PROVIDENCE, R.I. [Brown University] — Using finely tuned nanoscale building blocks, researchers from Brown University and the University of Michigan College of Engineering have stabilized a fleeting structural phase of matter that had been predicted theoretically but never before stabilized in a physical material. The new nanoparticle superlattice, described in the journal Science, freezes an elusive intermediate state between two of nature’s most common crystal metallic arrangements. Beyond describing new details about how this transition works, the new structure exhibits extraordinary optical properties that could be useful in quantum computing or other quantum information systems. More broadly, the work provides a new recipe for using custom-shaped nanoparticles to engineer entirely new classes of materials with tailored properties. “Our work is a little bit like kids playing with LEGO blocks,” said Ou Chen, an associate professor of chemistry at Brown and a corresponding author of the research. “We synthesize unique nanoscale building blocks and stack them into interesting structures. In this case, we were able to stabilize these theorized transitional structures and demonstrate important quantum optical properties.” The crystal structures of many metals fall into one of two categories: face-centered cubic (FCC) or body-centered cubic (BCC). FCC is the tightest packing arrangement for spherical particles. When stacked together, spherical particles tend to arrange themselves into a repeating cubic pattern, with on particle at each corner and one particle in the center of each cube face. BCC is somewhat less tightly packed: Particles are present in each corner of a cube, with one particle at the center of the cube’s body (rather than on each face). Loosely speaking, these are the arrangements that atoms form in metallic crystals. With heating, some metals transition between the two structures. Iron, for example, goes from BCC to FCC when heated to 912 degrees Celsius. There