1,720,974 research outputs found
Study of new polar intermetallic compounds: synthesis, structural relations and real space chemical bonding analysis
No full textThe 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
The La2Pd3(Si, Ge)5 complete solid solution: Crystal structure, chemical bonding, and volume chemistry
No full textThe 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
Unpredicted but It Exists: Trigonal Sc2Ru with a Significant Metal-Metal Charge Transfer
No full textThe Sc2Ru compound, obtained by high-temperature synthesis, was found to crystallize in a new trigonal hP45 structure type [space group P3̅m1; a = 9.3583(9) Å and c = 11.285(1) Å]: 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
Topological coordination numbers and coordination reciprocity from electron-density distributions
No full textTriangulated surface data sets of quantum theory of atoms in molecules (QTAIM) interatomic surfaces have been employed to calculate solid angles subtended at the nuclear positions by each diatomic contact surface. On this basis, topological effective coordination numbers were evaluated. This corresponds to a generalization of the established Voronoi-Dirichlet partitioning (VDP) based procedure. The topological coordination number (tCN) approach developed includes coordination reciprocity requirements necessary to extract coordination-consistent sub-coordination scenarios for identification of chemically meaningful coordination numbers. The ranking between different sub-coordination scenarios is accomplished by weighting functions derived from purely geometrical properties of square and semicircle areas. Exemplary cases analyzed using theoretical electron-density distributions span the range from the face centered cubic, body centered cubic, hexagonal close packed and diamond types of element structures, to rocksalt, CsCl and zincblende types of structures, to compounds of the TiNiSi structure type. An important difference compared with VDP-based coordination numbers arises from the natural inclusion of the effect of different atomic sizes in the tCN approach. Even in highly symmetrical element structures, differences between VDP and tCN results are obtained as an effect of atomic electron-density decay utilizing still available degrees of freedom in the crystal structure. Especially in the TiNiSi type of examples, the advantage of numerically ranking between different sub-coordination scenarios of similar importance emerges. Instead of being obliged to choose only one of them, a more precise characterization contains a listing of different scenarios with their relative weights and associated effective coordination numbers. This seems to be generally the more appropriate way to analyze atomic coordination, especially in more complex structures such as intermetallic phases, opening up its possible use as input for AI applications on structure-property relationships. open access
rule for semiconducting main-group compounds with the TiNiSi-type of crystal structure
No full textApplication 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 − Neff 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 − Neff type bonding scheme. A systematic increase of partially covalent bonding from chalcogenides E16 to the tetrelides E14 is found, summing up to as much as 2 covalent bonds E14-A and E14-A′, and correspondingly remaining 4 lone pair type electrons on species E14. 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. © 2023 The Royal Society of Chemistry
Ge5 Clusters in the Trivalent Rare-Earth Compound Sm3Ge5
No full textThe compound Sm3Ge5adopts two modifications with Pearson symbols hP16 (AlB2-derivative) and oF64 (defect α-ThSi2-type) upon synthesis at ambient pressure. Synthesis at extreme conditions grants access to the modification oS32 (Pu3Pd5-type). High-pressure high-temperature treatment of prereacted element mixtures yields Pu3Pd5-type Sm3Ge5, space group Cmcm with lattice parameters a = 9.42813(9), b = 7.56296(7), and c = 9.67056(8) Å. The atomic arrangement refined from powder X-ray diffraction data is confirmed by transmission electron microscopy measurements. The crystal structure features Ge5square pyramidal units. The topology of the Electron Localizability Indicator (ELI-D) supports the formation of a bicyclo[1.1.1]pentagermanide cluster composed of two- and three-bonded Ge species, resulting in an electron balance comprising excess electrons. The bonding analysis in position space further reveals the presence of polar covalent interactions between both germanium and the rare-earth metal and among the Ge atoms constituting the base of the Ge5pyramidal units, pointing to a complex bonding scenario that is difficult to rationalize by electron counting rules. Sm3Ge5shows a metallic conductivity. Heat capacity and magnetization measurements indicate a 4f5electron configuration and thus the trivalent state of the Sm ions. The magnetic moments of Sm in Sm3Ge5order antiferromagnetically at 20.4 K
New Insights into the Crystal Chemistry of FeB-Type Compounds: The Case of CeGe
No full textSeveral alkaline earth or rare earth binary monosilicides and -germanides possess complex bonding properties, such as polycation formation exceeding the scope of classical electron counting rules. In this study, we present characterization by powder and single-crystal diffraction and thermal analysis of CeGe, one of the few monogermanides crystallizing in the FeB-type structure. Comparative computational investigations for structure types experimentally observed for monogermanides and alternative structures with different structural motifs were performed to gain energetical insights into this family of compounds, underlining the preference for infinite germanium chains over other structural motifs. Formation enthalpy calculations and structural chemical analysis highlight the special position of FeB-type compounds among the monogermanides
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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