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Sökning: swepub > Umeå universitet > Refereegranskat > (2000-2004) > Sundqvist Bertil

  • Resultat 1-10 av 56
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1.
  • Aleksandrovskii, A. N., et al. (författare)
  • Thermal expansion of single-crystal fullerite C60 at helium temperatures
  • 2000
  • Ingår i: Low temperature physics (Woodbury, N.Y., Print). - : AIP Publishing. - 1063-777X .- 1090-6517. ; 26:1, s. 75-80
  • Tidskriftsartikel (refereegranskat)abstract
    • The thermal expansion of single-crystal fullerite C60 has been studied in the range of liquid-helium temperatures (2–10 K). At temperatures below ~4.5 K the thermal expansion of fullerite C60 becomes negative, in agreement with the previous results on polycrystalline materials. A qualitative explanation of the results is proposed.
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2.
  • Soldatov, Alexander, et al. (författare)
  • Topochemical polymerization of C70 controlled by monomer crystal packing
  • 2001
  • Ingår i: Science. - : American Association for the Advanement of Science. - 0036-8075 .- 1095-9203. ; 293:5530, s. 680-683
  • Tidskriftsartikel (refereegranskat)abstract
    • Polymeric forms of C60 are now well known, but numerous attempts to obtain C70 in a polymeric state have yielded only dimers. Polymeric C70 has now been synthesized by treatment of hexagonally packed C70 single crystals under moderate hydrostatic pressure (2 gigapascals) at elevated temperature (300°C), which confirms predictions from our modeling of polymeric structures of C70. Single-crystal x-ray diffraction shows that the molecules are bridged into polymeric zigzag chains that extend along the c axis of the parent structure. Solid-state nuclear magnetic resonance and Raman data provide evidence for covalent chemical bonding between the C70 cages.
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3.
  • Sundqvist, Bertil, et al. (författare)
  • Structure and stability of pressure polymerized C60
  • 2000
  • Ingår i: Molecular Materials: Molecular Crystals and Liquid Crystals Section C, vol. 13. - : Overseas Publishers Association N.V.. ; 13:1-4, s. 117-122
  • Konferensbidrag (refereegranskat)abstract
    • We discuss the bond energy of the orthorhombic C60 polymer and the structure of the two-dimensional C60 polymers. For the orthorhombic polymer, measurements of the dissociation energy by different methods give results that differ by a factor of two. We show that optical excitations lead to a temporary weakening of the intermolecular bonds and optical measurements thus show a low apparent bond energy. We have also polymerized a single crystal of C60 into two-dimensional phases. X-ray diffraction studies of this crystal have enabled us to determine the stacking sequences of both the tetragonal and the rhombohedral structures.
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4.
  • Wågberg, Thomas, et al. (författare)
  • 2D polymerization and doping of fullerenes under pressure
  • 2000
  • Ingår i: High Pressure Research vol. 18. - : Gordon & Breach. ; 18:1-6, s. 139-143
  • Konferensbidrag (refereegranskat)abstract
    • Tetragonal polymeric C60 has been studied by Raman spectroscopy and other methods. Attempts have been made to transform samples from the tetragonal to the orthorhom-bic phase and vice versa. The results suggest that the transformation is direct with no intermediate stage with free molecules. Tetragonal C60 has also been intercalated by potassium metal.
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5.
  • McRae, Edward, et al. (författare)
  • Raman light scattering and c-axis resistivity evidence for a pressure-induced stage transformation in PdAl2Cl8 intercalated graphite.
  • 2000
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 62:20, s. 13757-13766
  • Tidskriftsartikel (refereegranskat)abstract
    • We have examined several samples of first- to third-stage PdAl2Cl8-intercalated graphite under hydrostatic pressures up to 1 GPa. In stage-1 highly oriented pyrolytic graphite–(HOPG) and single-crystal-graphite-based materials, the c-axis resistivity decreases sharply above a few kilobars; pressure release induces a reversible return to the initial value only in the case of the latter sample. Raman spectra taken in situ under pressure on a HOPG-based material show similarly irreversible effects. Analysis of the spectra taken on higher-stage samples leads to the conclusion that hydrostatic pressure beyond a few kilobars increases the density of the intercalate within the graphitic galleries, transforming the initial sample to a higher-stage material. Since there is no loss of intercalate, the overall intercalate-to-host charge transfer remains constant so that the Raman frequency is approximately the same for both first- and second-stage products. This is an unusual situation in which there is thus an apparent lack of Raman signature in spite of the stage change.
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6.
  • Aleksandrovskii, A. N., et al. (författare)
  • Effect of argon on the thermal expansion of fullerite C60 at helium temperatures
  • 2001
  • Ingår i: Low temperature physics (Woodbury, N.Y., Print). - : American Institute of Physics. - 1063-777X .- 1090-6517. ; 27:3, s. 245-246
  • Tidskriftsartikel (refereegranskat)abstract
    • The linear thermal expansion of compacted Ar-doped fullerite C60(ArxC60) is investigated at 2–12 K using a dilatometric method. The thermal expansion of ArxC60 is also studied after partial desaturation of argon from fullerite. It is revealed that argon doping resulted in a considerable change of the temperature dependence of the thermal expansion of fullerite. An explanation of the observed effects is proposed.
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7.
  • Aleksandrovskii, A. N., et al. (författare)
  • Low-temperature thermal expansion of pure and inert gas-doped fullerite C60
  • 2003
  • Ingår i: Fizika Nizkikh Temperatur (Kharkov). - : AIP Publishing. - 0132-6414 .- 1816-0328. ; 29:4, s. 432-442
  • Tidskriftsartikel (refereegranskat)abstract
    • The low temperature (2–24 K) thermal expansion of pure (single-crystal and polycrystalline) C60 and polycrystalline C60 intercalated with He, Ne, Ar, and Kr has been investigated using the high-resolution capacitance dilatometer. The investigation of the time dependence of the sample length variations deltaL(t) on heating by deltaT shows that the thermal expansion is determined by the sum of positive and negative contributions, which have different relaxation times. The negative thermal expansion usually prevails at helium temperatures. The positive expansion is connected with the phonon thermalization of the system. The negative expansion is caused by reorientation of the C60 molecules. It is assumed that the reorientation is of a quantum character. The inert gas impurities affect the reorientation of the C60 molecules very strongly, especially at liquid helium temperatures. A temperature hysteresis of the thermal expansion coefficient of Kr– and He–C60 solutions has been revealed. The hysteresis is attributed to orientational polyamorphous transformation in these systems.
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8.
  • Aleksandrovskii, A.N., et al. (författare)
  • Thermal expansion of fullerite C60 alloyed with argon and neon
  • 2001
  • Ingår i: Low temperature physics (Woodbury, N.Y., Print). - Woodbury : American Institute of Physics. - 1063-777X .- 1090-6517. ; 27:12, s. 1033-1036
  • Tidskriftsartikel (refereegranskat)abstract
    • The linear thermal expansion of compacted fullerite C60 alloyed with argon (ArxC60) and neon (NexC60) are investigated by a dilatometric method. The experimental temperature is 2–12 K. In the same temperature interval the thermal expansion of ArxC60 and NexC60 are examined after partial desaturation of the gases from fullerite. It is found that Ar and Ne alloying affects the temperature dependence of the thermal expansion coefficient of C60 quite appreciably. The libration and translation contributions to the thermal expansion of pure C60 are separated. The experimental results on the thermal expansion are used to obtain the Debye temperature of pure C60. The effects observed are tentatively interpreted.
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9.
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10.
  • Buga, Sergei G., et al. (författare)
  • Dissociation energy of 3D-polymeric C60 : Calorimetric study and structural analysis
  • 2001
  • Ingår i: Electronic Properties of Novel Materials - Science and Technology of Molecular Nanostructures. - Melville, NY : American Institute of Physics. - 0735400334 ; , s. 49-53
  • Konferensbidrag (refereegranskat)abstract
    • Annealing of 2D- and 3D-polymeric C[sub60] fullerene obtained under pressures of 9.5 and 12.5 GPa and temperatures of 670 and 770 K has been investigated by DSC in the range 240–640 K. An endothermal heat effect was observed with a peak maximum just below 540 K, a temperature characteristic for breakdown of (2+2) intermolecular links in dimers, 1D and 2D polymers. Exothermal effects, starting from 380 K, were observed for the first time in polymeric fullerenes. These effects are attributed to relaxation processes and to breakdown of other types of intermolecular bonds such as common four-sided rings and (3+3) interlinks.
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  • Resultat 1-10 av 56

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