An international team of more than 60 scientists reports the successful creation of an extreme form of matter known as superionic water, thereby solving the mystery of this unusual state’s internal structure.
Part of a joint initiative between the German Research Foundation (DFG) and the French research funding agency ANR, the new research could help scientists understand the evolution of ice giant planets like Neptune and Uranus that are common throughout the galaxy, and may mean that superionic water—which normally exists at the heart of massive planets—is the most abundant forms of water by mass within the solar system.
Exploring the Mysterious Structure of Superionic Water
According to a statement announcing the breakthrough, superionic water is an unusual state of water where oxygen atoms form a solid lattice, but hydrogen atoms can move freely throughout the structure. The team’s statement also notes that this state of water only forms under “extreme conditions,” including temperatures of several thousand degrees Celsius and pressures on the order of a million atmospheres.
While the new study was not the first to create superionic water using high-temperature, high-pressure facilities, those previous efforts were unable to fully characterize the material’s internal structure.
For example, previous studies suggested that the oxygen atoms in superionic ice were arranged in one of two variants of a cubic lattice. The first theory suggested the oxygen atoms formed a body-centered cubic structure where an additional atom rests at the cube’s center. In the second proposal, the atoms formed a face-centered cubic structure, with an additional oxygen atom on each cube face.
The new study has finally answered this question, with a solution that combines aspects of both proposals.
High-Pressure, High-Temperature Experiments Reveal Hybrid Structure
To conduct the experiments needed to create and characterize superionic water’s internal structure, the research team utilized two primary facilities: the Matter in Extreme Conditions (MEC) instrument at Linac Coherent Light Source (LCLS) on the campus of Stanford University in the US, and the High Energy Density (HED-HIBEF) instrument at European XFEL. Access to both facilities was critical, the researchers note, as they can compress ordinary water to more than 1.5 million atmospheres and heat it to several thousands of degrees Celsius.
Critically, the facilities allowed the team to record the atomic structure of their manufactured superionic water “within trillionths of a second” even under these extreme conditions.
When the researchers examined the results, they were surprised to find that neither of the original theories was true. Instead, they found that the atoms in superionic water form a structure combining both face-centered cubic and hexagonal close-packed stacking.
“Instead of arranging themselves in a single regular configuration, the oxygen atoms form a hybrid, misstructured sequence – a pattern that can only be made visible by high-precision measurements using state-of-the-art X-ray lasers,” they explained.
The Most Common Form of Water in Our Solar System?
When discussing the implications of their findings, the research team said the results confirmed advanced simulations suggesting that superionic water can exhibit a structural diversity “similar to” that of solid ice, which forms a variety of crystal structures depending on surrounding pressure and temperature.
“The work underscores that water—despite its apparent simplicity—continually reveals new and remarkable properties under extreme conditions,” they explain.
Because most planetary formation models predict the presence of superionic water at the center of ice giants, the researchers said understanding its internal structure should offer “valuable constraints” to the formation models of these planets, which are also “very common outside our solar system.”
For example, researchers believe superionic weather inside ice giants could account for their “unusual” magnetic fields. If correct, they suggest the amount of water inside Neptune could make superionic water “the most common form of water in our solar system.”
The study “Observation of a mixed close-packed structure in superionic water” was published in Nature Communications.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.