AuCd4: A Hume–Rothery Phase with VEC of 1.8 and Icosahedral and Trigonal-Prismatic Clusters as Building Blocks

The η phase in the Au–Cd binary system has been synthesized, and the structure has been analyzed by single-crystal X-ray diffraction. The compound η-AuCd4 crystallizes in the hexagonal space group P63/m (No. 176). The unit cell contains ∼273 atoms. The compound AuCd4 represents a √3a × √3a × c superstructure of the AgMg4 type. The structure can be well described by icosahedral and trigonal-prismatic clusters. A phase transition to the high-temperature ε phase occurs exothermically at around 578 K. The compound is formed at a sharp valence electron concentration of 1.8 e/a. The compound can be understood within the framework of the Hume-Rothery stabilization mechanism.