新しい発見が結晶に対する私たちの理解を変える

新しい研究によると、半導体や太陽電池パネルなどの材料科学技術にとって重要な結晶構造は、必ずしも規則正しく配置されているわけではないことが判明した。 彼らは、これまで遷移状態と考えられていた六角形層のランダムな積層（RHCP）が安定である可能性が高く、高電圧エレクトロニクスや防弾チョッキに使用される炭化ケイ素などの複数種の材料に新たな有用な特性を提供できる可能性があることを発見しました。

不規則に配置された構造の発見により、これまでの考えは覆されました。

多くの人にとって、「クリスタル」という言葉は、虹色のプリズムを生み出す輝く太陽のイメージや、治癒力があると信じられている半透明の石を思い浮かべます。 しかし、科学や工学では、結晶はより技術的な定義を持ちます。 それらは、原子であれ、分子であれ、ナノ粒子であれ、その構成要素が空間内に規則的に配置された物質として見なされます。 言い換えれば、結晶はその成分の規則的な配置によって識別されます。 身近な例としては、ダイヤモンド、食卓塩、角砂糖などが挙げられます。

イ・サンウさん。 クレジット: レンセラー工科大学

この広く受け入れられている定義に反して、レンセラー工科大学化学生物工学部の助教授であるサンウー・リー氏が主導した最近の研究では、結晶構造の興味深い側面が明らかになり、結晶内の成分の配置はそうではないことが明らかになりました。 常に必然的に規則的です。

この発見は材料科学の分野を進歩させ、そこで使用される材料に未実現の影響を与えます[{” attribute=””>semiconductors, solar panels, and electric vehicle technologies.

One of the most common and important classes of crystal structures is the close-packed structures of regular spheres constructed by stacking layers of spheres in a honeycomb arrangement. There are many ways to stack the layers to construct close-packed structures, and how nature selects specific stacking is an important question in materials and physics research. In the close-packing construction, there is a very unusual structure with irregularly spaced constituents known as the random stacking of two-dimensional hexagonal layers (RHCP). This structure was first observed from cobalt metal in 1942, but it has been regarded as a transitional and energetically unpreferred state.

Lee’s research group collected X-ray scattering data from soft model nanoparticles made of polymers and realized that the scattering data contains important results about RHCP but is very complicated. Then, Patrick Underhill, professor in Rensselaer’s Department of Chemical and Biological Engineering, enabled the analysis of the scattering data using the supercomputer system, Artificial Intelligence Multiprocessing Optimized System (AiMOS), at the Center for Computational Innovations.

“What we found is that the RHCP structure is, very likely, a stable structure, and this is the reason that RHCP has been widely observed in many materials and naturally occurring crystal systems,” said Lee. “This finding challenges the classical definition of crystals.”

The study provides insights into the phenomenon known as polytypism, which enables the formation of RHCP and other close-packed structures. A representative material with polytypism is silicon carbide, widely used for high-voltage electronics in electric vehicles and as hard materials for body armor. Lee’s team’s findings indicate that those polytypic materials may have continuous structural transitions, including the non-classical random arrangements with new useful properties.

“The problem of how soft particles pack seems straightforward, but even the most basic questions are challenging to answer,” said Kevin Dorfman of the University of Minnesota-Twin Cities, who is unaffiliated with this research. “This paper provides compelling evidence for a continuous transition between face-centered cubic (FCC) and hexagonal close-packed (HCP) lattices, which implies a stable random hexagonal close-packed phase between them and, thus, makes an important breakthrough in materials science.”

“I am particularly pleased with this discovery, which shows the power of advanced computation to make an important breakthrough in materials science by decoding the molecular level structures in soft materials,” said Shekhar Garde, dean of Rensselaer’s School of Engineering. “Lee and Underhill’s work at Rensselaer also promises to open up opportunities for many technological applications for these new materials.”

Reference: “Continuous transition of colloidal crystals through stable random orders” by Juhong Ahn, Liwen Chen, Patrick T. Underhill, Guillaume Freychet, Mikhail Zhernenkovc and Sangwoo Lee, 14 April 2023, Soft Matter.
DOI: 10.1039/D3SM00199G

Lee and Underhill were joined in research by Rensselaer’s Juhong Ahn, Liwen Chen of the University of Shanghai for Science and Technology, and Guillaume Freychet and Mikhail Zhernenkov of Brookhaven National Laboratory.