Intermetallic compounds close-packed layers

Intermetallic compounds can be "valence compounds, with structures corresponding to those of NaCl, CaF2, etc., or compounds of various compositions with all atoms in close-packed layers. Because of the deficiencies of electrons and delocalized bonding, metals are not limited by the valence rules for ionic and covalent compounds. Some intermetallic compounds have structures found only for metals (Chapter 9). 

New York, 1972. An excellent book for intermetallic compounds, excellent figures, gives occupancies and spacings for close-packed layers for many structures. 

At AnXa stoichiometry there are four families of binary intermetallic compounds, which represent different stacking schemes of the close-packed layer shown in Fig. 19.6(a) [34]. All stacking variants yield crystal structures in which a CN 12 polyhedron consists entirely of X atoms surrounding the An atom. The structures that occur are the AuCus, TiNis, MgCdj, and PuAls types. The AuCus-type structure, ordered fee, is the most prevalent, occurring particularly when the actinides combine with Rh and Pd the other three structure types are ordered variants of the hep structure. 

The stacking of close-packed layers in various close-packed sequences is expected to be the favored arrangement when the An atom and its X partner have similar atomic radii, since optimum space filling is achieved under this condition. But, in fact, CN 12 radii derived from the structures of the pure An and X metals differ by 30%, leading one to believe that the effective atomic radii displayed by the An atom and its partner in an AnXa intermetallic compound may differ appreciably from the pure-metal radii. 

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