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How do amorphous solids differ from crystalline solids at the atomic scale?
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The primary distinction between amorphous and crystalline solids lies in their atomic-scale structural organization. While both types of solids exhibit short-range order—meaning atoms maintain consistent local arrangements with their nearest neighbors due to chemical bonding—they differ significantly regarding long-range order.12
- Crystalline Solids: These materials possess translational periodicity, where atomic positions repeat in a regular, predictable array throughout the entire structure. This long-range regularity creates well-defined shells of neighboring atoms at specific distances, which can be observed as sharp peaks in a radial distribution function (RDF).23
- Amorphous Solids: Also known as glasses, these solids lack long-range order. While they maintain a well-defined local structure similar to their crystalline counterparts, the bond directions and atomic positions become uncorrelated at larger distances. Consequently, the undulations in their RDF curves become "washed out" beyond the nearest-neighbor peaks.123
Unlike gases, where atoms are randomly distributed, atoms in amorphous solids are not mobile; they oscillate about fixed points, forming a stable, noncrystalline solid structure.42