研究者は無秩序な化合物に隠された化学秩序を発見

Ba の完全な結晶構造₇Nb₄モオ₂₀. クレジット：東京工業大学 八島正智教授

材料分析の新しい技術は、共鳴 X 線回折と固体核磁気共鳴を組み合わせたものです。

東工大の研究者は、無秩序な化合物 Ba の Mo と Nb 原子のこれまで知られていなかった化学的順序を明らかにしました₇Nb₄モオ₂₀. この成果は、共鳴X線回折や固体核磁気共鳴などの高度な技術を利用することによって可能になりました。 この研究の発見は、材料の隠れた化学的秩序がイオン伝導などの特性に与える影響を強調しています。 これらの結果は、材料科学と工学の分野で大きな進歩をもたらすことが期待されています。

結晶性固体の正確な構造を明らかにすることは困難な作業です。 イオン伝導性や化学的安定性などの材料の特性は、化学的 (職業的) 秩序と無秩序によって大きく影響を受けます。 しかし、科学者が未知の結晶構造を解明するために通常使用する技術には、深刻な制限があります。

たとえば、X 線および中性子回折法は、結晶格子内の原子の位置と配置を明らかにする強力な手法です。 ただし、異なる原子を区別するのに十分ではない場合があります[{” attribute=””>species with similar X-ray scattering factors and similar neutron scattering lengths.

Infographic describing the research. Credit: Professor Masatomo Yashima of Tokyo Institute of Technology

To tackle this issue, a research team led by Professor Masatomo Yashima of the Tokyo Institute of Technology (Tokyo Tech) in Japan sought to develop a novel and more powerful approach to analyze the order and disorder in crystals. They combined four different techniques to analyze the crystal structure of an important ionic conductor, Ba₇Nb₄MoO₂₀. “We chose Ba₇Nb₄MoO₂₀ as Ba₇Nb₄MoO₂₀-based oxides and related compounds are a class of emerging materials with interesting properties such as high ionic conduction and high chemical stability,” explains Prof. Yashima. “However, given that both the Mo⁶⁺ and Nb⁵⁺ cations have similar scattering powers, all structural analyses of Ba₇Nb₄MoO₂₀ until now have been performed assuming complete Mo/Nb disorder.”

As described in their recent paper published in Nature Communications, the researchers used an approach that combined two experimental techniques, resonant X-ray diffraction (RXRD) and solid-state nuclear magnetic resonance (NMR) aided by computational calculations based on density functional theory (DFT). The NMR provided direct experimental evidence that the Mo atoms occupy only the crystallographic M2 site in Ba₇Nb₄MoO₂₀, indicating the chemical order of Mo atoms.

Next, the researchers used RXRD to quantify the occupancy factors of Mo and Nb atoms. They found that the occupancy factor of Mo atoms was 0.5 at the M2 site but zero at all other sites. Interestingly, the M2 site is close to the oxide-ion-conducting, oxygen-deficient layer of Ba₇Nb₄MoO₂₀. This suggests that the Mo atoms at the M2 site have a key role in the high ion conduction of Ba₇Nb₄MoO₂₀. Furthermore, DFT calculations indicated that the Mo ordering stabilizes Mo excess composition exhibiting high ionic conductivity. Positions, occupancy, and atomic displacements of protons and oxide ions were also determined by neutron diffraction.

“Our results demonstrate that the Mo order affects the material properties of Ba₇Nb₄MoO₂₀,” highlights Prof. Yashima. “In this regard, our work represents a major advance in our understanding of the correlation between the crystal structure and the material properties of ionic conductors.” Further, in contrast to single-crystal X-ray and neutron diffraction, the proposed approach can even be extended to other polycrystalline and powdered samples.

Overall, the methodology presented in this study can open up new avenues for an in-depth analysis of chemical order/disorder in materials. In turn, this could lead to the development of physics, chemistry, and materials science and technology.

Only time will tell what other hidden orders and disorders we will stumble upon!

Reference: “Hidden chemical order in disordered Ba7Nb4MoO20 revealed by resonant X-ray diffraction and solid-state NMR” by Yuta Yasui, Masataka Tansho, Kotaro Fujii, Yuichi Sakuda, Atsushi Goto, Shinobu Ohki, Yuuki Mogami, Takahiro Iijima, Shintaro Kobayashi, Shogo Kawaguchi, Keiichi Osaka, Kazutaka Ikeda, Toshiya Otomo, and Masatomo Yashima, 24 April 2023, Nature Communications.
DOI: 10.1038/s41467-023-37802-4