Ge5 Clusters in the Trivalent Rare-Earth Compound Sm3Ge5

The compound Sm3Ge5adopts two modifications with Pearson symbols hP16 (AlB2-derivative) and oF64 (defect α-ThSi2-type) upon synthesis at ambient pressure. Synthesis at extreme conditions grants access to the modification oS32 (Pu3Pd5-type). High-pressure high-temperature treatment of prereacted element mixtures yields Pu3Pd5-type Sm3Ge5, space group Cmcm with lattice parameters a = 9.42813(9), b = 7.56296(7), and c = 9.67056(8) Å. The atomic arrangement refined from powder X-ray diffraction data is confirmed by transmission electron microscopy measurements. The crystal structure features Ge5square pyramidal units. The topology of the Electron Localizability Indicator (ELI-D) supports the formation of a bicyclo[1.1.1]pentagermanide cluster composed of two- and three-bonded Ge species, resulting in an electron balance comprising excess electrons. The bonding analysis in position space further reveals the presence of polar covalent interactions between both germanium and the rare-earth metal and among the Ge atoms constituting the base of the Ge5pyramidal units, pointing to a complex bonding scenario that is difficult to rationalize by electron counting rules. Sm3Ge5shows a metallic conductivity. Heat capacity and magnetization measurements indicate a 4f5electron configuration and thus the trivalent state of the Sm ions. The magnetic moments of Sm in Sm3Ge5order antiferromagnetically at 20.4 K.