Noncrystalline

Noncrystalline is a term used to describe materials that lack long-range atomic order. In noncrystalline solids, atoms do not form a repeating three-dimensional lattice with translational symmetry. Noncrystalline states include amorphous solids, in which the disorder persists in the solid, and liquids, which are inherently disordered at the atomic scale.

Because of the absence of long-range order, noncrystalline materials typically exhibit short-range order, isotropy in many properties, and different mechanical and optical behaviors from crystalline materials. Their X-ray diffraction patterns show broad diffuse halos rather than sharp Bragg peaks, reflecting the lack of periodicity.

Common examples include glasses such as silica glass, many polymers such as polystyrene and poly(methyl methacrylate), and metallic glasses (amorphous metals) formed by rapid cooling of liquid alloys. Amorphous carbon, amorphous silicon and germanium, and certain gels are also noncrystalline. Even ordinary liquids are noncrystalline in their disordered state.

Formation and processing methods favor noncrystallinity where crystallization is suppressed. Rapid cooling or quenching prevents crystal nucleation and growth, while vapor deposition, solution processing, or mechanical alloying can produce amorphous films and powders. Upon heating, many noncrystalline materials exhibit a glass transition, where a rigid amorphous solid becomes a more viscous supercooled liquid; crystallization may occur if the material is held at higher temperatures long enough or under suitable conditions.

Applications exploit the uniform properties and optical clarity of many noncrystalline materials, as well as the soft molding and unique magnetic characteristics of some metallic glasses. Limitations include aging and potential crystallization over time, which can alter properties. Characterization typically uses diffraction, calorimetry, and spectroscopy to assess the absence of long-range order.