PhD Thesis Presentation: Resolving Short Range Order of Amorphous Solids

PhD Candidate

Aly Rahemtulla

Abstract

Determining the structure of amorphous solids has been an ongoing challenge. The lack of long range ordering allows multiple solutions to fit experimental data equally well. Models are created based on physical principles to generate possible solutions to reconcile experimental and theoretical data. This work introduces the Self-sorted Local Atomic Motif (SLAM) method as a robust tool for visually inspecting and identifying unique structural geometries present in a model. Through SLAMs differences in structure are identified between models of amorphous silicon (a-Si) and amorphous germanium (a-Ge) before and after annealing. Here, the 3-coordination known to exist in these systems is identified to be composed of two different motifs; the Semi-buckled and Buckled motifs. Upon annealing the occupation and the shape of these motifs are observed to change. The SLAM method is additionally applied to a 300,000 atom model of Cu₅₀Zr₅₀ metallic glass during a shearing process. This highly disordered system contains a large amount of icosahedra that convert to 9-coordinated motifs in the region where the shear band forms. Large networks of icosahedra are shown to permanently disappear in this region at a critical point creating a structural indicator of when a metallic glass is permanently deformed. High-resolution pair distribution function measurements are performed on a-Se when annealed at different times showcasing the capabilities of creating high resolution measurements at the Brockhouse Sector in the Canadian Light Source.

Examination Committee

• Dr. Eric Poisson, Chair
• Dr. Stefan Kycia, Advisor
• Dr. Robert Wickham
• Dr. Alejandro Marangoni
• Dr. Chris Benmore, External Examiner (Argonne National Laboratory)