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\u2013M\u2013Ge (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\u2013La polar-covalent interactions against the Ge\u2013M 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\u2013La and Ge\u2013M (M 60 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\u2013M 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 B\ue4rnighausen 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 \u3bceff close to 0.8 \u3bcB/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\u2013Lu, Ge\u2013Pd 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

A new glance on R2MGe6 (R = rare earth metal, M = another metal) compounds. An experimental and theoretical study of R2PdGe6germanides

The R2PdGe6series (R = rare earth metal) was structurally characterized, and the results achieved were extended for a comprehensive study on R2MGe6(M = another metal) compounds, employing symmetry-based structural rationalization and energy calculations. Directly synthesized R2PdGe6exists for almost all R-components (R = Y, La-Nd, Sm and Gd-Lu) and even if with La is probably metastable. Several single crystal X-ray analyses (R = Y, Ce, Pr, Nd, Er and Lu) indicated oS72-Ce2(Ga0.1Ge0.9)7as the correct structure. The alternative In-flux method, once optimized, produced three good quality R2PdGe6single crystals: La2PdGe6and Pr2PdGe6turned out to be mS36-La2AlGe6-type non-merohedrally twinned crystals and Yb2PdGe6is of oS72-Ce2(Ga0.1Ge0.9)7-type. The vacancy ordering phenomenon was considered as a possible cause of the symmetry reduction relations connecting the most frequently reported 2:1:6 structural models (oS18, oS72 and mS36) with the oS20-SmNiGe3aristotype. The detected twin formation is consistent with the symmetry relations, which are discussed even considering the validity of the different structural models. DFT total energy calculations were performed for R2PdGe6(R = Y and La) in the three abovementioned structural models, and for La2MGe6(M = Pt, Cu, Ag and Au) in the oS18 and oS72 modifications. The results indicate that the oS18-Ce2CuGe6structure, prevalently proposed in the literature, is associated with the highest energy and thus it is not likely to be realized in these series. The oS72 and mS36 polytypes are energetically equivalent, and small changes in the synthetic conditions could easily stabilize any of them, in agreement with experimental results obtained by direct and flux syntheses

Lu5Pd4Ge8 and Lu3Pd4Ge4: Two more germanides among polar intermetallics

In this study, two novel Lu5Pd4Ge8and Lu3Pd4Ge4polar intermetallics were prepared by direct synthesis of pure constituents. Their crystal structures were determined by single crystal X-ray diffraction analysis: Lu5Pd4Ge8is monoclinic, P21/m, mP34, a = 5.7406(3), b = 13.7087(7), c = 8.3423(4) \uc5, \u3b2 = 107.8(1), Z = 2; Lu3Pd4Ge4is orthorhombic, Immm, oI22, a = 4.1368(3), b = 6.9192(5), c = 13.8229(9) \uc5, Z = 2. The Lu5Pd4Ge8analysed crystal is one more example of non-merohedral twinning among the rare earth containing germanides. Chemical bonding DFT studies were conducted for these polar intermetallics and showing a metallic-like behavior. Gathered results for Lu5Pd4Ge8and Lu3Pd4Ge4permit to described both of them as composed by [Pd\u2013Ge]\u3b4\u2013three dimensional networks bonded to positively charged lutetium species. From the structural chemical point of view, the studied compounds manifest some similarities to the Zintl phases, containing well-known covalent fragment i.e., Ge dumbbells as well as unique cis-Ge4units. A comparative analysis of molecular orbital diagrams for Ge26\u2013and cis-Ge10\u2013anions with COHP results supports the idea of the existence of complex Pd\u2013Ge polyanions hosting covalently bonded partially polarised Ge units. The palladium atoms have an anion like behaviour and being the most electronegative cause the noticeable variation of Ge species charges from site to site. Lutetium charges oscillate around +1.5 for all crystallographic positions. Obtained results explained why the classical Zintl-Klemm concept can\u2019t be applied for the studied polar intermetallics

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

The thesis focuses on the syntheses, structural characterizations and chemical bonding analyses for several ternary R\u2013M\u2013Ge (R = rare earth metal; M = another metal) intermetallics. The challenges in understanding the main interactions governing the chemistry of these compounds, which lead to our inability to predict their formation, structure and properties, are what provided the motivation for this study. In particular, the R2MGe6 (M = Li, Mg, Al, Cu, Zn, Pd, Ag), R4MGe10-x (M = Li, Mg), R2Pd3Ge5, Lu5Pd4Ge8, Lu3Pd4Ge4 and Yb2PdGe3 phases were synthesized and structurally characterized. Much effort was put into the stabilization of metastable phases, employing the innovative metal flux method, and into the accurate structure solution of twinned crystals. Cutting-edge position-space chemical bonding techniques were combined with new methodologies conceived to correctly describe the Ge\u2013M, Ge\u2013La and also La\u2013M polar-covalent interactions for the La2MGe6 (M = Li, Mg, Al, Cu, Zn, Pd, Ag) series. The present results constitute a step forward in our comprehension of ternary germanide chemistry as well as providing a good playground for further investigations

Synthesis, spectroscopic characterization and chemical reactions of stable o-QM on solid phase

A novel approach towards quinone methides stabilization has been achieved by anchoring the reactive o-QM intermediate on solid phase (RTHP). The reactivity and selectivity of supported o-QM towards N and S centered nucleophiles have been explored

{Ca, Eu, Yb}₂₃Cu₇Mg₄ as a Step towards the Structural Generalization of Rare Earth-Rich Intermetallics

The R23Cu7Mg4 (R = Ca, Eu) intermetallics, studied by single-crystal X-ray diffraction, were found to be isostructural with the Yb23Cu7Mg4 prototype (hP68, k4h2fca, space group P63/mmc), forming a small group inside the bigger 23:7:4 family, otherwise adopting the hP68-Pr23Ir7Mg4 crystal structure. The observed structural peculiarity is connected with the divalent character of the R component and with a noticeable volume contraction, resulting in the clear clustering of title compounds inside the whole 23:7:4 family. The occurrence of fragments typical of similar compounds, particularly Cu-centered trigonal prisms and Mg-centered core–shell polyicosahedral clusters with R at vertices, induced the search of significant structural relationships. In this work, a description of the hexagonal crystal structure of the studied compounds is proposed as a linear intergrowth along the c-direction of the two types of slabs, R10CuMg3 (parent type: hP28-kh2ca, SG 194) and R13Cu6Mg (parent type: hR60-b6a2, SG 160). The ratio of these slabs in the studied structure is 2:2 per unit cell, corresponding to the simple equation, 2 × R10CuMg3 + 2 × R13Cu6Mg = 2 × R23Cu7Mg4. This description assimilates the studied compounds to the {Ca, Eu, Yb}4CuMg ones, where the same slabs (of p3m1 layer symmetry) are stacked in a different way/ratio and constitutes a further step towards a structural generalization of R-rich ternary intermetallics

The La2Pd3(Si, Ge)5 complete solid solution: Crystal structure, chemical bonding, and volume chemistry

The La2Pd3Si5 intermetallic and the La2Pd3(SixGe1-x)5 solid solution were targeted for structural and computational investigations. The ternary compound and quaternary alloys with varying silicon contents (x = 0.25, 0.50, 0.70, 0.75) were prepared by arc melting and turned out to crystalize with the oI40–U2Co3Si5 (Ibam, N. 72) type structure based on powder X-ray diffraction data. The crystal structure of La2Pd3Si5 was additionally solved through X-ray diffraction on single crystal grown by recrystallization in Sn flux. Chemical bonding investigations based on QTAIM effective charges and DOS/(I)COHP analysis indicate the formation of heteropolar interactions between Si and the surrounding La/Pd metals, and between La and Pd. Covalently bonded zigzag chains of Si are also formed and considered to be the main responsible for the higher melting point of La2Pd3Si5, measured by DSC, with respect to that of La2Pd3Ge5. The formation of a complete solid solution between La2Pd3Si5 and La2Pd3Ge5 was confirmed and refined unit cell parameters and volumes change linearly with composition, displaying a Vegard trend. The calculation of atomic volumes on a quantum chemical basis (QTAIM) provides detailed insights into the volume chemistry of La2Pd3(SixGe1-x)5. Through this analysis La is found to be responsible, together with the gradual substitution of Ge with Si, for the volume contraction

Polarity-extended 8 - Neff rule for semiconducting main-group compounds with the TiNiSi-type of crystal structure

Application of chemical bonding analysis in position-space techniques based on combined topological analysis of the electron density and electron-localizability indicator distributions has recently led to the formulation of a polarity-extended 8 - N-eff rule for consistent inclusion of quantum chemically obtained polar-covalent bonding data into the classical 8 - N scheme for main-group compounds. Previous application of this scheme to semiconducting main-group compounds of the cubic MgAgAs type of structure with 8 valence electrons per formula unit (8 ve per f.u.) has shown a covalent bonding tendency preferring one zinc blende type partial structure over the other one, which seems to corroborate the classical Lewis picture of maximally four covalent bonds per main-group element. In contrast to the MgAgAs type, the orthorhombic TiNiSi type of structure displays a much higher geometrical flexibility to incorporate different kinds of metal atoms. The analysis of polar-covalent bonding in semiconducting 8 ve per f.u. containing main-group compounds AA ' E of this structure type reveals a transition to non-Lewis type bonding scenarios of species E with up to ten polar-covalently bonded metal atoms. This kind of situation is consistently included into the extended 8 - N-eff type bonding scheme. A systematic increase of partially covalent bonding from chalcogenides E-16 to the tetrelides E-14 is found, summing up to as much as 2 covalent bonds E-14-A and E-14-A ', and correspondingly remaining 4 lone pair type electrons on species E-14. The familiar notion of this structure type consisting of a '[NiSi]'-type framework with 'Ti'-type atoms filling the voids cannot be supported for the compounds investigated

Unpredicted but It Exists: Trigonal Sc2Ru with a Significant Metal-Metal Charge Transfer

The Sc2Ru compound, obtained by high-temperature synthesis, was found to crystallize in a new trigonal hP45 structure type [space group P3\u305m1; a = 9.3583(9) \uc5 and c = 11.285(1) \uc5]: Ru@Sc8 cubes, Ru@Sc12 icosahedra, and uncommon Ru@Sc10 sphenocoronae are the building blocks of a unique motif tiling the whole crystal space. According to density functional theory studies, Sc2Ru is a metallic compound characterized by multicenter interactions: a significant charge transfer occurs from Sc to Ru, indicating an unexpectedly strong ionic character of the interactions between the two transition metals. Energy calculations support our experimental results in terms of stability of this compound, contributing to the recurrent discussion on the limits of the high-throughput first-principles calculations for metallic materials design