Bonding, Crystal Structure, and Phase Stability

Intermetallics form because the strength of bonding between the respective unlike atoms is larger than that between like atoms. Accordingly, intermetallics form 

The best-known electron compounds are the Hume-Rothery phases which have the cubic B2 structure, e.g. P-brass at high temperatures and the transition-metal alu-minides FeAl, NiAl and CoAl, or the complex cubic A13 structure (P-manganese-type), e.g. ZujCo, CusSi, or the close-packed hexagonal A3 structure, e.g. CUjGa, AgjAl, for the valence electron concentration VEC = 3/2, the complex 

It has to be concluded that the bonding character and crystal structure of an inter-metallic can be predicted in a significant way only on the basis of a quantum-mechanical, ab initio calculation for the re- 

From the discussion in this section it follows that the intermetallics do not form a homogeneous group of materials at all. Instead the term intermetallics comprises a huge variety of phases which differ drastically with respect to bonding, crystal structure and properties. Thus the properties of intermetallics cannot be discussed generally with respect to all intermetallics they can only be discussed by referring to specific groups of intermetallics. 

It may be supposed that the particular crystal structure of an intermetallic reflects the character and strength of bonding in a sensible way, and thus the crystal structure would be a good criterion for phase classification. However, this does not mean that intermetallics with the same crystal structure are similar with respect to bonding and properties since, e.g. the B2 structure is common to the intermetallic Ni Al and to the purely ionic salt CsCl, and in the case of the transition-metal disilicides with a 

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