Trigermanides AEGe3 (AE = Ca, Sr, Ba): chemical bonding and superconductivity

Abstract

Abstract The crystal structures of the trigermanides AEGe3(tI32) (AE = Ca, Sr, Ba; space group I4/mmm, for SrGe3: a = 7.7873(1), c = 12.0622(3) Å) comprise Ge2 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 Ge2 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 Ge2 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 AEGe3.