Ge₅ Clusters in the Trivalent Rare-Earth Compound Sm₃Ge₅

The compound Sm₃Ge₅ adopts 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 Ge₅ square 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 Ge₅ pyramidal units, pointing to a complex bonding scenario that is difficult to rationalize by electron counting rules. Sm₃Ge₅ shows a metallic conductivity. Heat capacity and magnetization measurements indicate a 4f⁵ electron configuration and thus the trivalent state of the Sm ions. The magnetic moments of Sm in Sm₃Ge₅ order antiferromagnetically at 20.4 K.