New crystal structure discovery challenges classical definition of crystals

Scientists and engineers have long understood crystals as materials with regular arrangements of their constituents. However, recent research conducted by a team led by Sangwoo Lee from Rensselaer Polytechnic Institute challenges this conventional definition. The team discovered that crystal structures can deviate from regular arrangements, which has significant implications for materials science, particularly in areas such as semiconductors, solar panels, and electric vehicles.

One of the most common types of crystal structures involves stacking layers of spheres in a honeycomb arrangement, known as close-packed structures. There are various ways to stack these layers, and understanding the selection process is crucial in materials and physics research. Among the close-packed structures, the random stacking of two-dimensional hexagonal layers (RHCP) stands out as an unusual structure with irregularly spaced constituents. Although this structure was initially observed in cobalt metal in 1942, it has been considered a transitional and energetically unfavorable state.

Lee’s research group used X-ray scattering data from soft model nanoparticles made of polymers to investigate the RHCP structure. However, the analysis of this data proved to be complex. To overcome this challenge, Patrick Underhill, a professor at Rensselaer, utilized the supercomputer system called Artificial Intelligence Multiprocessing Optimized System (AiMOS) at the Center for Computational Innovations.

The research team’s findings indicate that the RHCP structure is likely a stable configuration, contrary to previous assumptions. This discovery challenges the classical definition of crystals. Moreover, it sheds light on polytypism, a phenomenon that facilitates the formation of RHCP and other close-packed structures. Silicon carbide, a widely used material in high-voltage electronics for electric vehicles and body armor, exemplifies polytypic materials. The study suggests that these materials may exhibit continuous structural transitions, including non-classical random arrangements that possess unique properties.

Kevin Dorfman from the University of Minnesota-Twin Cities, who was not involved in the research, commended the study, noting that it provides compelling evidence for a continuous transition between face-centered cubic (FCC) and hexagonal close-packed (HCP) lattices. This implies the existence of a stable random hexagonal close-packed phase between them, signifying a significant breakthrough in materials science.

Shekhar Garde, the dean of Rensselaer’s School of Engineering, expressed satisfaction with the discovery, highlighting the power of advanced computation in decoding molecular-level structures in soft materials. This breakthrough is expected to open up new opportunities for technological applications involving these novel materials.

In addition to Lee and Underhill, the research team included Juhong Ahn and Guillaume Freychet from Rensselaer, Liwen Chen from the University of Shanghai for Science and Technology, and Mikhail Zhernenkov from Brookhaven National Laboratory.

Source: Rensselaer Polytechnic Institute

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