Intermetallic Compounds

chemical compounds of metals with each other. The compounds formed by the transition metals with some nonmetals border on the intermetallic compounds (for example, the transition metal compounds with hy drogen, boron, carbon, and nitrogen). Metallic bonds predominate in such compounds. Intermetallic compounds are produced by direct reaction of their components upon heating or by double decomposition reactions. The formation of intermetallic compounds is observed during the separation of an excess of a component from metallic solid solutions or as a result of positional ordering of the atoms of the components in solid solutions.

The composition of intermetallic compounds usually does not correspond to the formal valence of the components and may vary within wide limits. This is because ionic and covalent bonding are seldom encountered in intermetallic compounds, and metallic bonding predominates. In 1912–14, N. S. Kurnakov systematically applied physicochemical methods of analysis to the study of metallic systems and demonstrated the existence of two types of intermetallic compounds, which he named daltonides and berthollides. In composition-property diagrams, daltonides are characterized by a singular point, which corresponds to a constant, usually simple relationship between the numbers of atoms forming the compound. The absence of such a point, and also the variable composition of the solid phase, are characteristic of berthollides.

Daltonides are relatively scarce among the intermetallic compounds. Examples are compounds of magnesium with elements of the main subgroup in Groups IV and V of the Mendeleev periodic system. These intermetallic compounds are of the monosilane (H₄Si) type (Mg₂Si, Mg₂Ge, Mg₂Sn, and Mg₃Pb) and the phosphine (H₃P) type (Mg₃P₂, Mg₃As₂, Mg₃Sb₂, and Mg₃Bi₂). They are characterized by predominance of ionic and covalent bonding, virtual absence of solid solutions with the components of intermetallic compounds, high brittleness, and low electrical conductivity—that is, they resemble ionic compounds (salts) in their properties.

Numerous compounds formed by the transition metals of the copper subgroup with the elements of the main subgroups of Groups III, IV, V, and VI of the Mendeleev periodic system crystallize according to the structural type NiAs (hexagonal lattice with the coordination number 6) and have fairly wide regions of homogeneity in their phase diagrams (that is, they form solid solutions with their components). Both daltonides (for example, NiSb, CoSn, and MnSb) and berthollides (for example, FeSb_x, where x = 0.72–0.92) are found among the NiAs compounds.

In 1914, Kurnakov and his co-workers found that, after annealing and slow cooling, singular points that correspond to the formation of the specific compounds CuAu and CusAu appear on the composition-property diagrams of solid solutions of the CuAu system. Subsequently, the appearance of intermetallic compounds upon cooling of solid solutions was detected in a number of other metallic systems; in particular, the compounds CuPt, Cu₃Pt, FePt, FeV, FeCr, Mn₃Au, MnAu, and MnAu₂ were discovered. The intermetallic compounds formed during the interconversions of solid solutions are called Kurnakov compounds. An additional confirmation of the correctness of recognizing these intermetallic compounds as chemical compounds was given by X-ray diffraction analysis: the diagrams of composition versus degree of orderliness exhibit singular maxima, which correspond to the stoichiometric ratios of the components.

The most common class of intermetallic compounds consists of compounds in which metallic bonding predominates. It includes, above all, the intermetallic compounds formed by copper, silver, and gold, as well as the transition metals, with beryllium, magnesium, zinc, cadmium, mercury, aluminum, germanium, tin, and antimony. As the English scientist W. Hume-Rothery showed in 1926, the composition of these compounds is determined by the electron concentration h, which is equal to the ratio of the total number of valence electrons (that is, the electrons in the outer shell) to the total number of atoms in the unit cell (for example, in Cu₅Cd₈ 5 + 2 X 8 = 21 outer electrons and 5 + 8 = 13 atoms; h = 21/13). At h = 3/2, β-phases with a body-centered cubic lattice are formed; at h = 21/13, γ-phases with a face-centered cubic lattice; and at h = 7/4, hexagonal ε-phases. The Hume-Rothery phases, or electron compounds, are widespread in alloys of the bronze and brass types—for example, CuBe, CuZn, and Cu₅Sn (β-phases); Cu₅Zn₈ and CusiSng (γ-phases); and CuZn₃ and Cu₃Sn (ε-phases).

In 1934 the German scientist F. Laves showed that if the ratio of the atomic radii r_A/r_B is 1.1–1.3 and if the composition is described by the formula AB₂, very compact structures are formed, with coordination numbers 12 and 16 and an ordered arrangement of atoms. The Laves phases (MgCu₂, MgZn₂, and MgNi₂ structural types) include about two-thirds of all known intermetallic compounds in binary systems. (For information on the rarer types of intermetallic compounds and ternary intermetallic compounds, see below: References.) Many intermetallic compounds have acquired practical importance, both in the pure state and in the form of alloys, as magnets (particularly SmCo₅ in the production of permanent magnets), semiconductors, and superconductors. The intermetallic compounds are important components of refractory alloys, high-strength construction materials antifriction materials, and type metal.