NaK29Hg48: A Contradiction to or an Extension of Theoretical Concepts to Rationalize the Structures of Complex Intermetallics?

NaK29Hg48 is a new silver metallic, air-sensitive ternary alkali metal amalgam with metallic properties. It crystallizes in space group Pm n (No. 223, Z=2, a=1685.3(2) pm) and shows a superconducting phase transition at 2.5 K. Its crystal structure is characterized by novel icosahedral Hg12 clusters centered by Na atoms and hexagonal antiprismatic Hg12 clusters ("drums") centered by K atoms. Both cluster types are embedded in a matrix of K atoms. With respect to the topology of the cluster centers the three-dimensional arrangement corresponds to the A15 structure type (Nb3Sn). The crystal structure of NaK29Hg48 is closely related to that of the complex intermetallic phases A3Na29In48 (A=K, Rb, Cs) and others. It is an unexpected electron deficient borderline case with respect to electron counting rules (Wadeʹs rules and Zintl–Klemm concept), which were successfully applied to explain the occurence of icosahedral and hexagonal antiprismatic In12/Tl12 clusters in the latter two compounds.