Novel Sillén oxyhalides, exhibit high oxide-ion conductivity at low temperatures, chemical stability, and negligible chemical expansion. One of the materials achieves 0.01 S cm⁻¹ bulk conductivity at 233 °C—within the “Norby gap.” The discovery reveals an unexpected vacancy-mediated mechanism for oxide-ion transport despite excess oxygen, introducing a new materials-design concept for fast oxide-ion conductors and low-temperature solid oxide fuel cells. The present findings may open a new research field, “Oxide Ionics.”
Solid-oxide fuel cells (SOFCs), which convert chemical energy directly into electrical power, are promising energy-conversion technologies because they can operate on various fuels, including hydrogen, natural gas, and biogas. When hydrogen is used as the fuel, water is the main product of the electrochemical reaction. However, conventional SOFCs generally operate at high temperatures, creating challenges related to material degradation, manufacturing costs, and compatibility between cell components.
Commercially available SOFC electrolytes, such as yttria-stabilized zirconia (YSZ), typically require high operating temperatures of around 700°C–1,000 °C to maintain high ionic conductivity. However, such high temperatures can accelerate material degradation and increase costs. Considerable efforts have therefore been devoted to developing electrolyte materials that can operate at lower temperatures of 200 °C–500 °C. Yet, materials that simultaneously combine sufficient oxide-ion conductivity, chemical stability, and negligible chemical expansion are very rare in this temperature range. This gap between 200 and 500 °C, called the “Norby gap,” represents an important challenge not only for SOFCs, but also for various chemical and energy-conversion processes.
In a breakthrough, a research team led by Professor Masatomo Yashima of the Department of Chemistry, School of Science at Institute of Science Tokyo (Science Tokyo) in Japan, has discovered a novel Sillén oxybromide, Bi 1.9 Te 0.1 LaO 4.05 Br, that exhibits exceptional bulk oxide-ion conductivity within the Norby gap. “This oxide-ion conductor not only demonstrates significantly higher ion conductivity than leading proton conductors but also exhibits negligible chemical expansion and high chemical stability. We believe these findings could lay the foundation for a new research field, ‘Oxide Ionics.’” says Yashima.
The team included graduate students Yusuke Kanenori and Hidetoshi Udagawa, Assistant Professors Kei Saito, Yuichi Sakuda, and Kotaro Fujii from Science Tokyo; Professor Tatsumi Ishihara and Dr. Maksymilian Kluczny from Kyushu University, Japan; Professor Stephen Skinner and Dr. Mudasir A. Yatoo from Imperial College London, UK; as well as Professor Kazuhiro Mori from the High Energy Accelerator Research Organization (KEK) and J-PARC, Japan. Their study will be published in Nature Communications on October 14, 2026.
In a previous work, the present researchers reported high oxide-ion conduction in a related Sillén oxychloride, Bi 1.9 Te 0.1 LaO 4.05 Cl, but its conductivity was lower than the Norby gap. In the present work, they replaced the chloride ion Cl – of Bi 1.9 Te 0.1 LaO 4.05 Cl with a larger and more polarizable bromide ion, Br – , and synthesized the Bi 1.9 Te 0.1 LaO 4.05 Br sample and investigated its structural and transport properties.
Notably, Bi 1.9 Te 0.1 LaO 4.05 Br exhibits a high bulk oxide-ion conductivity of 0.01 S cm⁻¹ at 233 °C within the Norby gap, far surpassing that of representative electrolytes such as YSZ and lanthanum strontium gallium magnesium oxide (LSGM), which require temperatures of 644 and 534 °C, respectively, to achieve the same conductivity. Its oxide-ion conductivity also exceeds even the proton conductivity reported for several representative proton-conducting ceramics. Moreover, it exhibits high chemical stability in various atmospheres, including dry and wet oxidizing, reducing, and CO 2 atmospheres, at temperatures up to 300 °C. It also exhibits negligible chemical expansion.
Their crystal structure analysis shows a Sillén-type structure, with highly conductive triple fluorite-like slabs sandwiched by Br layers, providing a distinctive structural environment for fast oxide-ion transport. This sandwich structure provides a new material-design principle for high oxide-ion conduction.
The most surprising finding concerns how the oxide ions move. Remarkably, despite the excess oxygen of 0.05 in Bi 1.9 Te 0.1 LaO 4.05 Br, both oxygen vacancies and interstitial oxide ions exist at the lattice oxygen sites and interstitial oxygen sites, respectively. Through diffraction studies, first-principles calculations and ab initio molecular dynamics simulations, the team uncovered that the oxide-ion transport in this material proceeds through a vacancy-mediated mechanism. Namely, oxide ions migrate from a lattice site to a neighboring vacant lattice site through vacancies despite the excess oxygen atoms and interstitial oxide ions. This finding challenges the conventional expectation that excess-oxygen oxide-ion conductors should predominantly transport oxide ions through an interstitialcy diffusion mechanism.
“Our findings demonstrate that strategically engineered oxygen vacancies enable ultrafast oxide-ion transport even in materials containing excess oxygen atoms,” remarks Yashima. “This overturns a widely accepted picture of how oxide ions should move in excess-oxygen materials and opens a new direction for designing fast oxide-ion conductors.”
The team also synthesized other Sillén oxybromides and iodides that demonstrated high electrical conductivity, suggesting that this family of oxyhalides can offer a promising platform for fast oxide-ion conductors.
The researchers note that the total conductivity of the present material is lower than its bulk conductivity because of grain-boundary resistance. Further optimization of the material's microstructure, including grain growth and densification, will therefore be important for translating its exceptional bulk conductivity into practical electrolyte performance.
Looking ahead, Yashima says, “This discovery establishes a new foundation for designing novel oxide-ion conductors based on vacancy-mediated transport in materials with excess oxygen atoms. By sandwiching highly oxide-ion conductive triple fluorite-like slabs by Br layers, we may be able to develop a broader family of novel oxide-ion conductors for low-temperature SOFCs, solid oxide electrolysis cells, and other technologies.”
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About Institute of Science Tokyo (Science Tokyo)
Institute of Science Tokyo (Science Tokyo) was established on October 1, 2024, following the merger between Tokyo Medical and Dental University (TMDU) and Tokyo Institute of Technology (Tokyo Tech), with the mission of “Advancing science and human wellbeing to create value for and with society.”
Nature Communications
Experimental study
Not applicable
Vacancy-mediated ultrafast oxide-ion conduction in excess-oxygen Sillén oxybromides
14-Oct-2026
The authors declare no competing interests.