Discovery of significant expansion of a dense oxide

The National Institute for Materials Science (NIMS) discovered a thermal response phenomenon in which Ba₄Ru₃O₁₂, a dense, inorganic oxide formed by high-pressure synthesis, increases in volume by about 4.4% when heated, and this expanded state is retained even after cooling. Normally, such a large volume change is accompanied by substantial changes in the crystal structure or chemical reactions and alters the material properties. In this research, however, NIMS elucidated that “giant irreversible expansion” occurs through atomic rearrangement within a crystal, while retaining the basic crystal framework, i.e., the symmetry of the crystal structure, and chemical composition. The research result demonstrates a new thermal response mechanism in oxide materials, and is expected to lead to new design guidelines for ceramic materials used in a high-temperature environment or materials that control thermal deformation and stress.

Background: Difficulty of causing a large thermal response in a dense and stable oxide

In a component made by combining multiple materials, the differences in thermal expansion between the materials cause internal thermal stress, which could lead to cracks, poor bonding, and performance degradation. Therefore, it is important to freely control and fix the volume changes of the materials without significantly changing their properties, and control the internal stress according to use. However, inorganic oxides and ceramics only exhibit small volume changes by heat, and normally revert to their original state after cooling. In addition, when a large volume change occurs, it is often accompanied by substantial changes in the crystal structure or chemical reactions and alters the material properties. Accordingly, there was demand for inorganic materials that exhibit a large irreversible thermal response while retaining the basic crystal framework and chemical composition.

Key Findings: Discovery that an oxide expands significantly through internal atomic rearrangement

NIMS discovered that barium ruthenate (Ba₄Ru₃O₁₂) obtained by high-pressure synthesis increases in volume by about 4.4% when heated, and that the expanded state is retained even after cooling. This is an “irreversible expansion” phenomenon, which differs from normal thermal expansion (Figure).

As a result of analyses including precision structural analysis using synchrotron X-ray diffraction, it was found that this expansion was driven not by decomposition, redox processes, or electronic or magnetic phase transitions, but by the rearrangement of ruthenium (Ru) atoms within the crystal. This result is characterized in that a large, irreversible thermal response occurs while the symmetry of the crystal structure and chemical composition are retained.

Future Outlook

Going forward, NIMS will clarify the conditions under which this phenomenon occurs, and explore methods for controlling the expansion size and the temperature at which the phenomenon occurs. Moreover, by extending this idea to various oxide materials, NIMS aims to develop materials for which volume changes can be designed according to temperature changes.

In the future, this phenomenon is expected to be applied to materials that suppress thermal stress that poses a problem in electronic devices and power devices, thereby enhancing the reliability and lifespan of components.

Other Information

• This project was conducted by a research team consisting of Zhijun Li (Graduate Research Assistant, Quantum Solid State Materials Group (QSSMG), Research Center for Materials Nanoarchitectonics (MANA), NIMS), Hongbo Yuan (Graduate Research Assistant, QSSMG, MANA, NIMS), Alexei A. Belik (Chief Researcher, QSSMG, MANA, NIMS), Yoshihiro Tsujimoto (Principal Researcher, QSSMG, MANA, NIMS), Kazunari Yamaura (Group Leader, QSSMG, MANA, NIMS), and Terumasa Tadano (Group Leader, Spin Theory Group, Research Center for Magnetic and Spintronic Materials, NIMS).
• The work was supported by the Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research (Grant Nos.: JP25K01657, JP25K01507).
• This work was published online in Journal of the American Chemical Society on May 20, 2026.