General
The range of existence of the intermediate solid solutions described above is contained between two boundary values of the alloying element concentration. When the solid solution only exists in the ordered state, the compound is said to be a stoichiometric or intermetallic compound. Such compounds are generally only stable in much narrower concentration ranges than disordered intermediate solid solutions. Thus, they can be termed stoichiometric compounds, although the stoichiometric imbalance is sometimes very high. Depending on the case, these compounds have a metallic character, to a greater or lesser degree. This characteristic decreases when passing from intermediate solid solutions to more or less covalent (or ionic) intermetallic compounds. Metal-non metal compounds form another type of intermetallic compound.
Generally speaking, for intermetallic compounds we can distinguish:
electronic compounds (Hume-Rothary phases) such as: \(\ce{CuZn\beta}\), \(\ce{Cu3Al}\) or \(\ce{Cu5Sn}\),
Laves phases: \(\ce{A2B}\) ordered, such as: \(\ce{MgZn2}\), \(\ce{MgCu2},\) \(\ce{CuAl2}\).
For metal-non metal compounds:
Hägg compounds and complex carbides (compounds characterized by the presence of small-dimension interstitial atoms \(-\ce{C}\), \(\ce{B}\), \(\ce{N}\), \(\ce{H}\), \(\ce{O}-\)): if the ratio between the atomic radius of the alloying element and that of the metal is lower than 0.59, a \(\ce{FCC}\) or \(\ce{CPH}\) lattice is formed with an occupation of the octahedral sites (for example, \(\ce{FCC}\) : \(\ce{TiC}\), \(\ce{ZrC}\), \(\ce{VC}\), \(\ce{NbC}\), \(\ce{TiN}\), \(\ce{ZrN}\), \(\ce{VN}\), \(\ce{NbN}\) - and \(\ce{HC}\) : \(\ce{Ta2C}\), \(\ce{Mo2C}\) ...); if this ratio is higher than 0.59, complex carbides are formed (interstitial insertion in a close-packed structure is more difficult and the structure is more complex). Example: cementite \(\ce{Fe3C}\).