Interpenetrating Polar and Nonpolar Sublattices in Intermetallics: The NaCd 2 Structure

In the 1960s, Samson solved the structures of some of the most complicated intermetallic phases known, including those of NaCd 2 , Mg 2 Al 3 , and Cu 3 Cd 4 (each with over 1000 atoms per unit cell). Following remarkable earlier constructions by Samson and by Andersson, we use quantum‐mechanical calculations as a guide to describing and understanding these structures. Our electronic Aufbau begins with the relatively simple Mg 17 Al 12 structure and works up to Samson's NaCd 2 structure. In both structures, a division of the sites into electron‐rich and electron‐poor (with respect to an average electron count) reveals MgCu 2 ‐type fragments. Between the interiors and exteriors of these fragments, a change in bonding character takes place—the interiors are more polar, the interfaces relatively nonpolar. This electronic situation is traced to the geometry of the interface sites; they lie simultaneously on electron‐rich and electron‐poor networks. The resulting polar and nonpolar sites in NaCd 2 are separated by a minimal surface, the D surface. The driving force for assuming this structure is electronic: NaCd 2 features an interpenetration of polar and nonpolar bonding regions. This sort of thinking can be applied to other structures.