An Unusual Triple-Decker Variant of the Tetragonal BaAl₄-Structure Type: Synthesis, Structural Characterization, and Chemical Bonding of Sr₃Cd₈Ge₄ and Eu₃Cd₈Ge₄

Reported are the synthesis and the crystal structures of the new ternary phases Sr₃Cd₈Ge₄ and Eu₃Cd₈Ge₄. The structures of both compounds have been established by single-crystal and powder X-ray diffraction methods. They crystallize in the tetragonal space group I4/mmm (No. 139, own structure type, Pearson symbol tI30) with Z = 2, and lattice parameters as follows: a = 4.4941(14) Å; c = 35.577(7) Å for Sr₃Cd₈Ge₄, and a = 4.4643(12) Å; c = 35.537(9) Å for Eu₃Cd₈Ge₄, respectively. The most prominent feature of the structure is the complex [Cd₂Ge] polyanionic framework, derived by unique ordering of the Cd and Ge atoms in fragments that bear resemblance to the BaAl₄ structure type. Temperature dependent DC magnetization measurements indicate that Eu₃Cd₈Ge₄ displays Curie-Weiss paramagnetic behavior with no sign of magnetic ordering in the measured range. Theoretical considerations of the electronic structure on the basis of the tight-binding linear muffin-tin orbital (TB-LMTO-ASA) method are also presented and discussed.