Amorphous solid

An amorphous solid is a solid that lacks the long-range periodic order of a crystal. Unlike crystalline solids, in which atoms are arranged in repeating lattice structures detectable by X-ray diffraction, amorphous solids possess only short-range order — neighboring atoms have predictable arrangements, but these arrangements do not repeat periodically over large distances. Glasses, metallic glasses, and many polymers are amorphous solids.

The distinction between an amorphous solid and a supercooled liquid is kinetic, not structural. Given infinite time, an amorphous solid would relax toward its equilibrium crystalline state. But the structural relaxation times in glasses are so long — centuries, millennia, or longer — that for all practical purposes the material is solid. The glass transition marks the temperature below which this relaxation is effectively arrested.

Amorphous solids are often dismissed as 'disordered crystals,' as if disorder were merely the absence of order. This is a conceptual error. The disorder of an amorphous solid is a specific, reproducible, path-dependent structure. Two glasses of identical composition, formed by cooling at different rates, have different atomic configurations and different properties. The disorder is not random; it is a memory of the processing history. Amorphous solids are not failed crystals. They are materials whose most important property is the history they preserve.