Three-dimensional atomic packing in amorphous solids with liquid-like structure

Abstract

Liquids and solids are two fundamental states of matter. However, due to the lack of direct experimental determination, our understanding of the 3D atomic structure of liquids and amorphous solids remained speculative. Here we advance 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. We observe that pentagonal bipyramids are the most abundant atomic motifs in these amorphous materials. Instead of forming icosahedra, the majority of pentagonal bipyramids arrange into a novel medium-range order, named the pentagonal bipyramid network. Molecular dynamic simulations further reveal that pentagonal bipyramid networks are prevalent in monatomic amorphous liquids, which rapidly grow in size and form icosahedra during the quench from the liquid state to glass state. The experimental method and results are expected to advance 2 the study of the amorphous-crystalline phase transition and glass transition at the single-atom level.In 1952, Frank hypothesized that icosahedral order is the prevalent atomic motif in monatomic liquids 1 . Over the past six decades, there have been a great deal of experimental, computational, and theoretical studies to understand the structure of liquids and amorphous materials [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] . A polytetrahedral packing model was proposed to explain the 3D atomic structure of monatomic liquids and amorphous materials 22 , in which icosahedral order is a key feature. The icosahedral order has also been found to play a critical role in the structure of metallic glasses and quasicrystals [23][24][25][26][27][28][29] . Despite all these developments, however, no experimental method could directly determine the 3D atomic packing of liquids and amorphous materials due to the lack of long-range order. Atomic electron tomography (AET), allowing the determination of 3D atomic structure of materials without assuming crystallinity 30 , is uniquely positioned to address this challenge. Since its first demonstration in 2012 31 , AET has been applied to reveal a wide range of crystal defects in materials such as grain boundaries, dislocations, stacking faults, point defects, atomic ripples, bond distortion, strain tensors and chemical order/disorder in three and four dimensions 30,[32][33][34][35][36][37][38][39] . More recently, AET was used to determine the structure of a multi-component glass-forming nanoparticle and quantitatively characterize the short-and medium-range order of its 3D atomic arrangement 40 . Here we advance AET to reveal the 3D atomic structure of an amorphous Ta thin film and two amorphous Pd nanoparticles that are not metallic glasses but liquid-like solids. We observe that pentagonal bipyramids are the main atomic motifs in the monatomic amorphous materials. Instead of assembling icosahedra, most pentagonal bipyramids closely connect

Probing the atomically diffuse interfaces in core-shell nanoparticles in three dimensions

“…To address these critical issues of core-shell interfacial structure, here we synthesized polycrystalline pentagonal bipyramid and monocrystalline truncated octahedron palladium-platinum CS-NPs using two-step chemical reduction, and then probed the 3D atomic structures of CS-NPs at the single-atom level. The 3D atomic coordinates and chemical composition of Pd@Pt CS-NPs with different morphology and size were determined using atomic resolution electron tomography (AET), which has been a powerful tool for atomic-scale structural characterization in 3D and even 4D 19,[26][27][28][29] . We discovered a 3D atomically diffuse core-shell interface.…”

“…The final classification results are shown in Supplementary Table2.The structure of two five-fold twinned nanoparticles is analyzed by two methods, i.e., local bond orientational order (BOO) parameters and polyhedral template matching33 . Averaged local BOO parameters ( 4 and 6 ) and normalized BOO parameter for ideal face-centered cubic (fcc) structure are calculated based on the procedure published elsewhere29 , using the first-nearest-neighbor shell distance(3.35 Å) as a constraint. By applying polyhedral template matching , PB/EPB particle was separated into five fcc grain and a five-fold coaxial twin boundary which locally possessing hexagonally close-packed environment.…”

Multi-technique Approach to Unravel the (Dis)order in Amorphous Materials

“…Non-crystalline materials, such as amorphous metals, alloys, and glasses, are employed in a wide range of industrial processes and applications. , The atomic arrangement of disordered systems determines their physical, mechanical, chemical, and electrical properties. − Several studies showed a strong correlation between the configurational local order and the electronic, , magnetic, catalytic, and corrosion-resistance properties of disordered systems, as found, for example, for the high-entropy alloys. , The structural order in amorphous solids is difficult to characterize, and it is far from being understood . Despite the lack of translational periodicity, amorphous systems are not completely random, being organized in hierarchical local structures at different length scales. − The presence of microscopic-ordered arrangement, repetitive units, or defects is responsible for the macroscopic properties of the whole material.…”

Quantifying the Local Structure of Nanocrystals, Glasses, and Interfaces Using TEM-Based Diffraction

“…The authors were able to parse out the interconnected geometries of the polyhedra, finding that face-sharing is common in densely packed clusters. More recently, Voronoi analysis was combined with atomic electron tomography and ePDF for the study of monatomic amorphous Ta and Pd materials . The authors demonstrated that experimental PDFs of amorphous regions were consistent with a MD-generated PDF of Ta liquid, indicating that the packings of atoms in these two phases closely resemble each other.…”