Study of new polar intermetallic compounds: synthesis, structural relations and real space chemical bonding analysis

The syntheses, structural characterizations and theoretical DFT-based investigations for different R–M–Ge (R = rare earth metal; M = another metal) germanides are reported. The R2PdGe6 and R2LiGe6 series, together with La2CuGe6 and La2AgGe6 compounds, were structurally characterized by single crystal X-ray diffraction, indicating the oS72-Ce2(Ga0.1Ge0.9)7 modification as the correct one. The alternative In-flux method, once optimized, produced three good quality R2PdGe6 single crystals: Pr2PdGe6 and the metastable La2PdGe6, which turned out to be mS36-La2AlGe6-type non-merohedrally twinned crystals, and the Yb2PdGe6 of oS72-Ce2(Ga0.1Ge0.9)7-type. These results were extended for a comprehensive study on the R2MGe6 (M = Li, Mg, Al, Cu, Zn, Pd, Ag, Pt, Au) family of compounds, employing symmetry-based structural rationalization and total energy calculations, revealing that the highest energy is always associated to the more reported oS18-Ce2CuGe6 structure. The knowledge of the correct structural models allowed a comparative chemical bonding analysis for La2MGe6 (M = Li, Mg, Al, Zn, Cu, Ag, Pd) and Y2PdGe6 germanides. State of the art position-space techniques (QTAIM, ELI-D and their basin intersection) were employed together with the proposal of new approaches developed during this work; i.e. the penultimate shell correction (PSC0) method and the ELI-D fine structure based on its relative Laplacian. The former was crucial to balance Ge–La polar-covalent interactions against the Ge–M ones, whereas the latter allows to reveal polyatomic bonding features. With these new tools at hand, it was possible to go beyond the Zintl picture (formally fulfilled only with M = Mg2+ and Zn2+) revealing Ge–La and Ge–M (M ≠ Li, Mg) polar-covalent interactions. For M = Li, Mg a formulation as germanolanthanate M[La2Ge6] is appropriate. In addition, a consistent picture of La/Y–M polar interactions was also described. A systematic study on the existence of R2Pd3Ge5 (R = La-Nd, Sm, Gd-Lu) was conducted and the desired phase was revealed to exist with R = La-Nd, Sm, Yb crystallizing with the oI40-U2Co3Si5 structure. A Bärnighausen tree was constructed in order to rationalize the related crystal structures of the RPd2Ge2, RPdGe3 and R2Pd3Ge5 ternary compounds, enriching the large family of the BaAl4 derivatives. After magnetization and susceptibility measurements Yb2Pd3Ge5 was described as a paramagnet with μeff close to 0.8 μB/Yb-atom, suggesting a nearly divalent Yb state. The new Lu5Pd4Ge8 and Lu3Pd4Ge4 intermetallics were synthesized. The former crystallizes with non-merohedral monoclinic twinned crystals (P21/m, mP34) and the latter is orthorhombic (Immm, oI22). COHP- and preliminary ELI-D-based chemical bonding analysis revealed the expected Ge-covalent fragments and in addition Ge–Lu, Ge–Pd and Pd-Lu polar-covalent interactions. These findings, together with the aforementioned results for La2MGe6 compounds, indicate the importance of these interactions within ternary rare-earth germanides. Finally, the existence of R4MgGe10-x and R4LiGe10-x phases along the R series was investigated. X-ray single crystal diffraction experiments show that all the phases, obtained with R = La-Nd, Sm, Gd-Dy, are non-merohedrally twinned with mS60-La4MgGe10-x structure. The presented results constitute a step forward in the comprehension of composition-structure-properties relationships and a good playground for further studies on analogous systems.