Evidence of a spin-liquid state emerges in pressurized oxygen
Ingrid Fadelli
contributing writer
Sadie Harley
scientific editor
Robert Egan
associate editor
Oxygen, the colorless and odorless gas that is essential to the survival of humans and other living organisms, is estimated to make up around 21% of Earth's atmosphere. While the primary properties of oxygen are now well understood, the states that can emerge in it at extreme conditions (e.g., at high pressures) are still under investigation.
Researchers at Shanghai Advanced Research in Â鶹ÒùÔºical Sciences (SHARPS), the Center for High Pressure Science and Technology Advanced Research in China, the Italian National Institute of Optics of the National Council of Research (CNR-INO), the European Synchrotron Radiation Facility and University Montpellier carried out a study exploring the properties of a high-pressure phase of solid oxygen, known as epsilon oxygen (ε-O2).
Their paper, in Â鶹ÒùÔºical Review Letters, offers the first indirect evidence that a dynamic magnetic state, known as a spin-liquid state, emerges in epsilon oxygen.
"Oxygen, as one of the most common and important elements in nature, seems ordinary but solid oxygen is actually a very unique crystal," Federico Aiace Gorelli, co-first author of the paper, told Â鶹ÒùÔº.
"Among many simple diatomic molecule systems, oxygen is the only one with magnetic properties, and solid oxygen is the only elemental solid that is an antiferromagnetic insulator at low temperatures. Our investigation was inspired by an important theoretical study suggesting that pressure can cause the magnetic properties of oxygen molecules to disappear in the epsilon phase (10 GPa
As part of their study, Gorelli and his colleagues closely examined an ε-O2 sample using single crystal X-ray diffraction. This is a technique that can help to uncover atomic arrangements in crystals, by observing how X-rays scatter through them.
"Our objective was to use single crystal synchrotron X-ray diffraction on unprecedented quality epsilon oxygen crystals in order to reveal possible subtle changes able to experimentally verify this theoretical prediction," explained Philip Dalladay-Simpson, co-first author of the paper.
"Indeed, we observed small but substantial discontinuities in the structural parameters at 18.1 GPa, which can be reliably assigned to the spin collapse from a magnetic spin-liquid state to a non-magnetic state."
The experiments carried out by Gorelli, Dalladay-Simpson and their colleagues led to the observation of an isostructural phase transition in ε-O2. This essentially means that the overall symmetry of the high-quality pressurized oxygen crystals they examined remained the same, while lattice constants (i.e., repeating distances between atoms in the crystal) and the spacing within clusters of four oxygen molecules known as (O2)4 quartets changed.
Notably, this phase transition was found to coincide with a collapse in the sample's molecular magnetic moment. This indicates that the epsilon oxygen has shifted from a spin-liquid state to a spinless state.
"The most important achievement is having identified, even if indirectly, a spin liquid-state, which turns out to be an exotic state of matter," explained Gorelli.
"As a matter of fact, spin liquids are well known and popular in spin 1/2 lattices, while the S=1 'nutshell spin liquid' inside the (O2)4 quartets of the epsilon phase seems to be unique in solid state physics. One of the implications of this work is thus to have expanded to a new case the phenomenology of the spin liquid state."
The efforts by Gorelli, Dalladay-Simpson and their colleagues led to the first indirect evidence that a spin liquid state exists in highly pressurized oxygen, thus highlighting its potential as an experimental platform for studying unconventional quantum states.
In the future, others could build on this evidence and set out to further explore the state reported by the researchers.
"We would now like to move in the future to investigate the spin-liquid state of matter under extreme conditions from direct measurements of the magnetic responsivity, which is a very challenging goal," added Mario Santoro, senior author of the paper.
Written for you by our author , edited by , and fact-checked and reviewed by —this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a (especially monthly). You'll get an ad-free account as a thank-you.
More information: Federico Aiace Gorelli et al, Structural Evidence for the Spin Collapse in High Pressure Solid Oxygen, Â鶹ÒùÔºical Review Letters (2025). .
Journal information: Â鶹ÒùÔºical Review Letters
© 2025 Science X Network
