Crystals not always so regular

When most people think of crystals, they picture suncatchers that act as rainbow prisms or the semi-transparent stones that some believe hold healing powers. However, to scientists and engineers crystals are simply a form of material in which their constituent parts – atoms, molecules or nanoparticles – are arranged regularly in space. In other words, crystals are defined by the regular arrangement of their constituents. Common examples include diamonds, table salt and sugar cubes.

A team led by Rensselaer Polytechnic Institute’s Sangwoo Lee, associate professor in the Department of Chemical and Biological Engineering, has now discovered that crystal structures are not necessarily always regularly arranged. This discovery, reported in a paper in Soft Matter, advances the field of materials science and has implications for the materials used in semiconductors, solar panels and electric vehicle technologies.

One of the most common and important classes of crystal structure is the close-packed structure of regular spheres created by stacking layers of spheres in a honeycomb arrangement. There are many ways to stack the layers in close-packed structures, and how nature selects a specific stacking arrangement is an important question in materials and physics research. In the close-packing arrangement, 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 for cobalt metal in 1942 but 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. This scattering data contains important information about RHCP but is very complicated. With the help of Patrick Underhill, professor in Rensselaer’s Department of Chemical and Biological Engineering, the group analyzed the scattering data using a supercomputer system known as the 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 a phenomenon known as polytypism, which leads to the formation of RHCP and other close-packed structures. A representative material with polytypism is silicon carbide, which is widely used for high-voltage electronics in electric vehicles and as hard materials for body armor. Lee’s team’s findings indicate that these polytypic materials may have continuous structural transitions, including 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.”

This story is adapted from material from Rensselaer Polytechnic Institute, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.