noncrystal

Noncrystal, in materials science, refers to solids that do not exhibit long-range crystalline order. The term is largely superseded in modern literature by amorphous or glassy, but it is sometimes used to emphasize the absence of translational symmetry in a material. Noncrystalline solids include glasses, gels, and many polymers when in a disordered state.

At the atomic level, noncrystals show short-range order, but their atoms do not repeat periodically in three-dimensional space. The arrangement may resemble a random network, and diffraction patterns lack sharp Bragg peaks; instead they show broad halos corresponding to short-range correlations. This lack of periodicity gives rise to isotropic properties on macroscopic scales.

Noncrystal formation occurs when a liquid is cooled rapidly, preventing crystalline nuclei from growing, or by vapor deposition and rapid quenching of materials. Polymers can become amorphous when cooled in ways that avoid crystallization. Some materials can be vitrified or remain disordered under conditions that inhibit crystallization. The resulting structures may be metastable relative to crystalline forms.

Examples include silica glass, borosilicate glass, amorphous carbon, metallic glasses, amorphous silicon, and many organic polymers such as polystyrene. Properties are diverse but often include optical transparency in glasses, isotropy, and a tendency toward brittleness in rigid amorphous solids; certain amorphous metals can be strong and resilient depending on composition and processing. Thermally, noncrystals typically display a glass transition rather than a sharp melting point.

Characterization relies on diffraction showing broad features and thermal analysis revealing a Tg; these distinguish noncrystals from crystalline materials and from ordered yet nonperiodic structures such as quasicrystals.