Revolutionary Discovery Challenges Conventional Understanding of Crystals

In a groundbreaking study, our perception of crystal structures, which play a vital role in materials science and various technologies like semiconductors and solar panels, has been transformed. The study revealed that crystal arrangements are not always uniformly organized, overturning previous beliefs. The researchers discovered that the seemingly random stacking of hexagonal layers, previously regarded as a transitional phase, may actually be stable and offer new properties in polytypic materials such as silicon carbide. This discovery has significant implications for high-voltage electronics and body armor applications.

When most people think of crystals, they envision dazzling suncatchers casting a colorful prism or translucent stones associated with healing properties. However, in the realm of science and engineering, crystals possess a more technical definition. They are materials characterized by the regular arrangement of their atomic, molecular, or nanoparticle components in space. Familiar examples include diamonds, table salt, and sugar cubes.

Contrary to this widely accepted definition, Sangwoo Lee, an associate professor in the Department of Chemical and Biological Engineering at Rensselaer Polytechnic Institute, led a recent study that revealed an intriguing aspect of crystal structures. The research unveiled that the arrangement of components within crystals is not always strictly regular.

This discovery marks a significant advancement in materials science and has far-reaching implications for semiconductors, solar panels, and electric vehicle technologies.

Among the various crystal structures, close-packed structures composed of regularly stacked spheres in a honeycomb arrangement are particularly common and essential. The specific stacking of these layers has been a significant question in materials and physics research. Within this close-packed construction, there exists an unconventional structure with irregularly spaced constituents known as the random stacking of two-dimensional hexagonal layers (RHCP). This structure was initially observed in cobalt metal in 1942 but was considered a transitional and energetically unfavorable state.

Lee’s research team collected X-ray scattering data from soft model nanoparticles made of polymers. They soon realized that the data contained crucial information about RHCP, although it was highly complex. To analyze the scattering data, they employed the supercomputer system called Artificial Intelligence Multiprocessing Optimized System (AiMOS) at the Center for Computational Innovations, under the guidance of Patrick Underhill, a professor in Rensselaer’s Department of Chemical and Biological Engineering.

“Our findings strongly suggest that the RHCP structure is stable, which explains its widespread occurrence in various materials and naturally formed crystal systems,” explained Lee. “This discovery challenges the traditional definition of crystals.”

The study sheds light on polytypism, a phenomenon that facilitates the formation of RHCP and other close-packed structures. Silicon carbide, a representative material with polytypism, finds extensive use in high-voltage electronics for electric vehicles and as a hard material for body armor. Lee’s team’s findings suggest that these polytypic materials may undergo continuous structural transitions, including the emergence of non-classical random arrangements that offer new beneficial properties.

Kevin Dorfman from the University of Minnesota-Twin Cities, who is not associated with this research, remarked, “The problem of how soft particles pack seems straightforward, but even the most basic questions are challenging to answer. 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.”

Shekhar Garde, dean of Rensselaer’s School of Engineering, expressed his satisfaction with this groundbreaking discovery and its potential technological applications. He highlighted the power of advanced computation in decoding molecular-level structures in soft materials.

The research was conducted by Sangwoo Lee and Patrick Underhill from Rensselaer Polytechnic Institute, along with Juhong Ahn and Mikhail Zhernenkov from Rensselaer, Liwen Chen from the University of Shanghai for Science and Technology, and Guillaume Freychet from Brookhaven National Laboratory.

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

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• Rensselaer Polytechnic Institute: Department of Chemical and Biological Engineering
• Soft Matter: Continuous transition of colloidal crystals through stable random orders
• University of Minnesota-Twin Cities: Kevin Dorfman
• Center for Computational Innovations: Artificial Intelligence Multiprocessing Optimized System (AiMOS)
• Brookhaven National Laboratory: Materials Science
• University of Shanghai for Science and Technology: School of Materials Science and Engineering