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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 72 (1992), S. 4232-4239 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An alternative procedure is outlined for the synthesis of high quality, fine grained PbMo6S8. As the grain size plays an important role on the densification process, which in turn has an influence on the magnitude of Jc, an attempt has been made to produce dense samples from such powders by "Hot Pressing.'' The effect of the hot-pressing temperature on the superconducting, crystallographic, microstructural, and grain boundary characteristics of the ternary compound was evaluated. Scanning electron micrographs and ac-susceptibility measurements indicate that hot pressing (1000–1200 °C) improves the grain connection as a consequence of better densification. However, at higher temperatures (1250–1400 °C) it also precipitates MoS2 as an additional phase. Calorimetric data indicate a continuous broadening, as a function of hot-pressing temperature, of the specific heat jump at Tc. Preliminary investigation on the Tc distribution of the samples shows a progressive degradation, as indicated by a smearing in Tc down at least to 8 K. The deterioration was examined using Auger electron spectroscopy and the results suggest possible compositional variations rather than oxygen defects in the phase. The origin of such behavior is examined on the basis of nonstoichiometry or chemical heterogeneity at the grain surface. In addition, grain boundary contaminants and their role on the superconducting properties are considered. Finally, the often encountered problem of transport Jc limitation in these materials is discussed in terms of interconnectivity of the grains, phases, the presence of secondary phases, impurities, inhomogeneities, and the grain boundary phases.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 0044-2313
    Keywords: Rhenium complexes ; Nitrido compounds ; Mixed-ligand complexes ; X-ray crystal structure ; Ligand exchange ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Mixed-Ligand Complexes of Rhenium IV. The Reaction of [ReNCl2(Me2PhP)3] with Dithiocarbamates. X-Ray Crystal Structures of trans-Chloro-dimethyldithiocarbamato-bis(dimethylphenylphosphine) nitridorhenium(V), [ReN(Cl)(Me2PhP)2(Me2dtc)], and Bis(diethyldithiocarbamato)(dimethylphenylphosphine)nitridorhenium(V), [ReN(Cl)(Me2PhP)(Et2dtc)2][ReNCl2(Me2PhP)3] reacts with dialkyldithiocarbamates, R2dtc-, under a stepwise ligand exchange. Final products of these reactions are the well-known [ReN(R2dtc)2] bischelates. Intermediatelly, however, complexes of the general formulae [ReN(Cl)(Me2PhP)2(R2dtc)] and [ReN(Me2PhP)(R2dtc)2] can be isolated. Representatives have been structurally characterized.[ReN(Cl)(Me2PhP)2(Me2dtc)] crystallizes monoclinic in the space group P21/c, Z = 4. The dimensions of the unit cell are a = 13.071(3); b = 11.622(1); c = 15.667(3) Å; β = 97.09(1)°. The rhenium atom has a distorted octahedral environment; the Re≡N bond length is 1.71(1) Å. The Re—Cl bond distance is markedly lengthened (2.665(2) Å) as a consequence of the strong trans labilizing influence of the coordinated nitrido ligand.[ReN(Me2PhP)(Et2dtc)2] crystallizes monoclinic in the space group P21/c, Z = 4, a = 17.262(3); b = 14.915(2); c = 9.888(2); β = 76.35(8)°. The equatorial coordination sphere is occupied by one phosphorus atom and three sulphur atoms. One of the dithiocarbamate ligands is coordinated bidentately; the second one with two distinct Re—S bond lengths. The Re—S(4) distance is 2.7983(2) Å which can be discussed as a weak interaction with the metal.
    Notes: Dichlorotris(dimethylphenylphosphan)nitridorhenium(V), [ReNCl2(Me2PhP)3], reagiert mit Dialkyldithiocarbamaten, R2dtc-, unter schrittweisem Ligandenaustausch. Als Endprodukte entstehen Komplexe der allgemeinen Formel [ReN(R2dtc)2]. Als Intermediate dieser Reaktion konnten Komplexe der Zusammensetzung [ReN(Cl)(Me2PhP)2(R2dtc)] und [ReN(Me2PhP)(R2dtc)2] isoliert und strukturell charakterisiert werden.[ReN(Cl)(Me2PhP)2(Me2dtc)] kristallisiert monoklin in der Raumgruppe P21/c, Z = 4. Die Elementarzelle hat die Abmessungen a = 13,071(3); b = 11,622(1); c = 15,667(3) Å; β = 97,09(1)°. Rhenium befindet sich in einer verzerrt oktaedrischen Koordinationsumgebung. Der Re≡N-Bindungsabstand beträgt 1,71(1) Å. Die trans-ständige Re—Cl-Bindung ist infolge des trans-Effektes des koordinierten Nitridoliganden stark aufgeweitet (2,665(2) Å).[ReN(Me2PhP)(Et2dtc)2] kristallisiert monoklin in der Raumgruppe P21/c, Z = 4, a = 17,262(3); b = 14,915(2); c = 9,888(2); β = 76,35(8)°. Die äquatoriale Koordinationssphäre ist mit einem Phosphor- und drei Schwefelatomen besetzt. Ein Dithiocarbamat-Ligand ist als „normales“ Chelat koordiniert; der andere ist mit zwei sehr unterschiedlichen Re—S-Abständen gebunden, wobei auch für das zweite Schwefelatom (Abstand Re—S(4): 2,7983(2) Å) Wechselwirkungen mit dem Metall diskutiert werden können.
    Additional Material: 2 Ill.
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  • 3
    ISSN: 0044-2313
    Keywords: Nitrido Compounds, Nitrenes, Rhenium complexes, Metal induced condensation, X-ray diffraction ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Mixed-ligand Complexes of Rhenium. V. The Formation of Nitrene Complexes by Condensation of Acetone at Coordinated Nitrido Ligands. Syntheses and Structures of fac-[Re{NC(CH3)2CH2C(O)CH3}X3(Me2PhP)2] Complexes (X = Cl, Br)The reaction of rhenium(V)-mixed-ligand complexes of the general formula [ReN(Cl)(Me2PhP)2(R2tcb)] (HR2tcb = N—(N,N-dialkylthiocarbamoyl)benzamidine) with HCl or HBr in acetone initializes a condensation of the solvent and results in nitrene-like compounds as a consequence of a nucleophilic attack of the coordinated nitrido ligand on the condensed acetone. The chelate ligands are removed during this reaction and complexes of the type fac-[Re{NC(CH3)2CH2C(O)CH3}X3(Me2PhP)2] (X = Cl, Br) are formed.fac-[Re{NC(CH3)2CH2C(O)CH3}Cl3(Me2PhP)2] crystallizes triclinic in the space group P1, a = 8.575(4); b = 9.088(3); c = 18.389(9) Å; α = 75.67(3)°, β = 85.30(3)°, γ = 70.58(4)°; Z = 2. A final R value of 0.031 was obtained on the basis of 6011 independent reflections with I ≥ 2σ(I). Rhenium is coordinated in a distorted octahedral environment with the three chloro ligands in facial positions. The rhenium-nitrogen bond (1,68(1) Å) is only slightly longer than typical Re—N bonding distances in nitrido complexes.fac-[Re{NC(CH3)2CH2C(O)CH3}Br3(Me2PhP)2] is isomorphous with the chloro complex. Triclinic cell with a = 8.625(4); b = 9.198(3); c = 18.581(5) Å; α = 75.62(3)°, β = 85.40(3)°, γ = 70.91(3)°; Z = 2. The R value converged at 0.049 on the basis of 3644 independent reflections with I ≥ 2σ(I). fac-[Re{NC(CH3)2CH2C(O)CH3}Cl3(Me2PhP)2] as well as fac-[Re{NC(CH3)2CH2C(O)CH3}Br3(Me2PhP)2] crystallizes in the noncentrosymmetric space group P1.
    Notes: Die Umsetzung von Rhenium(V)-Gemischtligandkomplexen des Typs [ReN(Cl)(Me2PhP)2(R2tcb)] (HR2tcb = N—(N,N-Dialkylthiocarbamoyl)benzamidin) mit Halogenwasserstoffsäuren in Aceton führt zur Kondensation des Lösungsmittels und zur Ausbildung nitrenartiger Komplex-verbindungen durch Reaktion des Kondensationsproduktes mit dem nukleophilen Nitrid - Stickstoff. Die Chelatliganden werden im Verlauf der Reaktion abgespalten und es entstehen Komplexe der allgemeinen Zusammensetzung fac-[Re{NC(CH3)2CH2C(O)CH3}X3(Me2PhP)2] (X = Cl, Br).fac-[Re{NC(CH3)2CH2C(O)CH3}Cl3(Me2PhP)2] kristallisiert triklin in der Raumgruppe P1, a = 8,575(4); b = 9,088(3); c = 18,389(9) Å; α = 75,67(3)°, β = 85,30(3)°, γ = 70,58(4)°; Z = 2. Mit 6011 unabhängigen Reflexen I ≥ 2σ(I) konnte ein R-Wert von 0,031 erreicht werden. Rhenium befindet sich in einer verzerrt oktaedrischen Umgebung mit facial angeordneten Chloro-Liganden. Die Rhenium - Stickstoff-Bindung ist mit 1,68(1) Å nur geringfügig größer als typische Re—N-Abstände in Nitridokomplexen.fac-[Re{NC(CH3)2CH2C(O)CH3}Br3(Me2PhP)2] ist isostrukturell mit dem Chlorokomplex. Trikline Elementarzelle mit a = 8,625(4); b = 9,198(3); c = 18,581(5) Å; α = 75,62(3)°, β = 85,40(3)°, γ = 70,91(3)°; Z = 2. Der R-Wert konvergierte gegen 0,049 unter Verwendung von 3 644 unabhängigen Reflexen mit I ≥ 2σ(I). Ebenso wie fac-[Re{NC(CH3)2CH2C(O)CH3}Cl3(Me2PhP)2] kristallisiert fac-[Re{NC(CH3)2CH2C(O)CH3}Br3(Me2PhP)2] in der nicht-zentrosymmetrischen Raumgruppe P1.
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  • 4
    ISSN: 0044-2313
    Keywords: Thionitrosyl Compounds ; Rhenium Complexes ; X-Ray diffraction ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Mixed-ligand Complexes of Rhenium. VI. Synthesis and X-Ray Structures of the Rhenium Thionitrosyl Complexes mer-[Re(NS)Cl2(Me2PhP)3] · CH2Cl2 and trans-[Re(NS)Cl3(Me2PhP)2]mer-Dichlorotris(dimethylphenylphosphine)(thionitrosyl)rhenium(I), mer-[Re(NS)Cl2(Me2PhP)3], and trans-Trichlorobis(dimethylphenylphosphine)(thionitrosyl)rhenium(II), trans-[Re(NS)Cl3(Me2PhP)2], are formed during the reaction of rhenium(V) mixed-ligand complexes of the general formula [ReN(Cl)(Me2PhP)2(R2tcb)] with disulphur dichloride (HR2tcb = N-(N,N-dialkylthiocarbamoyl)benzamidine). The chelating ligands are substituted during the reaction.mer-[Re(NS)Cl2(Me2PhP)3] crystallizes monoclinic in the space group P21/n. The dimensions of the unit cell are a = 8.854(2); b = 31.295(3); c = 11.981(3) Å; β = 108.14(1)°; Z = 4. A final R value of 0.033 was achieved on the basis of 5 387 reflections with I ≥ 3σ(I). The rhenium atom is coordinated in a distorted octahedral environment. The Me2PhP ligands are arranged meridionally cis to the linear thionitrosyl group.trans-[Re(NS)Cl3(Me2PhP)2] crystallizes in the monoclinic space group C2/c with an unit cell of the dimensions a = 33.320(9); b = 8.446(1); c = 17.28(5) Å; β = 116.09(1)°, Z = 8. The R value converged at 0.026 on the basis of 5 460 independent reflections. The metal is octahedrally coordinated with the phosphine ligands in trans position to each other. The angle Re—N—S is 175.7(3)°.
    Notes: Die Rhenium—Thionitrosylkomplexe mer-Dichlorotris(dimethylphenylphosphan)(thionitrosyl)rhenium(I), mer-[Re(NS)Cl2(Me2PhP)3], und trans-Trichlorobis(dimethylphenylphosphan)(thionitrosyl)rhenium(II), trans-[Re(NS)Cl3(Me2PhP)2], entstehen bei der Umsetzung von Rhenium(V)-Gemischtligandkomplexen des Typs [ReN(Cl)(Me2PhP)2(R2tcb)] mit Dischwefeldichlorid (HR2tcb = N-(N,N-dialkylthiocarbamoyl)benzamidin). Die Chelatliganden werden im Verlauf der Reaktion abgespalten.mer-[Re(NS)Cl2(Me2PhP)3] kristallisiert monoklin in der Raumgruppe P21/n. Die Abmessungen der Elementarzelle sind a = 8,854(2); b = 31,295(3); c = 11,981(3) Å; β = 108,14(1)°; Z = 4. Auf der Grundlage von 5 387 unabhängigen Reflexen mit I ≥ 3σ(I) konnte ein R-Wert von 0,033 erreicht werden. Das Rhenium-Atom befindet sich in einer verzerrt oktaedrischen Koordinationsumgebung mit meridional angeordneten Me2PhP-Liganden. Die lineare Thionitrosylgruppierung befindet sich dazu in cis-Stellung.trans-[Re(NS)Cl3(Me2PhP)2] kristallisiert in der monoklinen Raumgruppe C2/c mit einer Elementarzelle der Abmessungen a = 33,320(9); b = 8,446(1); c = 17,281(5) Å; β = 116,09(1)°, Z = 8. Der R-Wert konvergierte gegen 0,026 unter Verwendung von 5 460 unabhängigen Reflexen. Das Metall-Atom ist oktaedrisch koordiniert, wobei sich die beiden Phosphan-Liganden zueinander in trans-Position befinden. Der Winkel Re—N—S beträgt 175,7(3)°.
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