7. Why can superionic ice remain solid at enormous temperature?
8. Why are Uranus and Neptune relevant?
9. How could conductivity affect planetary magnetism?
10. What remains uncertain?

Answer Key

Open after attempting the questions

1. Pressure changes the free-energy balance and favours denser atomic arrangements.
2. The oxygen sublattice.
3. The hydrogen ions diffuse rapidly.
4. Moving charged hydrogen ions carry current.
5. It directly tests whether long-range crystal order exists.
6. A high-pressure superionic water phase with a crystalline oxygen lattice, associated especially with an fcc structure in key experiments.
7. Extreme pressure stabilises the dense oxygen framework.
8. Their interior conditions overlap predicted superionic stability fields.
9. Conducting moving layers can participate in planetary dynamos.
10. The detailed phases, mixtures and magnetic-field-generating geometry inside real planets.

Primary Science Bridge

• water can exist in different states;
• pressure as well as temperature affects state;
• solids contain ordered particles;
• charged particles can carry electric current;
• extreme environments can reveal behaviour not seen in daily life.

Secondary and JC Bridge

Core idea	Higher-resolution route
States of matter	High-pressure phase diagrams
Crystal lattice	bcc and fcc oxygen structures
Ions	Proton diffusion and ionic conductivity
Evidence	Shock compression and X-ray diffraction
Planetary science	Ice-giant interiors
Magnetism	Conductive planetary dynamos

Deep Science Window — Two Timescales in One Phase

The oxygen atoms remain localised around lattice sites over times long enough to produce diffraction, while hydrogen hops rapidly between available positions. A material can therefore be crystalline when viewed through one species and diffusive when viewed through another.

Deep Science Window — Phase Is Collective Behaviour

A phase is not defined only by whether individual molecules “look solid” or “look liquid.” It is defined by collective symmetry, mobility, thermodynamics and response. Superionic ice is a powerful example because everyday categories split apart.

Evidence Boundaries

• Hot ≠ automatically liquid.
• Solid lattice ≠ every atom immobile.
• Superionic ≠ ordinary metallic conduction.
• Laboratory pure water ≠ exact planetary mixture.
• Possible planetary phase ≠ direct observation inside Neptune.
• “Solid and liquid at once” ≠ literal coexistence of freezer ice and liquid water.

Manual Summary — KNOW → CONNECT → EXPLAIN → APPLY → CHECK

KNOW: phase diagram, pressure, oxygen lattice, proton diffusion, ionic conductivity, ice XVIII.

CONNECT: pressure to lattice stability, heat to hydrogen mobility, and mobile ions to electrical conduction.

EXPLAIN: how one phase can be structurally solid and dynamically liquid-like.

APPLY: predict why extreme planetary interiors may contain water phases absent at Earth’s surface.

CHECK: separate direct high-pressure measurements from planetary interpretation.

Research Sources and Further Reading

• Lawrence Livermore National Laboratory — First Experimental Evidence for Superionic Ice
• Lawrence Livermore National Laboratory — Atomic Structure of Superionic Ice
• Nature — Nanosecond X-Ray Diffraction of Shock-Compressed Superionic Water Ice
• Nature Physics — Structure and Properties of Two Superionic Ice Phases