Cs₃LiGe₄, One Compound with Two Complementary Structural Descriptions: Isolated [Ge₄]^4- Tetrahedral Clusters...

Reported are the synthesis and structural characterization of Cs₃LiGe₄, the first structurally characterized lithium-containing cesium germanide. Single-crystal X-ray diffraction data indicate that Cs₃LiGe₄ crystallizes in an orthorhombic crystal system with the space group Cmcm (no. 63, Person symbol oS32) with unit cell parameters a = 6.950(2) Å, b = 15.503(3) Å, and c = 9.919(2) Å and V = 1068.65(4) ų. The structure consists of [Ge]₄^4- tetrahedral clusters with the Li atoms positioned in such a way that polyanionic [LiGe]₄^3- chains could be considered as well. Electronic structure calculations indicate an intrinsic semiconductor with a band gap of 0.76 eV. To understand the nuances of the chemical bonding, position-space techniques based on the quantum theory of atoms in molecules, the electron localizability indicator, and their basin intersections were employed confirming the covalent character of the Ge-Ge bonding. The strong polarity of the interactions between [Ge]₄^4- and the surrounding lithium and cesium cations suggests to interpret these as mainly ionic, further supporting the most basic structure rationalization [Cs⁺]₃[Li⁺][Ge]₄^4-, following the Zintl-Klemm concept. The electron localizability indicator topology affirms the striking similarities between [Ge]₄^4- and molecular As₄ tetrahedra in yellow arsenic, further supporting Klemm's pseudoatom concept on a quantum chemical basis.