[Paper Review] The local atomic packing of a single-component glass is quasi-crystalline

Farid F. Abraham|arXiv (Cornell University)|Apr 22, 2015

Material Dynamics and Properties3 citations

TL;DR

This study uses the Ackland-Jones topological analysis to investigate local atomic environments in a single-component Lennard-Jones glass. It reveals that despite the macroscopic amorphous nature, the local atomic packing is predominantly quasi-crystalline, with most atoms adopting FCC, HCP, or BCC-like configurations, challenging conventional views of glassy disorder.

ABSTRACT

Our earlier Monte Carlo simulations of metastable supercooled-liquid and glass phases of Lennard-Jones atoms found several distinct signatures for identifying the glass transition boundary; i.e., the density, enthalpy, and pair distribution function dependences on temperature and pressure are different for the two phases (F. F. Abraham, J. Chem. Phys., 72, 359 (1980)). In this extension of that study, we base our analysis on the Ackland-Jones (A-J) method for determining the local crystal packing about each atom (G. Ackland & A. Jones, PRB 73, 054104 (2006)). It focuses on the angular distribution of the local atoms surrounding each individual atom and compares it with the known FCC, HCP, BCC, icosahedron packings within a specified uncertainly from perfect packing. Remarkably, the A-J method to our simulated glass states indicates that the local atomic packings of the individual atoms are predominantly crystalline; i.e., quasi FCC, HCP or BCC.

Motivation & Objective

• To investigate the nature of local atomic order in a single-component glass using advanced topological analysis.
• To determine whether local atomic environments in a supercooled liquid and glass phase exhibit crystalline-like characteristics.
• To assess the validity of the glass transition boundary identified through thermodynamic signatures in earlier simulations.
• To explore whether the local structure of a glass can be described as quasi-crystalline rather than fully disordered.
• To reconcile discrepancies between macroscopic amorphous behavior and local atomic ordering in metallic glasses.

Proposed method

• Employing the Ackland-Jones (A-J) topological method to classify local atomic environments based on angular distribution of neighboring atoms.
• Comparing the angular distribution of each atom's neighbors to idealized crystal structures: FCC, HCP, BCC, and icosahedral packings.
• Applying a tolerance threshold to define 'quasi' crystalline order, allowing for deviations from perfect symmetry.
• Analyzing data from Monte Carlo simulations of a Lennard-Jones system in both supercooled liquid and glassy states.
• Using the A-J method to compute the fraction of atoms classified as FCC, HCP, or BCC-like in the simulated glass state.
• Validating results against known thermodynamic signatures of the glass transition (density, enthalpy, pair distribution function).

Experimental results

Research questions

• RQ1 Do local atomic environments in a single-component glass exhibit characteristics of crystalline packing?
• RQ2 To what extent do atoms in a glass adopt FCC, HCP, or BCC-like coordination geometries?
• RQ3 How does the local atomic structure of a glass compare to that of a supercooled liquid at the same thermodynamic state?
• RQ4 Can the glass transition be identified through local structural signatures using the A-J method?
• RQ5 Is the apparent disorder in glasses consistent with a quasi-crystalline local structure?

Key findings

• The Ackland-Jones analysis reveals that the majority of atoms in the simulated single-component glass adopt local configurations resembling FCC, HCP, or BCC structures.
• Despite the macroscopic amorphous character, the local atomic packing in the glass is predominantly quasi-crystalline, with significant fractions of atoms classified as FCC-like or HCP-like.
• The local structure of the glass is distinct from the icosahedral order typically associated with metallic glasses, indicating a different local motif.
• The A-J method successfully identifies a clear structural signature that differentiates the glassy state from the supercooled liquid, even when thermodynamic properties are similar.
• The results suggest that the glass transition is associated with a structural reorganization toward a more ordered local configuration, even if long-range order is absent.
• The study demonstrates that local atomic environments in a glass can be highly ordered in a topological sense, challenging the traditional view of glasses as purely disordered materials.