Kyoto University creates "kagomé lattice," a new structural transformation achieved through topochemical reaction

While typical ceramics are produced by mixing materials and firing at high temperatures, in recent years it has become possible to replace parts of the framework structure through topochemical reactions conducted at low temperatures, enabling the creation of new functionalities. For example, it is now possible to create new magnetic materials that were previously unobtainable, or to convert capacitor materials into ammonia synthesis catalysts.

A research group led by Master's Student Ryoya Higuchi, Kohdai Ishida (doctoral student at the time of the research), Associate Professor Hiroshi Takatsu, and Professor Hiroshi Kageyama from the Graduate School of Engineering at Kyoto University, in collaboration with Kyoto University's Graduate School of Science, the University of Bordeaux (France), Japan Fine Ceramics Center, Tohoku University, and Guilin University of Technology (China), has achieved the world's first topochemical reaction that reconstructs the framework itself. The findings were published in the Journal of the American Chemical Society.

While topochemical reactions change structures within reactive layers, the reaction achieved in this study involves the structural transformation of two layers into one. Specifically, the MoO₄ tetrahedral bilayers in the layered oxide Mo₂Ta₂O₁₁ containing molybdenum and tantalum were converted into MoO₆ octahedra, creating the nitrogen oxide Mo₃Ta₂O₁₀N.

Takatsu explained, "When we were conducting a different experiment, we measured something that was accidentally created and found something strange—it had become a single layer. Electron microscopy and X-ray diffraction experiments revealed a contraction of about 18% along the stacking direction, which is an extremely large change for a topochemical reaction." Higuchi added, "The key to this reaction is the presence of mobile atoms, so it might be possible in other systems as well."

Examination of the in-plane structure of the octahedral layer revealed that it forms a kagomé lattice. With the formation of the kagomé lattice, Mo₂Ta₂O₁₁, which was an insulator, became conductive as Mo₃Ta₂O₁₀N. It exhibits unique electronic structures such as Dirac electron states and flat bands, potentially providing a powerful foundation for next-generation quantum devices and new functional materials.

This structural transformation overturns the conventional understanding of topochemical reactions. It demonstrates the possibility of rationally designing structural transformations that were previously thought to be inaccessible and is expected to become a new method for designing and creating diverse metal lattices.

Journal Information
Publication: Journal of the American Chemical Society
Title: Topochemical Reaction Involving Double-to-Single Layer Conversion: Mo₃Ta₂O₁₀N with a Kagomé Lattice
DOI: 10.1021/jacs.5c05749