Superionic water - the hot, black and strangely conductive form of ice that exists in the center of distant planets - was predicted in the 1980s and first recreated in a laboratory in 2018. With each closer look, it continues to surprise researchers.
In a recent study published in Nature Communications, a team including researchers at the Department of Energy's SLAC National Accelerator Laboratory made a surprising discovery: Multiple atomic packing structures can coexist under identical conditions in superionic water.
When the Voyager 2 spacecraft flew by Neptune and Uranus in the late 1980s, scientists were puzzled by what they observed. Unlike the tidy, rotationally aligned, dipolar magnetic fields found on Earth and other planets in our solar system, these ice giants had lumpier, complex magnetic fields with multiple poles that didn't align neatly with the planets' rotational axes.
The explanation for this magnetic chaos, scientists now believe, lies near the center of these distant planets. Where Earth has molten iron churning in its outer core, Neptune and Uranus have superionic water - a form of ice that exists only under the extreme temperatures and pressures found deep within giant ice planets.
In this exotic state of water, oxygen atoms stack into a rigid crystalline lattice, while hydrogen ions flow freely through the lattice structure and conduct electricity - behaving more like electrons than typical hydrogen ions. This strange conductivity could be the reason for the planets' peculiar magnetic fields, and for years, researchers have tried to better understand its unusual structure and behavior.
The unique structure of superionic water likely gives rise to its conductive properties, which in turn influences magnetic fields on the planetary scale, so understanding and measuring these packing structures on the atomic scale is key to understanding these dynamics.
This research team had intended to investigate the phase diagram of superionic water. Phase diagrams chart out the temperatures and pressures at which substances transition between different states. Water at sea level, for instance, boils at 100 degrees Celsius, but adjust the pressure and that boiling point shifts accordingly. A phase diagram of water outlines all those variables in a simple chart.
Rather than mapping solid-liquid-gas transitions, this research team wanted to plot the boundaries between different crystalline structures within superionic water - specifically, how the oxygen atoms arrange themselves in various patterns.
As with any phase transition, molecules reorganize themselves into the most thermodynamically efficient configuration as conditions change. This experiment hoped to identify at what conditions superionic water would reshuffle itself from a body-centered cubic (BCC) packing structure - where the oxygen molecules are packed with 68% efficiency - to even more tightly stuffed structures like face-centered cubic (FCC) or hexagonal close-packed (HCP), each with a 74% efficiency.
With dozens of competing theoretical models and some conflicting results from earlier experiments, the team hoped that recent advances in experimental technologies would allow them to draw clean boundaries between these packing structures.
First, the team had to design an experiment that allowed them to create and study superionic water in a lab. Not only did they have to drive water to these extreme pressures and temperatures, they also had to do so in a vacuum and take measurements incredibly quickly, before everything began falling apart.
Researchers used the Matter in Extreme Conditions instrument at SLAC, paired with ultrafast X-rays from the Linac Coherent Light Source, an X-ray free-electron laser. They then replicated the experiment at the European XFEL using the High Energy Density scientific instrument. Each experiment used laser-driven shocks that reverberated through water samples, creating progressively higher pressures while powerful lasers were used to create the temperature conditions found inside these planets; this approach allowed researchers to replicate the conditions found deep within ice giant planets. Using X-ray diffraction, the team traced the material's crystalline packing structures across ranges of temperature, pressure and time.
Achieving superionic conditions and collecting high-resolution diffraction data were key to this experiment. Together, they allowed the researchers to resolve subtle details at the origin of a rich and complex behavior.
When the data arrived, researchers were surprised to see clear signatures of different packing structures existing simultaneously under uniform conditions. Under certain conditions, they frequently observed the coexistence of BCC and FCC structures. Under other conditions, they observed an even more peculiar multiple packing of FCC and HCP structures. Unlike the clear phase transitions observed in virtually all other known materials, superionic water appeared to exhibit a mix of packing structures throughout the compound at once.
After the first experiment at LCLS, the team was so puzzled by the results that they replicated the study at another XFEL to exclude any potential error sources. The follow-up experiments at the European XFEL showed the very same results.
It is unclear why superionic water exhibits this strange characteristic, but because the rigid oxygen lattice and free-flowing hydrogen atoms give the material its conductive property, the details of these packing structures have profound implications at planetary, and even cosmic, scales.
Though ice giants make up the minority of the planets in our solar system - just two of eight - they represent a significant fraction of planets in the observable universe. Their magnetic fields - which arise from minuscule details of the planet's makeup - can tell scientists a lot about their formation and evolution, and the evolution of the universe.
The team will incorporate these findings into computer simulations to better understand the phenomenon. In the future, researchers want to probe the electrical conductivity of superionic water's packing structures more directly and explore how different chemical compositions, such as water mixed with other compounds likely present in planetary interiors, might affect the material's behavior.
This work was funded in part by the U.S. Department of Energy Office of Science, Fusion Energy Sciences and Basic Energy Sciences. LCLS is a DOE Office of Science user facility.
Research Report:Observation of a mixed close-packed structure in superionic water
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