Discovering Hidden Order in Disordered Crystals

Researchers at Tokyo Tech have discovered hidden chemical order of the Mo and Nb atoms in disordered Ba₇Nb₄MoO₂₀, by combining state-of-the-art techniques, including resonant X-ray diffraction and solid-state nuclear magnetic resonance. This study provides valuable insights into how a material's properties, such as ionic conduction, can be heavily influenced by its hidden chemical order. These results would stimulate significant advances in materials science and engineering.

Determining the precise structure of a crystalline solid is a challenging endeavor. Materials properties such as ion conduction and chemical stability, are heavily influenced by the chemical (occupational) order and disorder. However, the techniques that scientists typically use to elucidate unknown crystal structures suffer from serious limitations.

For instance, X-ray and neutron diffraction methods are powerful techniques to reveal the atomic positions and arrangement in the crystal lattice. However, they may not be adequate for distinguishing different atomic species with similar X-ray scattering factors and similar neutron scattering lengths.

To tackle this issue, a research team led by Professor Masatomo Yashima of 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 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!