On the nature of Ge-Pb bonding in the solid state. synthesis, structural characterization, and electronic structures of two unprecedented germanide-plumbides

Reported are the syntheses and the crystallographic characterization of two structurally related solid-state compounds: (Eu _1-xCa _x) ₂Ge ₂Pb (space group Pbam) and (Sr _xEu _1-x) ₂Ge ₂Pb (space group Cmmm). Both structures boast anionic sublattices with fully ordered Ge and Pb at the atomic level, which is unusual for elements of the same group. Despite the nearly identical formulas and the similar chemical makeup, the nature of the chemical bonding in the two compounds is subtly different; in the (Eu _1-xCa _x) ₂Ge ₂Pb structure, Ge and Pb are positioned at a relatively shorter distance from one another (<3.0 Å). The close proximity of the atoms leads to interactions, which are seen for the first time in an extended structure and can be suggested to have a covalent character. This conjecture is supported by extensive electronic band-structure calculations using first principles. Magnetic susceptibility measurements reveal Eu ²⁺ ground state ([Xe]4f ⁷ configuration) and the presence of an antiferromagnetic ordering at cryogenic temperatures.