Atomordnung

Atomordnung, or atomic ordering, is the degree to which different atomic species in a solid occupy specific lattice sites rather than being randomly distributed. In crystalline alloys, ordering results from energetic preferences that favor particular arrangements of atoms on the lattice. The concept contrasts with complete chemical disorder, where species are randomly mixed, and with perfect long-range order, where a well-defined periodic pattern repeats throughout the crystal.

Two important concepts are long-range order and short-range order. Long-range order (LRO) refers to a periodic arrangement that extends across the entire crystal, often producing superlattice reflections in diffraction experiments and characteristic superstructures such as ordered intermetallics. Short-range order (SRO) describes local preferences in atom pairs or small clusters without a repeating long-range pattern. Real materials may exhibit SRO at high temperatures and develop LRO upon cooling through an order-disorder transition.

Common ordered structures appear in substitutional alloys and intermetallics. Examples include B2 (CsCl-type), L1_0 (tetragonal, as in some Fe–Pt systems), and L1_2 (Cu3Au-type), as well as the DO3 variant in Fe–Al systems. Heusler and other intermetallic families also show characteristic ordering phenomena that affect properties.

Measurement and analysis rely on diffraction methods (x-ray and neutron), which reveal superlattice reflections indicative of order, as well as microscopy, calorimetry, and atom-probe techniques to study transition temperatures and local ordering. Atomic ordering influences mechanical strength, diffusion, magnetic behavior, and phase stability, making it a central concept in materials science.