Flux Growth, Crystal Structure, and Chemical Bonding of Yb2PdGe3, an AlB2 Superstructure within the Rare-Earth Series

The complete structure revision of the RE2PdGe3 (RE = rare-earth metal) series revealed that Yb2PdGe3 is the only AlB2 ordered superstructure. Good-quality single crystals of this compound were successfully grown from molten indium flux, enabling accurate single-crystal investigations. Yb2PdGe3 crystallizes with the Ce2CoSi3-type structure in the hexagonal space group P6/mmm (no. 191) with lattice parameters a = 8.468(1) angstrom and c = 4.0747(7) angstrom. This structure is a four-order derivative of AlB2, composed of planar (2)(infinity)[PdGe3] honeycomb layers spaced by Yb species, located at the center of Ge-6 and Ge4Pd2 hexagons. A superconducting transition is observed below the critical temperature of 4 K. A divalent state of Yb is deduced from magnetic susceptibility measurements below room temperature, which indicate an almost nonmagnetic behavior. A charge transfer from Yb to Pd and Ge was evidenced by the Quantum Theory of Atoms in Molecules (QTAIM) effective charges; polar four-atomic Ge-Pd/Yb and two-atomic Pd-Yb bonds were observed from the ELI-D (electron localizability indicator), partial ELI-D, and ELI-D/QTAIM intersections. The bonding interactions between Ge atoms within regular Ge-6 hexagons are found to be intermediate between single bonds, as in elemental Ge, and higher-order bonds in the hypothetic Ge6H6 and Ge-6(6-) aromatic molecules.

Automated summary

The RE2PdGe3 (RE = rare-earth metal) series was studied and it was found that Yb2PdGe3 is the only compound with an AlB2 ordered superstructure. Single crystals of Yb2PdGe3 were grown from molten indium flux for accurate investigation. The crystal structure of Yb2PdGe3 was determined to be a four-order derivative of AlB2, with superconducting behavior below 4 K. Magnetic susceptibility measurements indicate a divalent state for Yb, while bonding interactions between Ge were found to be intermediate between single bonds and higher-order bonds.