SODIUM AMALGAMS - PHASE-DIAGRAM, STRUCTURAL CHEMISTRY AND THERMODYNAMIC DATA, A SUMMARY OF RECENT DEVELOPMENTS

A summary of recent results concerning the binary system Na-Hg is give n. Characterized by six thermodynamically stable solids: NaHg4, NaHg2, NaHg (alpha,beta,gamma), Na3Hg2, Na8Hg3 (alpha,beta,gamma) and Na3Hg (alpha,beta), their crystal structures could be solved by single cryst al X-ray studies, except those of NaHg4 and beta-NaHg. The structural characterization of beta-NaHg is based on high temperature powder data , whereas NaHg4 has an incommensurate superstructure which has not bee n solved up to now. NaHg2 crystallizes in a modified AlB2 structure wi th hexagonal nets of mercury atoms and sodium in hexagonal prismatic c oordination. alpha-NaHg has a distorted CsCl-type structure with zigza g chains of Hg-4 rectangles, beta- and gamma-NaHg show Zintl-type NaTl structures (beta-NaHg slightly distorted). Na3Hg2 has a unique struct ure with isolated square-planar Hg-4 clusters (d(Hg-Hg)approximate to 300 pm) embedded in a matrix of Na atoms. The structures of the three modifications of Na8Hg3 and the two modifications of Na3Hg are closely related and characterized by eutactic hexagonal nets of Hg atoms (d(H g-Hg)>500 pm) with different stacking sequences, specific distortions and Na atoms in tetrahedral and octahedral holes. Calorimetric data ba sed on DSC and c(p) measurements are presented for all phases. In addi tion, important aspects of chemical bonding as well as some associated physical properties are referred to.