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: (Eu1–xCax)2Ge2Pb (space group Pbam) and (SrxEu1–x)2Ge2Pb (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 (Eu1–xCax)2Ge2Pb 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 Eu2+ ground state ([Xe]­4f7 configuration) and the presence of an antiferromagnetic ordering
at cryogenic temperatures.