Recent studies have provided new insights into the molecular and structural behavior of water across different temperature and pressure conditions, including its supercooled liquid state, superionic ice phase, and ambient liquid form. These findings, published separately in Nature Physics, Nature Communications, and Science, expand scientific understanding of water's complex phase behavior.
Water's Two Distinct Local Structures in Liquid Form
Researchers led by Xiao Cheng Zeng and Liwen Li at the City University of Hong Kong used unsupervised deep learning to analyze molecular dynamics simulations of liquid water. The simulations tracked hundreds of thousands of water molecules and generated tens of millions of data points.
The AI analysis identified evidence that water exists in two distinct local structures: high-density and low-density forms. The researchers identified reaction coordinates describing the transition between the two structures. Two transition pathways were identified: a "semi-loop" with one energy barrier and a "full-loop" with three barriers near the coexistence boundary.
The two-state hypothesis proposes that water's anomalies—such as maximum density at 4°C, expansion upon freezing, and resistance to temperature change—arise from the coexistence of two liquid forms. Previous experimental and theoretical work suggested such behavior in supercooled water, but direct molecular evidence was previously lacking.
According to the researchers, traditional analysis of the data would have taken approximately a decade; with AI, the analysis took approximately 1.5 years. The team plans to build a more rigorous machine-learning model and connect the structural findings to properties like density, viscosity, and temperature. Experimental confirmation will require sensitive techniques, such as those developed at Pacific Northwest National Laboratory.
The findings were published June 4 in Nature Physics.
Critical Point Identified in Supercooled Water
Two separate research teams have identified a previously hidden critical point in supercooled water—water that remains liquid below its standard freezing temperature without turning into ice.
Using X-ray lasers, an international research team observed indicators of a long-theorized transition between two liquid states in supercooled water. The experiments relied on rapid heating using infrared lasers and swift X-ray snapshots. Researchers engineered ice and manipulated it through the liquid-liquid state, past the critical point, and into a fluctuating state, capturing observations on extremely short timescales before crystallization occurred.
Chemical physicist Anders Nilsson from Stockholm University stated that rapid X-ray imaging allowed observations before ice formation, revealing the liquid-liquid transition's vanishing point and the emergence of a new critical state. He noted that the existence of such a critical point has been theorized for decades.
The critical point was estimated to be around -63 °C (-81.4 °F) and 1,000 atmospheres. The system dynamics of the liquid slow down as it approaches this critical point, making the transition unavoidable.
This state is characterized by significant instability and fluctuations between the two liquid states, which researchers noted contribute to water's unique properties.
The findings were published on March 26 in the journal Science.
Observing Superionic Ice Structure
Scientists have obtained observations of "superionic ice," a state of water where oxygen atoms form a solid crystal lattice while hydrogen atoms flow freely like a liquid.
Experimental Method
Using X-ray lasers, researchers compressed water samples to pressures up to 180 gigapascals (1.8 million times Earth's atmospheric pressure) and heated them to thousands of degrees. The experiments involved sandwiching thin water layers, approximately half the width of a human hair, between diamond windows. Laser pulses were used to generate shock waves that incrementally compressed the water. Each experiment lasted nanoseconds and destroyed the sample, necessitating repeated trials for verification.
Structural Findings
Under these conditions, the team observed that oxygen atoms formed a complex structure mixing two different crystal arrangements that constantly interconvert. At moderate pressures (below 120 gigapascals), experiments detected two different crystal structures coexisting. This suggests the energy cost for forming either structure is nearly identical, leading water to crystallize into whichever pattern is locally favorable. As pressure increased beyond 150 gigapascals, one arrangement became dominant, but a significant portion (25-32%) of layers stacked in an alternative pattern, creating disorder within the crystal. Machine learning simulations, trained on quantum calculations, independently confirmed these disordered patterns.
Implications for Planetary Science
Uranus and Neptune are believed to contain vast oceans of this superionic water in their deep interiors. The extreme pressures and temperatures within these planets transform water into this superionic state. This unique state may account for their tilted, off-center magnetic fields that do not align with their rotational axes. The flow of charged hydrogen ions through the oxygen lattice is believed to contribute to these magnetic fields.
Resolving Past Contradictions
Previous experiments attempting to study water under these extreme conditions had produced conflicting results, with different crystal structures being reported at similar pressures and temperatures. The new measurements, published in Nature Communications, utilized ultrafast X-ray pulses lasting just 50 femtoseconds. This technique allowed researchers to capture diffraction patterns with high resolution, effectively "freezing" the water's structure before it could change.
The nature of this stacking disorder—whether temporary or permanent—remains under investigation. If stable, such defects could influence how heat and electricity move through the material, which is critical for understanding planetary dynamo action and magnetic field generation.