Flux Growth, Crystal Structure, and Chemical Bonding of Yb 2 PdGe 3 , an AlB 2 Superstructure within the Rare-Earth Series.

The complete structure revision of the RE 2 PdGe 3 ( RE = rare-earth metal) series revealed that Yb 2 PdGe 3 is the only AlB 2 ordered superstructure. Good-quality single crystals of this compound were successfully grown from molten indium flux, enabling accurate single-crystal investigations. Yb 2 PdGe 3 crystallizes with the Ce 2 CoSi 3 -type structure in the hexagonal space group P 6/ mmm (no. 191) with lattice parameters a = 8.468(1) Å and c = 4.0747(7) Å. This structure is a four-order derivative of AlB 2 , composed of planar ∞ 2 [PdGe 3 ] honeycomb layers spaced by Yb species, located at the center of Ge 6 and Ge 4 Pd 2 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 Ge 6 H 6 and Ge 6 6- aromatic molecules.