Solving A Century-Old Problem—Three-Dimensional Atomic Structure of Amorphous Materials

Amorphous materials such as glass, rubber and plastics are ubiquitous in our daily life and have broad applications ranging from telecommunications to electronics. However, due to the lack of long-range order, the 3D atomic structure of amorphous materials had eluded direct experimental determination for more than a century. One experimental method that can in principle address this grand challenge is atomic electron tomography (AET). AET combines high-resolution tomographic tilt series with advanced computational algorithms to resolve the 3D atomic structure of materials without assuming crystallinity, which has been applied to image grain boundaries, anti-phase boundaries, stacking faults, dislocations, point defects, chemical order/disorder, atomic-scale ripples, bond distortion and strain tensors with unprecedented 3D detail. Recently, we advanced AET to determine the 3D atomic positions of monatomic amorphous materials, namely a Ta thin film and two Pd nanoparticles. We observed that pentagonal bipyramids are the most abundant atomic motifs in these amorphous materials. Instead of forming icosahedra, the majority of pentagonal bipyramids arrange into pentagonal bipyramid networks with medium-range order. Molecular dynamics simulations further reveal 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. Furthermore, we also used AET to determine the 3D atomic positions of a multi-component metallic glass and quantitatively characterized the short- and medium-range order. We discovered that, although the 3D atomic packing of the short-range order is geometrically disordered, some short-range-order structures connect with each other to form crystal-like superclusters and give rise to medium-range order. We identified four types of crystal-like medium-range order - face-centred cubic, hexagonal close-packed, body-centred cubic and simple cubic - coexisting in the amorphous sample, showing translational but not orientational order. These observations provide direct experimental evidence to support the general framework of the efficient cluster packing model for metallic glasses. We expect that these results pave the way for the determination of the 3D atomic structure of a wide range of amorphous materials, which could transform our fundamental understanding of non-crystalline materials and related phenomena.<br/><br/>1. Y. Yang, J. Zhou, F. Zhu, Y. Yuan, D. Chang, D. S. Kim, M. Pham, A. Rana, X. Tian, Y. Yao, S. Osher, A. K. Schmid, L. Hu, P. Ercius and J. Miao. Determining the three-dimensional atomic structure of an amorphous solid. Nature 592, 60–64 (2021).<br/>2. Y. Yuan, D.S. Kim, J. Zhou, D.J. Chang, F. Zhu, Y. Nagaoka, Y. Yang, M. Pham, S. J. Osher, O. Chen, P. Ercius, A. K. Schmid and J. Miao. Three-dimensional atomic packing in amorphous solids with liquid-like structure. Nature Mater. 21, 95–102 (2022).