110 research outputs found
Alloying behaviour of mg with transition elements and rare earth metals (R): the R-T-Mg systems (T=Ni, Cu, Ag, Au)
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The 400 °C Isothermal Section of the La-Co-Mg Ternary System
No full textThe isothermal section of the La-Co-Mg system at 400 °C was determined by characterization of about thirty ternary alloys synthesised by induction melting in sealed Ta crucibles and then annealed. Scanning electron microscopy (SEM) coupled with energy dispersive x-ray spectroscopy (EDXS) and x-ray powder diffraction (XRPD) were used to analyze microstructures, identify phases, measure their compositions and determine their crystal structures. Phase equilibria are characterized by the absence of ternary solid solutions and by the presence of three ternary phases. The existence and the crystal structure of the La4-xCoMg1 + x (τ1, 0 ≤ x ≤0.15, cF96-Gd4RhIn) were confirmed and its homogeneity region determined; the new phases La23-xCo7Mg4 + x (τ2, -0.50≤ x ≤0.60, hP68-Pr23Ir7Mg4) and ~La38Co55Mg7 (τ3, unknown crystal structure) were detecte
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
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
{Ca, Eu, Yb}<sub>23</sub>Cu<sub>7</sub>Mg<sub>4</sub> as a Step towards the Structural Generalization of Rare Earth-Rich Intermetallics
No full textThe 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
Crystal Chemistry of the New Families of Interstitial Compounds R6Mg23C (R = La, Ce, Pr, Nd, Sm, or Gd) and Ce6Mg23Z (Z = C, Si, Ge, Sn, Pb, P, As, or Sb)
No full textThe crystal chemical features of the new series of compounds R6Mg23C with R = La-Sm or Gd and Ce6Mg23Z with Z = C, Si, Ge, Sn, Pb, P, As, or Sb have been studied by means of single-crystal and powder X-ray diffraction techniques. All phases crystallize with the cubic Zr6Zn23Si prototype (cF120, space group Fm3m, Z = 4), a filled variant of the Th6Mn23 structure. While no Th6Mn23-type binary rare earth-magnesium compound is known to exist, the addition of a third element Z (only 3 atom %), located into the octahedral cavity of the Th6Mn23 cell (Wyckoff site 4a), stabilizes this structural arrangement and makes possible the formation of the ternary R6Mg23Z compounds. The results of both structural and topological analyses as well as of LMTO electronic structure calculations show that the interstitial element plays a crucial role in the stability of these phases, forming a strongly bonded [R6Z] octahedral moiety spaced by zeolite cage-like [Mg45] clusters. Considering these two building units, the crystal structure of these apparently complex intermetallics can be simplified to the NaCl-type topology. Moreover, a structural relationship between RMg3 and R6Mg23C compounds has been unveiled; the latter can be described as substitutional derivatives of the former. The geometrical distortions and the consequent symmetry reduction that accompany this transformation are explicitly described by means of the Bärnighausen formalism within group theory
The Y-Cu-Mg system in the 0-66.7 at % Cu concentration range: the isothermal section at 400°C
No full textSynthesis and characterization of about fifty alloys were performed in order to construct the isothermal
section of the Y–Cu–Mg ternary system at 400 C in the 0–66.7 at.% Cu concentration range. Scanning
electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS) and X-ray powder diffraction
(XRPD) techniques were used to examine microstructures, identify phases and define their compositions
and crystal structures. Phase equilibria in the investigated compositional region are characterized by the
absence of extended ternary solid solutions and by the presence of at least ten ternary phases. Crystal
structures of the previously reported Y2Cu2Mg, Y5Cu5Mg8, Y5Cu5Mg13, Y5Cu5Mg16 and YCuMg4 phases
were confirmed. A ternary phase with homogeneity range around the YCu4Mg stoichiometry was found,
crystallizing in the cF24–MgCu4Sn structure type; at 400 C this phase coexists with a ternary solid
solution based on the binary Laves phase Cu2Mg, which dissolves about 5 at.% Y. The equiatomic YCuMg
phase was also found to exist: from the analysis of X-ray powder patterns it is suggested to crystallize in
the hP9–ZrNiAl structure type (a= 0.74449(4) nm, c= 0.39953(2) nm). Two other stoichiometric ternary
phases were detected, of approximate compositions Y25Cu18Mg57 and Y13Cu9Mg78, whose crystal
structures are still unknown. In the Mg-rich region, a ternary phase forms characterized by a large
homogeneity region
Study of a doubly photochemically activated Paternò-Büchi Reaction through Design of Experiments
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