A thin film crystal has been discovered that can repeatedly release and reabsorb oxygen at relatively low temperatures while keeping its structure intact.
The study points to practical use in clean energy and electronics, where controlled oxygen movement changes how a material handles heat, light, and charge.
The material is a perovskite derived oxide of strontium, iron, and cobalt, a family whose lattice tolerates missing oxygen atoms known as oxygen vacancies.
Only cobalt changed its valence state during reduction, with the cobalt absorption edge shifting by 1.65 eV and the average valence moving from about 2.91 to 2.00, and the process reversed cleanly during reoxidation.
How the crystal breathes oxygen
The study was led by Professor Hyoungjeen Jeen at Pusan National University and co-authored by Professor Hiromichi Ohta at Hokkaido University.
Their thin films cycled between oxygen poor and oxygen replenished states under forming gas and oxygen without crumbling.
These changes rely on oxygen vacancies, small gaps in the lattice that tune valence, structure, and function across transition metal oxides.
In this system, vacancies prefer sites near cobalt and form under mild conditions, while iron stays largely inert, which helps the lattice resist collapse.
The team used X-ray absorption spectroscopy to track element specific changes, a technique that probes local electronic states by watching how core electrons absorb X-rays.
The spectra confirmed cobalt reduction and the growth of an oxygen deficient but stable defective perovskite rather than a vacancy ordered phase.
“It is like giving the crystal lungs and it can inhale and exhale oxygen on command,” said Professor Jeen. The films showed synchronized structural and transport changes during each oxygen cycle.
Why oxygen control in the film matters
In solid oxide fuel cells, oxygen movement through a ceramic electrolyte underpins efficient conversion of fuel to electricity with low on site emissions.
Materials that shift oxygen content at modest temperatures can reduce energy costs and simplify system design.
Engineers are also developing thermal transistors, three terminal devices where a control input modulates heat flow, and recent prototypes demonstrate gate controlled heat currents with large on off ratios.
Oxygen driven changes in bonding and lattice spacing add another practical handle for thermal switching.
For buildings, electrochromic smart windows change their light transmission with a small voltage, improving comfort and trimming cooling loads.
The studied films became more transparent after reduction, so oxygen tuning could support windows that adjust both heat and light.
From dark film to clear film
Optical measurements showed the bandgap widen from 2.47 eV to 3.04 eV as the film reduced, matching a visible jump in transparency.
That shift tracked the suppressed cobalt oxygen hybridization seen in spectroscopy and returned when oxygen was added back.
Greater transparency came with higher electrical resistance, a tradeoff that appears when oxygen leaves a mixed cobalt iron oxide.
The reversible swing between a clearer, more insulating state and a darker, more conductive state suggests modulators that tune both light and charge.
Importantly, the oxygen cycling preserved the interface with the substrate and the surface step terrace morphology across multiple runs.
The film thickness even increased slightly upon reduction, matching out of plane lattice expansion, and the new phase held steady during long anneals near 752 ºF.
What makes the oxygen film different
Prior oxide films toggled between brownmillerite and perovskite at 392 to 572 ºF with fast redox switching, but both cations usually participated or the lattice degraded under stress.
Here, cobalt carries the redox load while iron remains stable, creating an oxygen deficient yet robust structure that resists collapse.
Brownmillerite is a perovskite related framework with ordered oxygen vacancy channels that enable fast, anisotropic oxygen transport, and it often acts as a waypoint during oxygen insertion and removal in oxides.
The reduced phase reported here did not show the long range vacancy order of classic brownmillerites, pointing instead to a disordered defective perovskite.
“This finding is striking in two ways: only cobalt ions are reduced, and the process leads to the formation of an entirely new but stable crystal structure,” explained Professor Jeen.
That selectivity expands options for multi cation oxides where one element shuttles oxygen while another anchors the lattice.
Where this could go next
The films switched among three distinct states under controlled gases, and redox cycles repeated several times without structural degradation.
There is still a thermal ceiling near 932 ºF where the reduced phase becomes unstable, which sets a practical limit for devices.
Scaling will mean thicker films or bulk materials, stable switching in ambient air, and interfaces that tolerate repeated oxygen exchange.
Those steps will require materials engineering, yet the chemistry is compatible with standard oxide processing.
Looking ahead, oxygen tuned cobaltites could pair with resistive memory or neuromorphic circuits where redox states encode information, a line already established for valence change memristors.
The material here adds a clean, reversible oxygen switch that also modulates optics, which is rare in a single platform.
“This is a major step toward the realization of smart materials that can adjust themselves in real time,” said Professor Ohta. He pointed to clean energy, electronics, and building technologies as likely early adopters.
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Image: Schematic illustration of the oxygen-breathing in the new crystal. The scientists have developed a special type of crystal with oxygen-breathing abilities, which could be used in clean energy technologies and next-generation electronics. Credit: Professor Hyoungjeen Jeen/Pusan National University
The study is published in Nature Communications.
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