Packing of Russian doll clusters to form a nanometer-scale CsCl-type compound in a Cr–Zn–Sn complex metallic alloy

Abstract

The new complex intermetallic alloy, Cr₂₂Zn₇₂Sn₂₄, with a giant cubic unit cell (space group Fmc, Z = 8, Pearson symbol cF944 with a = 25.083(3) Å) is reported. The structure can be described as a CsCl-type packing of two “Russian Doll” type intermetallic clusters: a 99-atom cluster, Cr@(Cr₈Zn₆)(Sn)₂₄(Zn)₆₀, composed of a central Cr atom encapsulated by three shells: a distorted rhombic dodecahedron, a snub-cube, and a rhombi-icosidodecahedron; and a 55-atom cluster, Cr@(Zn)₁₂(Zn)₃₀(Zn)₁₂, also composed of a central Cr atom, but now surrounded by three different shells: an icosahedron, an icosidodecahedron, and a larger icosahedron. The two distinct clusters can be considered as “superatoms” that form the resulting nanometer-scale CsCl-type structure. Evaluating the electronic structure of this complex alloy structure yielded an alternative description, involving a 3-d network of vertex-connected, Cr-centered icosahedra and fragments of bcc metals. This description closely parallels the effective chemical bonding interactions, allowing for an understanding of the stability of this CMA. Magnetic and Seebeck coefficient measurements on self-flux grown crystals of Cr₂₂Zn₇₂Sn₂₄ show it to be paramagnetic and metallic, in agreement with the results of electronic structure calculations.