1,720,974 research outputs found
The 500 °C Isothermal Section of the Al-Fe-Pr Ternary System
No full textPhases and equilibria involved in the isothermal section at 500 degrees C of the ternary Al-Fe-Pr system have been here investigated in the whole composition range by means of X-ray diffraction, optical and scanning electron microscopy, and electron probe microanalysis. Phase equilibria are characterised by the formation of two binary solid solutions, Pr2(FexAl1-x)17 with 0.34 <= x <= 1.00 (hR57-Th2Zn17) and Pr(FexAl1-x)2 with 0 <= x <= 0.20 (cF24-Cu2Mg), and four ternary compounds, tau 1-PrFe2Al10 (oS52-YbFe2Al10), tau 2-PrFe2Al8 (oP44-CeFe2Al8), tau 3-Pr(FexAl1-x)12 with 0.28 <= x <= 0.44 (tI26-ThMn12) and tau 4-Pr6(FexAl1-x)14 with 0.63 <= x <= 0.81 (tI80-La6Co11Ga3), which have been confirmed. The solid solubility ranges of the binary and ternary compounds have been considered and trends of their lattice parameters studied. The complete isothermal section has been obtained and the general features of the section are discussed and compared with those of other ternary systems of light R (La, Ce, Pr, Nd, Sm) with Al and Fe
Cs3LiGe4, One Compound with Two Complementary Structural Descriptions: Isolated [Ge4]4- Tetrahedral Clusters Coordinated by Li+ and Cs+ Cations or One-Dimensional [LiGe4]3- Polyanions Packed within a Matrix of Cs+ Cations?
No full textReported are the synthesis and structural characterization of Cs3LiGe4, the first structurally characterized lithium-containing cesium germanide. Single-crystal X-ray diffraction data indicate that Cs3LiGe4 crystallizes in an orthorhombic crystal system with the space group Cmcm (no. 63, Person symbol oS32) with unit cell parameters a = 6.950(2) & Aring;, b = 15.503(3) & Aring;, and c = 9.919(2) & Aring; and V = 1068.65(4) & Aring;3. The structure consists of [Ge]4 4- tetrahedral clusters with the Li atoms positioned in such a way that polyanionic [LiGe]4 3- chains could be considered as well. Electronic structure calculations indicate an intrinsic semiconductor with a band gap of 0.76 eV. To understand the nuances of the chemical bonding, position-space techniques based on the quantum theory of atoms in molecules, the electron localizability indicator, and their basin intersections were employed confirming the covalent character of the Ge-Ge bonding. The strong polarity of the interactions between [Ge]4 4- and the surrounding lithium and cesium cations suggests to interpret these as mainly ionic, further supporting the most basic structure rationalization [Cs+]3[Li+][Ge]4 4-, following the Zintl-Klemm concept. The electron localizability indicator topology affirms the striking similarities between [Ge]4 4- and molecular As4 tetrahedra in yellow arsenic, further supporting Klemm's pseudoatom concept on a quantum chemical basis
The Missing Link in the Monogermanide Series: YbGe
No full textHigh-pressure, high-temperature synthesis at 12 GPa between 750 and 1000 °C for 30 to 300 min yields the last missing rare-earth metal monogermanide, YbGe. Powder and single-crystal X-ray diffraction measurements reveal that the compound crystallizes in a FeB-type structure (space group Pnma, a=7.901(2) Å, b=3.8981(9) Å, and c=5.873(2) Å). The results of the chemical bonding study, while supporting the presence of polyanionic Ge chains interacting with the surrounding Yb through multi-atomic polar bonds, suggest a transitional scenario between the monogermanides formed by alkaline-earth elements and those formed by trivalent rare-earth metals
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
The 500 °C Isothermal Section of the Al-Co-Nd Ternary System
No full textThe Al-Co-Nd isothermal section at 500 degrees C has been investigated in the whole composition range by means of x-ray diffraction, optical and scanning electron microscopy and electron probe microanalysis. The following ternary phases have been identified or confirmed: tau(1)-Nd4CoAl (cF96-Gd4RhIn), tau(2)-NdCo2Al8 (oP44-CeFe2Al8), tau(3)-NdCo3Al11 (unknown), tau(4)-Nd2Co3Al9 (oS56-Y2Co3Ga9) tau(5)-Nd5Co2Al2 (unknown), tau(6)-Nd2Co6Al19 (mS108 U2Co6Al19), tau(7)-Nd7Co6Al7 (tP40-Pr7Co6Al7), tau(8)-Nd2Co2Al (oI10-W2CoB2), tau(9)-Nd13Co14-xAl4+x (0 <= x <= 0.7, oS28-W3CoB3). The complex system of phase equilibria linking the large number of binary and ternary Al-Co-Nd phases at 500 degrees C has been completely determined. The general characteristics of the section have been discussed in comparison with those of other Al-Co-R systems
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
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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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