Just as in ordinary metals, defects in intermetallic compounds fall into three basic categories: point defects (vacancies, substitutional and interstitial atoms), line defects (dislocations), and planar defects (stacking faults, interfaces, grain boundaries). Also like ordinary metals, many important physical and chemical properties of intermetallic compounds are governed by the presence of these defects and the effects on them from temperature, composition, chemical environment, elastic stress state, and so on.

What ultimately distinguishes an intermetallic compound from ordinary metals is its superlattice crystal structure. A two-dimensional analogue of the actual three-dimensional superlattice structure is shown in Figure 1a where the superlattice (unit cell marked by full lines) is made up of two identical sublat-tices (unit cells marked by dotted lines). A property of the sublattice is that it is exclusively occupied by one atom species, and accordingly sublattices are named after the atoms that occupy them, for example, the A and B sublattices in Figure 1a. In three dimensions, a super-lattice may consist of several sublattices. For example, the L12 superlattice of Ni3Al consists of four interpenetrating cubic sublattices, one occupied by Al atoms (Al sublattice), the other three by Ni atoms (Ni sublattices). When the sub-lattices are occupied exclusively by their designated atoms, the crystal is said to be fully ordered. The crystal will be partially ordered if a certain fraction of the sublattice sites is taken up by atoms that would otherwise sit at other sublattices; this fraction is used to describe the degree of long-range order.