1,720,971 research outputs found
Solid state interactions in the La-Au-Mg system: phase equilibria, novel compounds and chemical bonding
No full textGold intermetallic chemistry is very rich, covering different classes of compounds ranging from the Hume-Rothery to Zintl phases to polar intermetallics to quasicrystals. Au's relativistic effects are frequently mentioned as responsible for the peculiar structural and physical properties of its compounds, nonetheless the aspects of chemical bonding are far to be clearly understood. In this work, the La-Au-Mg system was targeted for the discovery of new gold intermetallics and their structural and chemical bonding characterization. Studies on solid state interactions resulted in the construction of a partial La-Au-Mg isothermal section at 400 °C. The high reactivity between the constituents is reflected by the formation of five intermetallic compounds in the concentration range of less than 50 at% of Au. A complete crystallographic study was conducted for four of them, namely La1.82Au3+xMg14.36−x(0 ≤x≤ 0.90,hP42-3.64-CeMg10.3), La3Au4−xMg12+x(0 ≤x≤ 0.75,hP38-Gd3Ru4Al12), LaAuMg2(oS16-MgCuAl2) and LaAu1+xMg1−x(0 ≤x≤ 0.15,hP9-ZrNiAl). A unifying description based on the different stacking sequences of equal slabs along thec-axis is proposed for these intermetallics. Chemical bonding in LaAuMg2was studied by following the position space approach and including relativistic effects. Among the peculiarities of this LaMg2Au auride, there are two-atomic La-Au bonds showing a classical polar covalent character and that form distorted hexagonal planar layers and multi-atomic bonds involving Mg species. One of these is interpreted as a Mg-Mg bond supported by the neighbouring La and Au atoms, explaining the Mg reduced oxidation state (close to +1) in this compound
A machine learning approach for predicting formation enthalpy: A case study of Mackay-type approximants of icosahedral quasicrystals
No full textWe present a machine learning approach for evaluating thermodynamic properties within model compositional/configurational spaces (CCSs) of complex intermetallics with Mackay-type building units. Recently, by performing high-throughput density functional theory calculations, we determine and analyze the formation energies for the recently synthesized 1/1 approximant of icosahedral quasicrystals in the Sc-Pd system possessing structural disorder. Here, sequentially increasing training sets are used in regression models based on compositional and topological descriptors for predicting formation enthalpies. We investigate different learning strategies. The obtained R2 dependencies show higher learning rates for the models using real features (composition and contact counters of defects) with small training set sizes, whereas using categorical features (topological types of defect structure fragments) result in a slightly higher prediction quality when the training set size increases. In connection with the obtained results, the proposed approach is considered as a tool for investigations of disordering phenomena in complex intermetallics as well as for a subsequent searching for new stable phases of Mackay-type approximants of quasicrystals
Architecture of new {Ca, Eu, Yb}4CuMg complex intermetallics based on polyicosahedral clusters
No full textThe crystal structures of R4CuMg (R––Ca, Eu, Yb) complex intermetallics, solved by single crystal X-ray diffraction, were found to represent a new trigonal prototype (hR144-h7c2ba, space group R 3 m), being the first examples of 4:1:1 rare earth-rich phases not crystallizing as cF96-Gd4RhIn, adopted by more than 200 compounds
known so far. The divalent character of the R component, together with Cu atomic size could contribute to explain the observed structural peculiarity. Indeed, the occurrence of fragments typical of compositionally similar compounds, namely Cu-centered trigonal prisms and Mg-centered core-shell polyicosahedral clusters with R at vertices, prompted a search for significant structural relationships. In this work, a description of the rhombohedral R4CuMg crystal
structure is proposed as a linear intergrowth along the c-direction of the two types of slabs R9.5CuMg3.5 (parent type: hP28-kh2ca, SG 194) and R13Cu6Mg (parent type:hR60-b6a2, SG 160). The ratio of these slabs in the studied structure is 2:1, corresponding to the simple equation 6×R9.5CuMg3.5+3×R13Cu6Mg = 24×R4CuMg,normalized to the unit cell content. According to the proposed description, symmetry properties of the slabs (depicted by the p3m1 layer group) were used for screening of possible space groups for other potential members of the same/similar intergrowth families
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
RCuMg2 compounds (R = Dy–Tm, Lu): crystal structure, chemical bonding and magnetic properties
No full textThe RCuMg2 intermetallics (R = Dy-Tm, Lu) were found to crystallize in a new structure type (DyCuMg2, oP32, space group: Pmma, a = 13.5397(4), b = 3.7594(1), c = 13.7985(4) & Aring;). This unique crystal space is characterized by the presence of clusters formed by four Cu-centred trigonal prisms sharing edges to create a cubic-like cavity occupied by a Mg atom; 2D wavy slabs of these star-like fragments and of Mg with a bcc topology are alternated along the c-direction. An architecture based on similar moieties was recognized for the related R2Cu2Mg (tP10-Mo2FeB2) and RCuMg4 (oS48-TbCuMg4) compounds, proposing a generalization scheme. Electronic structure calculations and a chemical bonding analysis in the position space were applied to LuCuMg2, as a representative. The calculated effective charges indicated that Lu and Cu act as the QTAIM cation and anion, respectively, and the Mg species show a slight positive charge, except that at the centre of the bcc-fragment, showing a tiny negative one. In general, a complex bonding scenario was revealed, dominated by both hetero and homoatomic (within the Mg bcc-slabs) multicentre interactions. The temperature and field dependencies of magnetic susceptibility and specific heat capacity were measured for RCuMg2. The nonmagnetic LuCuMg2 phonon reference compound was identified as a Pauli paramagnet. The {Ho, Er, Tm}CuMg2 compounds ordered antiferromagnetically at critical temperatures between about 5 and 8 K, whereas DyCuMg2 revealed multiple magnetic transitions. The complexity of the magnetic behaviour of the studied compounds was indicated by the temperature evolution of magnetic entropy
Flux Growth, Crystal Structure, and Chemical Bonding of Yb2PdGe3, an AlB2 Superstructure within the Rare-Earth Series
No full textThe complete structure revision of the RE2PdGe3 (RE = rare-earth metal) series revealed that Yb2PdGe3 is the only AlB2 ordered superstructure. Good-quality single crystals of this compound were successfully grown from molten indium flux, enabling accurate single-crystal investigations. Yb2PdGe3 crystallizes with the Ce2CoSi3-type structure in the hexagonal space group P6/mmm (no. 191) with lattice parameters a = 8.468(1) Å and c = 4.0747(7) Å. This structure is a four-order derivative of AlB2, composed of planar ∞2[PdGe3] honeycomb layers spaced by Yb species, located at the center of Ge6 and Ge4Pd2 hexagons. A superconducting transition is observed below the critical temperature of 4 K. A divalent state of Yb is deduced from magnetic susceptibility measurements below room temperature, which indicate an almost nonmagnetic behavior. A charge transfer from Yb to Pd and Ge was evidenced by the Quantum Theory of Atoms in Molecules (QTAIM) effective charges; polar four-atomic Ge–Pd/Yb and two-atomic Pd–Yb bonds were observed from the ELI-D (electron localizability indicator), partial ELI-D, and ELI-D/QTAIM intersections. The bonding interactions between Ge atoms within regular Ge6 hexagons are found to be intermediate between single bonds, as in elemental Ge, and higher-order bonds in the hypothetic Ge6H6 and Ge66– aromatic molecules
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
Interaction of the components in the Ce-Cu-Zn ternary system at 200 °c
No full textThe isothermal section of the Ce-Cu-Zn phase diagram was constructed at 200 °C over the whole concentration range as a result of X-ray powder diffraction, wavelength dispersive spectrometry and electron probe microanalysis. The existence of one new ternary compound Ce2Cu 5-5.7Zn2-1.3 was observed at this temperature and its crystal structure was refined from X-ray single crystal data (structure type Ce2Ni5Zn2, space group R3̄m, Pearson code hR18, a = 0.4998(1)-0.5078(1) nm, c = 3.6924(5)-3.6987(4) nm). The homogeneity regions were determined for a number of solid solutions on the basis of binary phases. © 2005 Elsevier B.V. All rights reserved
Crystal Structure and Magnetism of Noncentrosymmetric Eu2Pd2Sn
No full textThe new intermetallic compound Eu2Pd2Sn has been investigated. A single crystal was selected from the alloy and was analyzed by single-crystal X-ray diffraction, revealing that this compound possesses the noncentrosymmetric Ca2Pd2Ge structure type being, so far, the only rare-earth-based representative. Bonding analysis, performed on the basis of DOS and (I)COHP, reveals the presence of strong covalent Sn-Pd bonds in addition to linear and equidistant Pd-Pd chains. The incomplete ionization of Eu leads to its participation in weaker covalent interactions. The magnetic effective moment, extracted from the magnetic susceptibility χ(T) is μeff = 7.87 μB, close to the free ion Eu2+ value (μeff = 7.94 μB). The maximum of χ(T) at TN ∼13 K indicates an antiferromagnetic behavior below this temperature. A coincident sharp anomaly in the specific heat CP(T) emerges from a broad anomaly centered at around 10 K. From the reduced jump in the heat capacity at TN a scenario of a transition to an incommensurate antiferromagnetic phase below TN followed by a commensurate configuration below 10 K is suggested
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