構造常識を覆すトポケミカル反応の発見～カゴメ格子を持つ新しい2次元量子物質の創製に成功～

2025-07-25 京都大学,ファインセラミックスセンター,東北大学,科学技術振興機構

本研究の発見：四面体構造の 2 層が八面体の 1 層へと変換される新しいトポケミカル反応

＜関連情報＞

• https://www.jst.go.jp/pr/announce/20250725-2/index.html
• https://www.jst.go.jp/pr/announce/20250725-2/pdf/20250725-2.pdf
• https://pubs.acs.org/doi/10.1021/jacs.5c05749

二重層から単一層への変換を伴うトポ化学反応：カゴメ格子を有するMo3Ta2O10N Topochemical Reaction Involving Double-to-Single Layer Conversion: Mo3Ta2O10N with a Kagomé Lattice

Ryoya Higuchi,Kohdai Ishida,Cédric Tassel,Baptiste Vignolle,Daichi Kato,Kantaro Murayama,Hsin-Hui Huang,Akihide Kuwabara,Shunsuke Kobayashi,Yusuke Nambu,Hiroyasu Matsudaira,Shunsaku Kitagawa,Kenji Ishida,Congling Yin,Hiroshi Takatsu,and Hiroshi Kageyama
Journal of the American Chemical Society  Published: July 24, 2025
DOI:https://doi.org/10.1021/jacs.5c05749

Abstract

Topochemical reactions in transition metal oxides, typically involving oxygen removal or anion exchange, provide a versatile platform for creating metastable phases with diverse functionalities. While these reactions often modify the valence and coordination environment of transition metals, the underlying metal frameworks are usually preserved, maintaining a 1:1 structural correspondence. A representative example is SrFeO₃ → SrFeO₂, in which each octahedral layer transforms into a single square-planar layer. In this study, we report an unprecedented topochemical transformation in Mo₂Ta₂O₁₁, composed of alternating MoO₄ tetrahedral bilayers and TaO₆ octahedral bilayers. Ammonolysis at 500 °C in the presence of Mo(CO)₆ collapses the MoO₄ tetrahedral bilayer into a single MoO₆ octahedral layer, thereby breaking the conventional 1:1 structural correspondence. This collapse leads to an 18% contraction along the c axis, substantially exceeding the 10% contraction seen in SrFeO₂. The resulting compound, Mo₃Ta₂O₁₀N (space group R–3m), has an Mo-based kagomé lattice with finite d-electrons (Mo^4.33+), in sharp contrast to the d⁰ configuration of the precursor (Mo⁶⁺₂Ta⁵⁺₂O₁₁). Magnetic susceptibility and NMR measurements suggest that Mo₃Ta₂O₁₀N is an itinerant kagomé system. This study demonstrates that topochemical reactions can induce dynamic and extensive structural reorganizations, pushing the boundaries of what was previously considered accessible by such low-temperature routes.