Sm26.25Ge22.75O5: Oxidic Sm30Ge4O5 Superclusters Embedded in a Zintl Polyanionic Framework

The heteroanionic rare-earth germanide oxide Sm26.25Ge22.75O5 was obtained by arc melting. The crystal structure determined by single-crystal X-ray diffraction (space group P4/nmm, a = 14.9838(2), c = 10.5353(1) Å) features [Sm6O] octahedra and capped [Sm8Ge] prisms that are condensed to Sm30Ge4O5 superclusters. These clusters form an interpenetrating framework with polyanionic Ge chains. Chemical bonding analysis reveals the substantial role of oxygen in stabilizing the structure: in the hypothetical oxygen-free compound, significant electron localization appears at the centers of Sm6 octahedra, coinciding with the experimentally observed oxygen positions. Consistently, DFT calculations show that the incorporation of oxygen lowers the enthalpy of formation in comparison to the hypothetical intermetallic compound. Electronic structure calculations indicate metallic conductivity for both the suboxide and the hypothetical oxygen-free intermetallic compound.