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The occupational order and disorder in solid materials have a crucial influence on its properties. There are numerous crystalline materials exhibiting occupational order/disorder of atoms with similar X-ray scattering factors and similar neutron scattering lengths. It is difficult to analyze the occupancies of such atoms in materials by conventional diffraction methods. Herein, a combined technique of resonant X-ray diffraction (RXRD), solid-state nuclear magnetic resonance (NMR), and first-principle calculations is used to determine the Mo/Nb order quantitatively in the remarkable ionic conductor Ba7Nb4MoO20. NMR provided direct experimental evidence that Mo occupy only the M2 site adjacent to the oxygen-deficient ion-conductive layer. Analysis of RXRD data showed the fractional occupancies of Mo atoms at the M2 and other sites to be 0.50 and 0.00, respectively. First-principle calculations helped the assignment of NMR peaks and indicated that the crystal structure with Mo located at the M2 site is the most energetically stable, supporting the experimental results. These findings provide a concrete basis for the development of ion conductors. This combined technique of RXRD and NMR (RXRD/NMR method) would open a new avenue for further investigation of the hidden occupational order/disorder in materials (Yasui et al. in Nat Commun 14:2337, 2023).

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Hidden Occupational Order in Disordered Ba7Nb4MoO20 Revealed by a Combined Technique of Resonant X-ray Diffraction and Solid-State NMR

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  • 发表日期：

    2026年

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  • Yuta Yasui

摘要

The occupational order and disorder in solid materials have a crucial influence on its properties. There are numerous crystalline materials exhibiting occupational order/disorder of atoms with similar X-ray scattering factors and similar neutron scattering lengths. It is difficult to analyze the occupancies of such atoms in materials by conventional diffraction methods. Herein, a combined technique of resonant X-ray diffraction (RXRD), solid-state nuclear magnetic resonance (NMR), and first-principle calculations is used to determine the Mo/Nb order quantitatively in the remarkable ionic conductor Ba7Nb4MoO20. NMR provided direct experimental evidence that Mo occupy only the M2 site adjacent to the oxygen-deficient ion-conductive layer. Analysis of RXRD data showed the fractional occupancies of Mo atoms at the M2 and other sites to be 0.50 and 0.00, respectively. First-principle calculations helped the assignment of NMR peaks and indicated that the crystal structure with Mo located at the M2 site is the most energetically stable, supporting the experimental results. These findings provide a concrete basis for the development of ion conductors. This combined technique of RXRD and NMR (RXRD/NMR method) would open a new avenue for further investigation of the hidden occupational order/disorder in materials (Yasui et al. in Nat Commun 14:2337, 2023).