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- Spijksma, G. I., et al.
(author)
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Chemistry of 2,2,6,6,-tetramethyl-3,5-heptanedione (Hthd) modification of zirconium and hafnium propoxide precursors
- 2006
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In: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 45:13, s. 4938-4950
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Journal article (peer-reviewed)abstract
- The modification of different zirconium propoxide and hafnium propoxide precursors with 2,2,6,6,-tetramethyl-3,5-heptanedione (Hthd) was investigated by characterization of the isolated modified species. The complexes [Zr(O nPr)3(thd)]2, [Zr(OnPr)(O jPr)2(thd)]2, Zr(OjPr)(thd) 3, [Hf(OnPr)3(thd)]2, and Hf(O jPr)(thd)3 were isolated and characterized. The structure of the n-propoxide analogue of Zr(OjPr)(thd)3 could not be refined, but its existence was clearly demonstrated by XRD and 1H NMR. The modification of the propoxide precursors involves mono- and trisubstituted intermediate compounds and does not involve a disubstituted compound; thus, the commercial product that is claimed to be "Zr(O jPr)2(thd)2" and is most commonly used for the MOCVD preparation of ZrO2 does not exist. No evidence was found for the presence of such a compound in either zirconium- or hafnium-based systems. Formation of the dimeric hydroxo-di-thd-substituted complex, [Hf(OH)(OjPr)(thd)2]2, which could be isolated only for hafnium-based systems, occurs on microhydrolysis. All heteroleptic intermediates are eventually transformed to the thermodynamically stable Zr(thd)4 or Hf(thd)4. The compounds obtained from isopropoxide precursors showed a higher stability than those with n-propoxide ligands or a combination of both types. In addition, it is important to note that residual alcohol facilitates the transformation and strongly enhances its rate. The unusually low solubility and volatility of MIV(thd) 4 has been shown to be due to close packing and strong van der Waals interactions in the crystal structures of these compounds.
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| 2. |
- Ahniyaz, Anwar, et al.
(author)
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Preparation of iron oxide nanocrystals by surfactant-free or oleic acid-assisted thermal decomposition of a Fe(III) alkoxide
- 2008
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In: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853 .- 1873-4766. ; 320:6, s. 781-787
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Journal article (peer-reviewed)abstract
- A new non-hydrolytic, alkoxide-based route was developed to synthesize iron oxide nanocrystals. Surfactant-free thermal decomposition of the iron 2-methoxy-ethoxide precursors results in the formation of uniform iron oxide nanocrystals with an average size of 5.6 nm. Transmission electron microscope study shows that the nanocrystals are protected against aggregation by a repulsive surface layer, probably originating from the alkoxy-alkoxide ligands. Addition of oleic acid resulted in monodisperse nanocrystals with an average size of 4 nm. Mössbauer analysis confirmed that the nanocrystals mainly consisted of maghemite. Analysis of the magnetic hysteresis loop measurements and the zero field and field cooled measurements displayed an excellent fit to established theories for single-domain superparamagnetic nanocrystals and the size of the magnetic domains correlated well to the crystallite size obtained from transmission electron microscope.
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| 4. |
- Kessler, Richard, et al.
(author)
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First-Year Sloan Digital Sky Survey-II Supernova Results : Hubble Diagram and Cosmological Parameters
- 2009
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In: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 185:1, s. 32-84
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Journal article (peer-reviewed)abstract
- We present measurements of the Hubble diagram for 103 Type Ia supernovae (SNe) with redshifts 0.04 < z < 0.42, discovered during the first season (Fall 2005) of the Sloan Digital Sky Survey-II (SDSS-II) Supernova Survey. These data fill in the redshift "desert" between low- and high-redshift SN Ia surveys. Within the framework of the MLCS2K2 light-curve fitting method, we use the SDSS-II SN sample to infer the mean reddening parameter for host galaxies, RV = 2.18 ± 0.14stat ± 0.48syst, and find that the intrinsic distribution of host-galaxy extinction is well fitted by an exponential function, P(AV ) = exp(-AV /τV), with τV = 0.334 ± 0.088 mag. We combine the SDSS-II measurements with new distance estimates for published SN data from the ESSENCE survey, the Supernova Legacy Survey (SNLS), the Hubble Space Telescope (HST), and a compilation of Nearby SN Ia measurements. A new feature in our analysis is the use of detailed Monte Carlo simulations of all surveys to account for selection biases, including those from spectroscopic targeting. Combining the SN Hubble diagram with measurements of baryon acoustic oscillations from the SDSS Luminous Red Galaxy sample and with cosmic microwave background temperature anisotropy measurements from the Wilkinson Microwave Anisotropy Probe, we estimate the cosmological parameters w and ΩM, assuming a spatially flat cosmological model (FwCDM) with constant dark energy equation of state parameter, w. We also consider constraints upon ΩM and ΩΛ for a cosmological constant model (ΛCDM) with w = -1 and non-zero spatial curvature. For the FwCDM model and the combined sample of 288 SNe Ia, we find w = -0.76 ± 0.07(stat) ± 0.11(syst), ΩM = 0.307 ± 0.019(stat) ± 0.023(syst) using MLCS2K2 and w = -0.96 ± 0.06(stat) ± 0.12(syst), ΩM = 0.265 ± 0.016(stat) ± 0.025(syst) using the SALT-II fitter. We trace the discrepancy between these results to a difference in the rest-frame UV model combined with a different luminosity correction from color variations; these differences mostly affect the distance estimates for the SNLS and HST SNe. We present detailed discussions of systematic errors for both light-curve methods and find that they both show data-model discrepancies in rest-frame U band. For the SALT-II approach, we also see strong evidence for redshift-dependence of the color-luminosity parameter (β). Restricting the analysis to the 136 SNe Ia in the Nearby+SDSS-II samples, we find much better agreement between the two analysis methods but with larger uncertainties: w = -0.92 ± 0.13(stat)+0.10 -0.33(syst) for MLCS2K2 and w = -0.92 ± 0.11(stat)+0.07 -0.15 (syst) for SALT-II.
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| 5. |
- Nikonova, Olesya A, et al.
(author)
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Electrochemical Synthesis, Structural Characterization, and decomposition of Rhenium Oxoethoxide, Re4O4(OEt)12. Ligand Influence on the Structure and Bonding in the High-Valent Tetranuclear Planar Rhenium Alkoxide Clusters
- 2008
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In: Inorganic Chemistry. ; 47, s. 1295-1300
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Journal article (peer-reviewed)abstract
- Anodic oxidation of rhenium in ethanol in the presence of LiCl as a conductive additive results with high yield information of a new oxoethoxide cluster, Re4O4(OEt)12. The structure of the planar centrosymmetric metal–oxygencore of this molecule is composed of four edge-sharing Re(V)O6 octahedra. Eight electrons are available for theformation of metal-metal bonds indicated by five relatively short Re-Re distances within the Re4-rhombus, a“planar butterfly” type cluster. The theoretical calculations are indicating relatively low contribution of metal-metalbonding in the stability of the core. The stability of the +V-oxidation state, unusual for rhenium alkoxides can beat least partially attributed to the size effects in the packing of ligands. The X-ray powder study indicates thattreatment of Re4O4(OEt)12 in ambient atmosphere rapidly transforms it into a mixed-valence derivativeRe4O6(OEt)10swith a structure related to the earlier investigated cluster Re4O6(OiPr)10. Thermal decomposition ofthe latter rhenium oxoethoxide results in reduction to rhenium metal at as low temperatures as 380 °C, producingaggregates of metal nanoparticles with the average size of 3 nm.
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| 9. |
- Smith, D. L., et al.
(author)
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Lifetime measurements of the negative-parity 7(-) and 8(-) states in Cd-122
- 2008
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In: Physical Review C. Nuclear Physics. - 0556-2813 .- 1089-490X. ; 77:1, s. 014309-
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Journal article (peer-reviewed)abstract
- The Advanced Time-Delayed beta gamma gamma(t) method was used to measure lifetimes of selected high-spin states in Cd-122 populated from beta(-) decay of the J(pi)=(9(-)) isomer in Ag-122. From the gamma gamma coincidences, a new energy level was established at 2616.6 keV with a suggested spin-parity assignment of 8(-). Lifetimes were determined for the high-spin states at 2616.6 and 2502.7 keV as T-1/2=1.35(29) ns and 0.24(6) ns, respectively. The transition rates for gamma rays de-exciting the 7(-) states in the N=74 isotones of Cd-122, Sn-124, and Te-126 were found to be very similar.
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| 10. |
- Spijksma, Gerald I., et al.
(author)
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Nona-coordinated MO6N3 centers M = Zr, Hf as a stable building block for the construction of heterometallic alkoxide precursors
- 2007
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In: Inorganica Chimica Acta. - : Elsevier BV. - 0020-1693 .- 1873-3255. ; 360:6, s. 2045-2055
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Journal article (peer-reviewed)abstract
- The modification of zirconium or hafnium alkoxides with diethanolamine, H(2)dea, leads to the formation of unique nona-coordinated M{mu-eta(3)-NH(C2H4O)(2)}(3) cores. The mechanism is used to develop a self-assembly approach to the first thermodynamically stable zirconium-titanium and hafnium-titanium precursors, Zr{mu-eta(3)-NH(C2H4O)(2)}(3)[Ti((OPr)-Pr-i)(3)](2) (1) and Hf{mu-eta(3)-NH(C2H4O)(2))(3)[Ti((OPr)-Pr-i)(3)](2) (2). Mass spectrometric characterization of these compounds demonstrates their volatility. In addition to the solution stability of these compounds the volatility makes them attractive single source precursors for MOCVD and ALD applications. These precursors are also interesting candidates for application in sol-gel synthesis of microporous materials as the stability of the core prevents self-assembly of ligands on the outer surface of the primary particles formed during the hydrolysis. A n-propoxide analog of 1 can be prepared from zirconium n-propoxide but does not yield any crystalline material. It is demonstrated that 1 can be prepared from [Zr((OPr)-Pr-n)((OPr)-Pr-i)(3) ((PrOH)-Pr-i)](2), however, with a lower yield compared to the use of zirconium isopropoxide. The single crystals obtained from systems containing zirconium isopropoxide, titanium isopropoxide and triethanolamine H(3)tea turned out to be Ti-2((OPr)-Pr-i)(2)(mu-eta(4)-NH(C2H4O)(3)}(2))(2) (4). Theoretical calculations indicate that the octacoordinate M{mu-eta(4)-N(C2H4O)(3)}(2) core, anticipated in reaction with H(3)tea, will have metal-nitrogen bonds that are too long for its stabilization. This explains why the formation of 4 is thermodynamically favored over the formation of heterometallic species.
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