Ge Clusters in the Trivalent Rare-Earth Compound SmGe.

The compound SmGe adopts two modifications with Pearson symbols 16 (AlB-derivative) and 64 (defect α-ThSi2-type) upon synthesis at ambient pressure. Synthesis at extreme conditions grants access to the modification 32 (PuPd-type). High-pressure high-temperature treatment of prereacted element mixtures yields PuPd-type SmGe, space group with lattice parameters = 9.42813(9), = 7.56296(7), and = 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 Ge 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 Ge pyramidal units, pointing to a complex bonding scenario that is difficult to rationalize by electron counting rules. SmGe shows a metallic conductivity. Heat capacity and magnetization measurements indicate a 4 electron configuration and thus the trivalent state of the Sm ions. The magnetic moments of Sm in SmGe order antiferromagnetically at 20.4 K.