Noncrystallinity

Noncrystallinity refers to the absence of long-range periodic atomic order in a solid. In crystalline materials, atoms occupy repeating lattice sites that extend in all directions. Noncrystalline, or amorphous, solids lack this translational symmetry, though they may retain short-range order among a few nearest neighbors. As a result, a noncrystalline material does not exhibit a well-defined crystal lattice throughout its volume.

The structure of noncrystalline materials is typically inferred from diffraction data. X-ray and electron diffraction of amorphous solids produce broad, diffuse halos rather than sharp Bragg peaks, indicating the absence of long-range periodicity. This structural difference underpins many physical properties: isotropy in mechanical and thermal behavior, absence of well-defined cleavage planes, and distinct thermal transitions compared with crystals. In polymers, noncrystallinity arises when chain mobility and packing prevent organized, long-range arrangement; some polymers are semicrystalline, containing both crystalline and amorphous regions.

Formation of noncrystalline solids often results from rapid solidification or inherently disordered building blocks. Quenching a liquid melt quickly can trap atoms in a disordered arrangement. Certain alloys can form metallic glasses; many glasses are oxide-based, such as silica, or polymer-based, such as polystyrene, polycarbonate, and PMMA.

Common examples include glass (silica and soda-lime variants), fused silica, and various amorphous metals and polymers. Noncrystallinity influences properties relevant to optics, glass manufacturing, packaging, pharmaceuticals, and coatings, and is a central concept in the study of disordered materials.