What is Plastic Ice VII? An Exotic Phase of Water

Water exists in various states beyond the commonly known solid, liquid, and gas. Scientists have long theorised about the presence of exotic phases of water under extreme temperature and pressure conditions. One such phase, Plastic Ice VII, has now been observed experimentally for the first time using advanced neutron spectrometry at the Institut Laue-Langevin (ILL). This discovery provides crucial insights into molecular dynamics and planetary sciences.

Key Takeaways

1. Plastic Ice VII is a hybrid phase with both solid and liquid-like properties.
2. Observed at 450-600K and 0.1-6 GPa using advanced neutron spectrometry.
3. Contradicts initial MD simulations, revealing a four-fold rotational mechanism.
4. Relevant to planetary science, especially for icy celestial bodies.
5. Neutron scattering emerges as a crucial tool for studying extreme-pressure water phases.

What is Plastic Ice VII?

Plastic Ice VII is a unique phase of water initially predicted through molecular dynamics (MD) simulations over 15 years ago. It combines the properties of both solids and liquids, creating a hybrid state where molecules form a rigid lattice while maintaining rotational motion.

What are the key characteristics of Plastic Ice VII?

Plastic Ice VII comprises a cubic crystalline lattice that has rotational motion and is formed at its highest temperature around 450–600 K. Below in table, all its features are given in detail: 

Scientific Methods and Observations

Neutron Scattering and Identification of Phases

The study utilised Quasi-Elastic Neutron Scattering (QENS) to analyse molecular movement across different states. Three distinct phases were identified:

1. Liquid Water: Molecules exhibit both translational and rotational motion.
2. Solid Ice VII: Both types of motion are completely frozen.
3. Plastic Ice VII: Molecules retain rotational motion but lose translational movement.

How were experiments set for observing an exotic phase of water?

For this, neutron experiments were conducted using time-of-flight spectrometers IN5 and IN6-SHARP at the ILL. Sample conditions reached up to 60,000 times the atmospheric pressure to stabilise Plastic Ice VII. Advanced thermodynamic control and sample environments enabled the successful observation of this exotic phase.

What were the breakthrough findings from this experiment on an exotic phase of water?

As per Maria Rescigno, a Ph.D. student at Sapienza University, two suggestions were revealed, which are:

1. Unexpected Molecular Rotation Mechanism

Contrary to initial MD predictions, the study revealed that the water molecules in Plastic Ice VII follow a four-fold rotational model, common in jump-rotor plastic crystals. This suggests a more complex molecular behaviour than previously assumed.

2. Phase Transition Insights

Additional neutron and X-ray diffraction experiments explored how Ice VII transitions into Plastic Ice VII. Findings suggest that:

• The transition could be either first-order or continuous, depending on simulation methods.
• A continuous transition hints at a possible precursor to the superionic phase, where hydrogen diffuses freely through the oxygen lattice at even higher temperatures and pressures.

How will the discovery of Plastic Ice VII be significant for planetary science?

The discovery of Plastic Ice VII has significant implications for understanding the internal structure of icy planets and moons.

Potential Applications

Neutron Scattering in Planetary Science

• Traditionally underutilised, neutron scattering is proving invaluable for mapping hydrogen dynamics in planetary ice structures.
• The ability to replicate planetary-relevant pressures in experiments enhances our understanding of extraterrestrial water behaviour.

Conclusion

The first experimental observation of Plastic Ice VII marks a major milestone in the study of water’s exotic phases. The findings challenge existing molecular models and offer new perspectives in planetary science, particularly in understanding the composition of icy moons and planets. Future research may uncover even more exotic water phases, potentially reshaping our knowledge of planetary interiors and high-pressure physics. As experimental techniques advance, the study of exotic water phases will continue to unveil new dimensions of physics and planetary science.