Chemical Engineering Tips – Metal Intermetals

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Also known as intermetallic compounds. It generally refers to compounds formed by two or more metal elements combining with each other in an integer ratio of their atomic numbers under certain conditions; examples include Al2Zn3, CuZn, etc. However, even if some of the components in a compound are semimetals or nonmetals, as long as the resulting compound exhibits significant metallic properties, it is usually classified as a metal intercalide, such as Cu5Si. The latter category includes useful materials such as magnets and semiconductors. Under normal circumstances, the composition of metal intermetallics has nothing to do with the valence charges or covalency of their constituent elements, and oxidation states are even less meaningful. There is a class of metal interhalides whose composition and structure follow the Hume-Rothery rules. That is, compounds for which the ratio of the number of electrons to the number of atoms calculated based on their formula is a specific value such as 3:2, 21:13, or 7:4 are stable, and they possess a common crystal structure. The number of electrons here is generally the usual number of valence electrons, and it corresponds to the group number in the periodic table; exceptions are the metals in Groups VII and VIII, for which the electron count is considered to be 0. For example, the calculations for several 21:13 γ-brass-type compounds are as follows: Since the structure and properties of such intermetallic compounds depend on the ratio of electrons to atoms, they are also known as electron compounds. Most electronic compounds are brittle and have high resistance. Nevertheless, the properties and structure of these electronic compounds are still very similar to those of metals; for example, Cu3Al is not significantly different from Cu and Al, and it can be considered to be in a completely disordered state. Therefore, Cu3Al is merely a literal compositional formula; relationships such as those in ordinary compounds regarding composition and valence cannot be assumed. In intermetallic compounds formed between metals with a large difference in electronegativity, the covalent bond framework composed of the metal with the higher electronegativity determines the composition and structure of the entire compound. For example, without distinguishing the metals involved, NaT1 has a body-centered cubic lattice, while when only T1 is considered, it has a diamond-shaped lattice. If only Na is considered, although there is the same diamond lattice and the same arrangement, it is not impossible to assume that T1, with 4 valence electrons in its outermost shell (Na0.9, T11.8), forms the diamond lattice, while Na+ fills in their interstices. Positively charged metal ions penetrate into the interstices of a three-dimensional framework composed of highly electronegative metals; typical examples of this include compounds with the same crystal structure such as NaZn13, KCd13, CaZn13, CaBe13, LaBe13, ThBe13, etc. These are the well-known Zintl phases. In intermetallic compounds formed between metals with a small difference in electronegativity, the atomic radius ratio is an important factor determining their composition and structure. An example of this is an AB2-type compound with an atomic radius ratio of about 1.2, known as the Laves phase. These phases all have the same crystal structure, which consists of 3 types; to date, over 100 compounds of this kind have been discovered. The ratio of the instance to its radius (in parentheses) is: MgZn2 (1.20), KNa2 (1.21), CaLi2 (1.29), CeAl2 (1.28), AgBe2 (1.27), CaAg2 (1.37), TiFe2 (1.19), etc.