Large Area, High Resolution Mapping of Approximate Rotational Symmetries in a Pd_77.5Cu₆Si_16.5 Metallic Glass Thin...

Area of Science:

• Materials Science
• Electron Microscopy
• Crystallography

Background:

• Amorphous materials lack long-range order, making their local structures challenging to characterize.
• Understanding local symmetries in amorphous materials is crucial for predicting their properties.

Purpose of the Study:

• To develop and validate a method for mapping approximate rotational symmetries in amorphous materials using 4D STEM.
• To establish the relationship between experimental parameters and the quality of symmetry mapping.
• To link symmetry coefficients to atomic cluster symmetries in metallic glasses.

Main Methods:

• Densely spaced four-dimensional scanning transmission electron microscopy (4D STEM) acquisition.
• Analysis using correlation symmetry coefficients.
• Electron diffraction simulations on atomic models of metallic glass.
• Experimental validation on a Pd77.5Cu6Si16.5 metallic glass thin film.

Main Results:

• Identified key experimental parameters (noise, electron counts, coherence, sampling, interaction volume) affecting 4D STEM symmetry mapping.
• Established an experimental parameter envelope for reliable symmetry mapping.
• Demonstrated a direct correlation between symmetry coefficients and local atomic cluster symmetries.
• Successfully imaged the types, sizes, volume fractions, and spatial correlations of rotationally symmetric regions in a metallic glass.

Conclusions:

• 4D STEM with correlation symmetry analysis is a powerful technique for characterizing local order in amorphous materials.
• The study provides a framework for optimizing 4D STEM experiments for symmetry mapping.
• This method enables detailed imaging of structural heterogeneities in metallic glasses.