Determining the three-dimensional atomic structure of a metallic glass (2004.02266v2)

Abstract: Amorphous solids such as glass are ubiquitous in our daily life and have found broad applications ranging from window glass and solar cells to telecommunications and transformer cores. However, due to the lack of long-range order, the three-dimensional (3D) atomic structure of amorphous solids have thus far defied any direct experimental determination without model fitting. Here, using a multi-component metallic glass as a proof-of-principle, we advance atomic electron tomography to determine the 3D atomic positions in an amorphous solid for the first time. We quantitatively characterize the short-range order (SRO) and medium-range order (MRO) of the 3D atomic arrangement. We find that although the 3D atomic packing of the SRO is geometrically disordered, some SRO connect with each other to form crystal-like networks and give rise to MRO. We identify four crystal-like MRO networks - face-centred cubic, hexagonal close-packed, body-centered cubic and simple cubic - coexisting in the sample, which show translational but no orientational order. These observations confirm that the 3D atomic structure in some parts of the sample is consistent with the efficient cluster packing model. Looking forward, we anticipate this experiment will open the door to determining the 3D atomic coordinates of various amorphous solids, whose impact on non-crystalline solids may be comparable to the first 3D crystal structure solved by x-ray crystallography over a century ago.

• The paper leverages AET to accurately determine 3D atomic positions in a multi-element metallic glass with 97.37% identification accuracy.
• It employs high-resolution imaging and iterative algorithms to differentiate short-range from medium-range order in the amorphous structure.
• The findings validate the efficient cluster packing model and provide actionable insights for designing advanced amorphous materials.

Determining the Three-Dimensional Atomic Structure of a Metallic Glass

The paper presents a significant advancement in the characterization of amorphous solids by determining the three-dimensional (3D) atomic structure of a metallic glass using atomic electron tomography (AET). Traditionally, the characterization of amorphous materials has been challenging due to the absence of long-range atomic order, a limitation that hinders direct and precise experimental determination of their atomic structures.

Methodological Framework

The authors employed a multi-component metallic glass comprising elements such as Co, Ni, Ru, Rh, Pd, Ag, Ir, and Pt. The paper leveraged AET, a sophisticated imaging technique that combines high-resolution tomographic tilt series with advanced iterative algorithms, to trace the 3D atomic positions without the assumption of crystallinity. The samples were synthesized using a carbothermal shock method that ensured high cooling rates, conducive to retaining the amorphous nature of the nanoparticles. A scanning transmission electron microscope with an annular dark-field detector was utilized, capturing a series of 55 images from a highly disordered nanoparticle.

Key Findings

The research reveals several significant insights into the atomic-level structure of the metallic glass:

1. Short-Range Order (SRO): The utilization of local bond orientational order (BOO) parameters enabled the differentiation of disordered atomic arrangements from crystal-like structures. Despite the overall disordered packing, some SRO regions were identified to form crystal-like networks that give rise to medium-range order (MRO).
2. Medium-Range Order (MRO): The paper discerned the existence of four crystal-like MRO networks — face-centered cubic (fcc), hexagonal close-packed (hcp), body-centered cubic (bcc), and simple cubic (sc) — coexisting within the amorphous matrix. These networks show translational but no orientational order, aligning with the efficient cluster packing model.
3. Atomic Packing and Coordination: Detailed analysis of the radial distribution function (RDF) and partial pair distribution functions (PDFs) showed bond length discrepancies indicative of complex packing and coordination. Voronoi tessellation was utilized to characterize SRO, revealing various polyhedral configurations and icosahedral motifs, which are common in metallic glasses.

Implications and Future Prospects

The paper offers direct experimental evidence supporting the efficient cluster packing model for metallic glasses, highlighting the intricate balance between disordered and ordered atomic arrangements. The methodologies outlined in this paper could be extended to various amorphous solids, opening new vistas in understanding their structure-property relationships.

In a theoretical context, this advancement enhances our comprehension of the non-crystalline phase transitions and glass-forming abilities of metallic glasses. Practically, these insights could inform the design and engineering of novel amorphous materials with tailored properties for applications in electronics, optics, and materials science.

The precision of AET, highlighted by the identification accuracy of 97.37% with a 3D precision of 21 picometers, suggests the potential application of this method to other complex material systems, including shear transformation zones in metallic glasses. The paper sets a precedent for future explorations and methodological innovations in unraveling the atomic intricacies of amorphous materials that were once deemed elusive due to conventional limitations. As we further explore this line of inquiry, AET stands as a pivotal tool in the arsenal for materials characterization.