Barrelane-like germanium clusters in Eu3Ge5: Crystal structure, chemical bonding and physical properties

Formation and crystal structure of the binary germanide Eu3Ge5 were investigated in detail. The compound forms peritectically at 1008deg. C and does not undergo any phase transition down to room temperature. The crystal structure was determined first from X-ray powder diffraction data and was later confirmed by single-crystal X-ray diffraction: structure type Pu3Pd5, space group Cmcm (no. 63), a=9.7675(4)A, b=7.9681(3)A, c=9.8562(3)A. The main building blocks are Ge56- cluster anions surrounded by Eu2+ cations. The nearly tetragonal-pyramidal shape is suggested by the interatomic distances. Contrary to that, the bonding analysis with the electron localization function (ELF) reveals only two- and three-bonded germanium atoms forming a strongly distorted [1.1.1]-barrelane-like cluster. Despite the formal electron deficiency, compared to the barrelane C5H8, the electron counting in the cluster anion and its conformation cannot be interpreted applying the Wade's rules. In accordance with the calculated electronic density of states, Eu3Ge5 shows a metal-like temperature dependence of the electrical resistivity with a sharp change of ρ(T) slope at the Neel point. Above the Neel point the inverse magnetic susceptibility reveals Curie-Weiss behavior with an effective moment of 8.11μB (Eu2+, 4f7 configuration) in agreement with the analysis of the chemical bonding. The 4f7 electronic configuration of europium is confirmed by Eu-LIII X-ray absorption spectroscopy