Liquids and solids are two fundamental states of matter. Although the structure of crystalline solids has long been solved by crystallography, our understanding of the 3D atomic structure of liquids and amorphous materials remained speculative due to the lack of direct experimental determination. Now, a collaborative team from UCLA, Lawrence Berkeley National Lab and Brown University has advanced atomic electron tomography to determine for the first time the 3D atomic positions in monatomic amorphous materials, including a Ta thin film and two Pd nanoparticles. Despite different chemical composition and synthesis methods, they observed that pentagonal bipyramids are the most abundant atomic motifs in these amorphous materials. Contrary to traditional understanding, most pentagonal bipyramids do not assemble icosahedra, but are closely connected to form networks extending to medium-range scales. Molecular dynamics simulations further revealed that pentagonal bipyramid networks are prevalent in monatomic metallic liquids, which rapidly grow in size and form more icosahedra during the quench from the liquid to the glass state. These results expand our fundamental understanding of the atomic structure of amorphous solids and will encourage future studies on amorphous-crystalline phase and glass transitions in non-crystalline materials with three-dimensional atomic resolution.
