#superionic

Superionic compound with liquid-like dynamics shows promise as solid-state battery electrolyte

Superionic materials are a class of materials that simultaneously present properties that are characteristic of solids and liquids. Essentially, a set of ions in these materials exhibits liquid-like mobility, even if the materials' underlying atomic structure maintains a solid-like order. Due to their unique ionic conductivity patterns, superionic materials could be promising for developing solid-state batteries. These are batteries that contain electrolytes based on solid materials instead of liquid electrolytes. While various past studies have explored the potential of superionic materials as solid-state electrolytes, the physics underpinning their rapid ionic diffusion is not yet fully understood. Specifically, it is unclear whether this property results from liquid-like motion in the material or from the conventional lattice phonons (i.e., atom vibrations) in the material.

Comparing the Gas Giants Our solar system has 4 large planets made up mostly of gas: Jupiter, Saturn, Uranus, and Neptune. Each of them have some things in common; large numbers of moons, rings present, gaseous outer layers in their atmosphere, and it is thought each likely has some sort of rocky core around which the gases originally were collected as the planets formed.

Study unveils new superionic states of helium-water compounds

Helium and water are known to be abundant throughout the universe, particularly in giant planets such as Uranus and Neptune. Although helium is typically unreactive at common atmospheric conditions, past studies have found that it can sometimes react with other elements and compounds under high pressure.

Researchers at Nanjing University and the University of Cambridge have recently carried out a study investigating the reaction between helium and water under high pressure conditions such as those on other planets. In their study, featured in Nature Physics, they unveiled two previously unknown types of superionic states, which they refer to as SI-I and SI-II. Superionic states are essentially phases of matter in which a compound can simultaneously exhibit both some properties of a liquid and a solid.

"Helium is the most inert element in the periodic table and generally considered to be unreactive under ambient conditions," Jian Sun, one of the researchers who carried out the study, told Phys.org. "However, helium has been found react with some elements and compounds at high pressure. We wanted to understand whether helium and water can react with each other under high pressure and the nature of the states that may emerge under planetary conditions."

First experimental evidence for superionic ice

Among the many discoveries on matter at high pressure that garnered him the Nobel Prize in 1946, scientist Percy Bridgman discovered five different crystalline forms of water ice, ushering in more than 100 years of research into how ice behaves under extreme conditions.

One of the most intriguing properties of water is that it may become superionic when heated to several thousand degrees at high pressure, similar to the conditions inside giant planets like Uranus and Neptune. This exotic state of water is characterized by liquid-like hydrogen ions moving within a solid lattice of oxygen.

Since this was first predicted in 1988, many research groups in the field have confirmed and refined numerical simulations, while others used static compression techniques to explore the phase diagram of water at high pressure. While indirect signatures were observed, no research group has been able to identify experimental evidence for superionic water ice -- until now.

In a paper published today in Nature Physics, a research team from Lawrence Livermore National Laboratory (LLNL), the University of California, Berkeley and the University of Rochester provides experimental evidence for superionic conduction in water ice at planetary interior conditions, verifying the 30-year-old prediction.