Topochemical Reaction Involving Double-to-Single Layer Conversion: Mo₃Ta₂O₁₀N with a Kagomé Lattice

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 <i>c</i> axis, substantially exceeding the 10% contraction seen in SrFeO₂. The resulting compound, Mo₃Ta₂O₁₀N (space group <i>R</i>-3<i>m</i>), has an Mo-based kagomé lattice with finite <i>d</i>-electrons (Mo^4.33+), in sharp contrast to the <i>d</i>⁰ 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.