krystallitet

Krystallitet, or crystallite, is a single coherent crystal domain within a polycrystalline solid. Each crystallite has a continuous lattice that extends through its volume and is separated from neighboring crystallites by grain boundaries where the lattice orientation changes. The concept is widely used in materials science and powder diffraction to describe the size of coherent scattering domains that contribute to X-ray diffraction signals.

Size and relationship to grains. The crystallite size reflects the extent of a region that behaves as a single crystal for diffraction purposes. It is not always identical to the visible grain size observed by microscopy; a grain may contain multiple crystallites, and conversely a single crystallite can appear as part of a larger grain depending on the imaging or interpretation method. In many materials, especially after processing, crystallites range from a few nanometers up to several micrometers.

Formation and evolution. Crystallites form during nucleation and growth in solidification, annealing, or sintering. Processing conditions such as temperature, time, and impurities control the average crystallite size. With annealing or extended processing, crystallites can grow and coarsen, reducing the total grain boundary area.

Measurement and analysis. Crystallite size is commonly estimated from X-ray diffraction by analyzing peak broadening with the Scherrer equation, while correcting for instrumental effects. Electron or transmission electron microscopy can provide direct imaging of crystallite boundaries. Techniques like Williamson–Hall analysis separate size effects from microstrain contributions to peak broadening.

Implications. Smaller crystallites increase grain boundary area, influencing mechanical strength (often via the Hall-Petch effect), diffusion, optical properties, and corrosion resistance. Nanocrystalline materials, with very small crystallites, exhibit distinct mechanical and functional characteristics compared with coarse-grained counterparts.

See also: Grain boundary, polycrystal, nanocrystal, Scherrer equation.