Study of New Ternary Rare-Earth Intermetallic Germanides with Polar Covalent Bonding

The syntheses, structural characterizations and chemical bonding analyses for several ternary R–M–Ge (R = rare-earth metal; M = another metal) intermetallic compounds are reported. Each chapter of the thesis, the introductory ones part, is dedicated to the obtained achievements for a specific series of investigated compounds, which are: R2MGe6 (M= Li, Mg, Al, Cu, Zn, Pd, Ag), R4MGe10-x (M = Li, Mg), R2Pd3Ge5, Lu5Pd4Ge8, Lu3Pd4Ge4 and Yb2PdGe3. Preparation techniques included both traditional and innovative methods, like the metal flux synthesis, which turned out to be crucial for crystal growth and stabilization of some metastable compounds. Accurate crystal structure determinations were performed on the basis of both single crystal and powder diffraction data. In the case of R2LiGe6, R4MGe10-x and Lu5Pd4Ge8, the presence of non-merohedrally twinned crystals was successfully faced. The obtained structures for the R2MGe6, R2Pd3Ge5, Lu5Pd4Ge8 and Yb2PdGe3 were concisely described and rationalized according to the group–subgroup formalism. These results, combined with total energy calculations, allow presenting a correct distribution of structure modifications among the large family of the R2MGe6 compounds, leading to a deep revision of many controversial literature data. The knowledge of the correct structural models was the essential starting point to perform accurate and reliable chemical bonding investigations, mainly focusing on the far from trivial interactions between the Ge-polyanionic networks and the surrounding metals, revealing in all cases strong deviation from the Zintl description. In this framework, a comparative chemical bonding analysis for La2MGe6 (M = Li, Mg, Al, Zn, Cu, Ag, Pd) and Y2PdGe6 germanides was performed by means of cutting-edge position-space quantum chemical techniques based on QTAIM, ELI-D and their basins intersection. The accurate description of the bonding scenario required also the proposal of two new approaches: the penultimate shell correction (PSC0) and the ELI-D fine structure analysis based on its relative Laplacian. Hence, the Ge–La/Y and Ge–M (M 6¼Li, Mg) bonding were described as polar covalent. The Li- and Mg-containing phases were described as germanolanthantes M[La2Ge6]. Finally, thanks to these tools, a consistent picture for La/Y–M polar bonds was also presented.