Long-Range and Short-Range Order
orderliness in the arrangement of atoms and molecules in solids and liquids. Orderliness over distances comparable tointeratomic distances is called short-range order, whereas orderliness repeated over infinitely great distances is called long-range order. In an ideal gas. the arrangement of an atom at any point in space is independent of the arrangement of otheratoms. Thus, both long-range and short-range order are absent in the ideal gas, but liquids and amorphous solids exhibitshort-range order—a certain regularity in the arrangement of the neighboring atoms. At great distances the order becomes“blurred” and gradually gives way to “disorder.” which means that long-range order does not exist in liquids and amorphoussolids.
In crystals, the atoms are arranged in regular rows or networks (in three-dimensional lattices), and a regular alternation ofatoms separated by the same distances is repeated for atoms separated by any distance—that is, both long-range andshort-range order exist. The basic criteria of long-range order are the symmetry and regularity of arrangement of particles,which repeat at any distance from a given atom. The presence of long-range and short-range order is due to the interactionbetween the particles. A completely ordered state, however, is only possible at absolute zero, since order is disturbed bythermal motion.
The concepts of long-range and short-range order are important in the theory of alloys, in which they characterize the degreeof ordering of an alloy—for example. in an alloy consisting of two components, complete ordering leads to alternation of thetwo types of atoms; in other words, the nearest neighbors of each atom are only atoms of the other type. Incomplete orderis reflected by the fact that atoms of the same type appear among the nearest neighbors. Depending on the thermal andmechanical treatment, various degrees of ordering may be attained in an alloy; in this case. the physical properties of thealloy are also changed.
Long-range and short-range order exist in terms other than the mutual arrangement of the particles (coordination order). Forexample, a liquid consisting of asymmetric molecules exhibits short-range order (in the case of liquid crystals, long-rangeorder) in the orientation of molecules (orienta-tional order). Ferromagnetic and antiferromagnetic materials exhibit short-rangeand long-range order in the orientation of magnetic moments.
