Kyoto University reconstructs crystal framework of inorganic oxides: Applications expected in quantum devices and other areas

A research group from Kyoto University and other institutions has succeeded, to an unprecedented extent, in significantly reconstructing the crystal framework of hard inorganic oxides formed by high-temperature firing using "topochemical reactions" that occur at relatively low temperatures. The team caused the reaction by treating molybdenum and tantalum oxides with ammonia. The resulting oxynitrides were confirmed to have conductivity that the original oxides did not possess. This development is expected to enable applications in new quantum devices and other areas.

A topochemical reaction is a method that selectively removes and introduces specific ions while maintaining the crystal framework. Research on using this reaction to create new materials based on inorganic crystalline materials synthesized at high temperatures of 1000 to 2000 degrees Celsius is, like ceramics, advancing. One-to-one changes where each polyhedron becomes planar have been discovered, but it was believed that reconstruction of crystal frameworks that change the number or arrangement of metal sites was not possible.

Professor Hiroshi Kageyama, who studies solid state chemistry at Graduate School of Engineering of Kyoto University, and his colleagues focused on molybdenum and tantalum oxides with layered structures. While researching whether nitridation could be achieved using the characteristic feature of having two tetrahedral structures, they discovered that when molybdenum hexacarbonyl [Mo(CO)₆] was added to ammonia gas and heated to 500 degrees Celsius, the two-layer tetrahedral structure became compressed into a single layer of octahedral structures. Electron microscopy confirmed that the layer spacing was reduced by about 20%.

When the arrangement of the octahedra that changed from two layers to one layer was examined, there was a crystal lattice with regularly arranged hexagons and triangles like those seen in bamboo basket weaving patterns, taking on a structure called a "kagome lattice." The kagome lattice is expected to be a new functional material because it has the potential to exhibit electrical and magnetic properties different from ordinary metals. The oxynitrides generated were also confirmed to have conductivity.

Kageyama said, "Solid materials are far more flexible than previously thought. This is the discovery of a topochemical reaction that brings about more dramatic structural changes than those creating ceramics and other oxides would expect." He noted that it has the potential to become a fundamental technology for designing next-generation quantum devices and new functional materials.

The research was conducted jointly with the University of Bordeaux in France, the Japan Fine Ceramics Center, Tohoku University, and Guilin University of Technology in China, with support from the Japan Society for the Promotion of Science (JSPS) and the Japan Science and Technology Agency (JST), among others. It was published online in the Journal of the American Chemical Society (JACS), an international journal, on July 24.