Abstract

The crystal structures of the trigermanides AEGe(3) (tI32) (AE = Ca, Sr, Ba; space group I4/mmm, for SrGe3 : a = 7.7873(1), c = 12.0622(3)angstrom) comprise Ge-2 dumbbells forming layered Ge substructures which enclose embedded AE atoms. The chemical bonding analysis by application of the electron localizability approach reveals a substantial charge transfer from the AE atoms to the germanium substructure. The bonding within the dumbbells is of the covalent two-center type. A detailed analysis of SrGe3 reveals that the interaction on the bond-opposite side of the Ge-2 groups is not lone pair-like-as it would be expected from the Zintl-like interpretation of the crystal structure with anionic Ge layers separated by alkaline-earth cations-but multi-center strongly polar between the Ge-2 dumbbells and the adjacent metal atoms. Similar atomic interactions are present in CaGe3 and BaGe3. The variation of the alkaline-earth metal has a merely insignificant influence on the superconducting transition temperatures in the s,p-electron compounds AEGe(3).