Cs3LiGe4, One Compound with Two Complementary Structural Descriptions: Isolated [Ge4]4- Tetrahedral Clusters Coordinated by Li+ and Cs+ Cations or One-Dimensional [LiGe4]3- Polyanions Packed within a Matrix of Cs+ Cations?

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) & Aring;, b = 15.503(3) & Aring;, and c = 9.919(2) & Aring; and V = 1068.65(4) & Aring;3. 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.