| 1. |
- Aleksandrovskii, A. N., et al.
(author)
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Negative thermal expansion of fullerite C60 at helium temperatures
- 1997
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In: Low temperature physics (Woodbury, N.Y., Print). - : AIP Publishing. - 1063-777X .- 1090-6517. ; 23:11, s. 943-946
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Journal article (peer-reviewed)abstract
- The thermal expansion of fullerite C60 has been measured in the temperature range 2–9 K. A compacted fullerite sample with a diameter of about 6 mm and height of 2.4 mm was used. It was found that at temperatures below ~ 3.4 K the linear thermal expansion coefficient becomes negative. At temperatures above 5 K our results are in good agreement with the available literature data. A qualitative explanation of the results is proposed
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| 2. |
- Andersson, Ove, et al.
(author)
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Reorientational relaxation in C60 following a pressure induced change in the pentagon/hexagon equilibrium ratio
- 1995
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In: Physics Letters A. - : Elsevier Science B.V.. - 0375-9601 .- 1873-2429. ; 206:3-4, s. 260-264
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Journal article (peer-reviewed)abstract
- The orientational structure of C60 depends on pressure and temperature. Pressurization below the glass transition temperature Tg can freeze in non-equilibrium orientational structures. The relaxation of such structures on heating through Tg has been studied through thermal conductivity measurements and the effects observed are explained in a simple model.
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| 3. |
- Lundin, Anders, et al.
(author)
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Compressibility and Structure of C70
- 1995
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In: Europhysics letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 30:8, s. 469-474
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Journal article (peer-reviewed)abstract
- The compressibility of C70 has been investigated by means of direct piston and cylinder measurements in the temperature range 150 to 365 K and up to 1 GPa. At 296 and 343 K we find a very rapid change of volume with pressure below 0.15 GPa. We tentatively interpret this as a continuous molecular reorientation with pressure, probably resulting in a transformation from a mixture of face-centred cubic (f.c.c.) and rhombohedral (r.h.) phases to mainly pure r.h. phase. At 365 K as well as at 236 K and below we see no anomalies indicating any structural or rotational anomalies in the pressure range investigated. The zero-pressure bulk modulus decreases with increasing temperature, from 13.1 GPa at 185 K to 7.9 GPa at 365 K. Our volumetric measurements confirm a recently proposed phase diagram for C70.
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| 4. |
- Lundin, Anders, et al.
(author)
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Compressibility of C61D2 up to 1 GPa in the temperature range 175 - 345 K
- 1996
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In: Carbon. - : Elsevier Science Ltd.. - 0008-6223 .- 1873-3891. ; 34:9, s. 1119-1121
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Journal article (peer-reviewed)abstract
- We have measured the bulk modulus K for C61D2 up to 1 GPa in the temperature range 175–343 K. For face-centered cubic C61D2 above 290 K, we find an anomalously low value for K below about 0.15 GPa, possibly indicating pressure-induced changes in the structure. The (extrapolated) zero-p bulk modulus K(0) decreases with increasing T from 6.7 GPa at 175 K to 5.2 GPa at 343 K. A comparison with hypothetical expanded f.c.c. C60 with the same lattice constant shows that K(0) values are similar, indicating that the main intermolecular interactions are still between molecular bellies, not the sidegroups.
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| 5. |
- Nagel, Peter, et al.
(author)
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C60 one- and two-dimensional polymers, dimers, and hard fullerite: Thermal expansion, anharmonicity, and kinetics of depolymerization
- 1999
-
In: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 60:24, s. 16920-16927
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Journal article (peer-reviewed)abstract
- We report on high-resolution thermal expansion measurements of high-temperature-pressure treated C60 [one-dimensional (1D) and (2D) polymers and “hard fullerite”], as well as of C60 dimers and single crystal monomer C60 between 10 and 500 K. Polymerization drastically reduces the thermal expansivity from the values of monomeric C60 due to the stronger and less anharmonic covalent bonds between molecules. The expansivity of the “hard” material approaches that of diamond. The large and irreversible volume change upon depolymerization between 400 and 500 K makes it possible to study the kinetics of depolymerization, which is described excellently by a simple activated process, with activation energies of 1.9±0.1 eV (1D and 2D polymers) and 1.75±0.05 eV (dimer). Although the activation energies are very similar for the different polymers, the depolymerization rates differ by up to four orders of magnitude at a given temperature, being fastest for the dimers. Preliminary kinetic data of C70 polymers are presented as well.
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| 6. |
- Persson, Per-Axel, et al.
(author)
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NMR and Raman characterization of pressure polymerized C-60
- 1996
-
In: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 258:5-6, s. 540-546
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Journal article (peer-reviewed)abstract
- Bulk C60 has been treated at 1.1 GPa and 550–585 K, producing a dense insoluble material which on heating to above 600 K reverts to normal C60. Raman and IR studies on modified material show a large number of new lines, and the Raman pentagon pinch mode shifts from 1469 to 1458 cm−1 as on photopolymerization. MAS NMR shows one broadened line at the original C60 shift 144 ppm and a small peak at about 77 ppm due to the bridging carbons. None of the new resonances observed for C60 polymerized by other methods were observed. The results verify previously suggested polymeric structures where the fullerence cages are connected by four-membered rings.
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| 7. |
- Persson, Per-Axel, et al.
(author)
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The physical properties of high-pressure polymerized C60
- 1997
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In: Journal of Physics and Chemistry of Solids, volume 58, issue 11. - : Elsevier B.V.. ; 58:11, s. 1881-1885
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Conference paper (peer-reviewed)abstract
- We have studied the structural, thermophysical, and spectroscopic properties of polymeric C60 obtained by high pressure treatment at pressures and temperatures near 1 GPa and 600 K. We present here a brief overview of our results for the structural and thermophysical properties and a more detailed report on recent results obtained by Raman spectroscopy on both thin films, polycrystalline, and single crystal material. The results presented include a comparison between Raman results for photopolymerized and pressure polymerized thin films and a preliminary estimate of the binding energy of polymeric C60.
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| 8. |
- Sing, M., et al.
(author)
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Electronic structure studies of pressure-polymerized C60
- 1999
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In: Synthetic Metals vol. 103. - : Elsevier B.V.. ; 103:1-3, s. 2454-2455
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Conference paper (peer-reviewed)abstract
- We have investigated the electronic structure of one and two-dimensional C60 polymers with regard to both their low-lying excitations and C 1s excitation spectra by means of electron energy-loss spectroscopy in transmission. We compare the results with those for pristine C60. In general, the spectra for the polymers resemble those for pristine C60 but show a broadening due to both the lowering of the symmetry and the increased intermolecular overlap. This is also reflected by a reduction of the optical gap in comparison with pristine C60.
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| 9. |
- Soldatov, Alexander, et al.
(author)
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Molecular rotation in C70 at high pressures: a thermal conductivity study
- 1996
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In: Journal of Physics and Chemistry of Solids. - : Elsevier Science Ltd. - 0022-3697 .- 1879-2553. ; 57:9, s. 1371-1375
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Journal article (peer-reviewed)abstract
- We have measured the thermal conductivity ε and the heat capacity per unit volume varrhocp of highly pure C70 in the temperature interval 100–450 K under pressures up to 1 GPa. Anomalies indicating freezing of uniaxial molecular rotation were observed in λ and varrhocp upon both cooling and increasing pressure. The phase boundary for this transition has an approximate slope dT/dp = 70 K Gpa−1. The temperature and pressure dependence of λ indicate a substantial amount of structural defects in the sample and strong metastability effects.
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| 10. |
- Soldatov, Alexander, et al.
(author)
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Polymeric fullerenes: from C60 to C70
- 1999
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In: AIP Conference Proceedings vol. 486. - Melville, NY : American Institute of Physics (AIP). - 1563969009 ; , s. 12-15
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Conference paper (peer-reviewed)abstract
- For the first time polymerization of both powder and single crystals of C70 fullerene was established after their subjection to high pressure (1.1 - 2 GPa) at elevated temperature (500 - 580 K). High-resolution capacitance dilatometry, FTIR/Raman spectroscopy and thermal conductivity were employed to characterise the polymeric phase of C70. The results demonstrate drastic changes in the physical properties of C70 on polymerization. We report on a reverse transformation to the monomeric state on heating the polymer to 500 K at ambient pressure. The activation energy of depolymerization was determined to be 1.8(1) eV. We discuss our results in terms of existing structural models for polymerization of C70 and compare the physical properties of C70 and C60 polymers.
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| 11. |
- Soldatov, Alexander, et al.
(author)
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Transport and vibrational properties of pressure polymerized C60
- 1998
-
In: Molecular Materials (Molecular Crystals and Liquid Crystals, Section C), vol. 11. - Amsterdam : Overseas Publishers Association. ; 11:1-2, s. 1-6
-
Conference paper (peer-reviewed)abstract
- The vibrational spectrum of polycrystalline C60 polymerized at 1.1 GPa and 585 K was studied by inelastic neutron scattering. We find drastic changes in the spectrum compared to the vibrational spectrum of pristine C60: the appearance of a new, broad vibrational band at low energies, and splitting and significant changes in the peak positions of various modes. The thermal conductivity lambda of polymerized C60 was measured in the temperature range 150-320 K and was found to increase with a rise in temperature, which reflects strong phonon scattering. A high degree of structural disorder in the crystalline lattice of the polymeric phase is most probably responsible for the glass-like beahviour of lambda(T).
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| 12. |
- Sundqvist, Bertil, et al.
(author)
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Phase diagram, structure, and disorder in C60 below 300 K and 1 GPa
- 1995
-
In: Solid State Communications. - : Elsevier Science Ltd. - 0038-1098 .- 1879-2766. ; 93:2, s. 109-112
-
Journal article (peer-reviewed)abstract
- Earlier structural studies have shown that the pentagon-to-hexagon orientation ratio in the orientationally ordered simple cubic phase of C60 decreases under pressure. From anomalies observed in the compressibility and thermal conductivity of C60 under pressure we have deduced a pressure-temperature phase diagram for this substance in the range below 300 K and 1 GPa (10 kbar). We conclude that C60 forms a new, completely “hexagon” ordered structural phase above about 0.6 GPa at 150 K (1.2 GPa at 300 K), and that the glass transition shifts upwards in T under pressure by 54 K GPa−1. However, above 0.1 GPa, pentagon-to-hexagon orientation relaxation seems to occur on heating at an almost pressure independent temperature near 100 K.
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| 13. |
- Sundqvist, Bertil, et al.
(author)
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Structural and physical properties of pressure polymerized C60
- 1998
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In: Carbon, vol. 36 issue 5-6. - : Elsevier B.V.. ; 36:5-6, s. 657-660
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Conference paper (peer-reviewed)abstract
- We discuss the structural and dynamic properties of C60 polymerized under low-P, low-T conditions, and suggest that the disordered mixed orthorhombic-tetragonal-rhombohedral phases produced under these conditions arise from nucleation of molecular chains in random directions because of the quasi-free molecular rotation under standard reaction conditions in the fcc phase of C60. Polymerization in He gives results qualitatively different from those obtained in other media.
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| 14. |
- Andersson, Ove, et al.
(author)
-
Thermal conductivity of C60 at pressures up to 1 GPa and temperatures in the range 50-300 K
- 1996
-
In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 54:5, s. 3093-3100
-
Journal article (peer-reviewed)abstract
- The thermal conductivity λ of C60 shows anomalies near 260 K and 90 K which are associated with the well-established phase transition and glass transition, respectively. Both transition temperatures increase with pressure, at the rates 120 K GPa-1 and 62 K GPa-1, respectively. With increasing temperature, λ of the simple cubic (sc) phase increased below 170 K (glasslike behavior) but decreased above. The glasslike behavior of λ is probably due to a substantial amount of lattice defects. Possible reasons for the change of sign of dλ/dT near 170 K are discussed. In the face centered cubic (fcc) phase (T≳260 K at atmospheric pressure) λ was almost independent of temperature, a behavior which is far from that of an ordered crystal (λ∝T-1 for T≳Debye temperature). This result can be attributed to the molecular orientational disorder of the fcc phase. The relaxation behavior associated with the glassy state and its unusually strong dependence on thermal history are discussed briefly, and data which support a previously reported relaxation model are presented. At room temperature, the density dependencies of λ, (∂ lnλ/∂ lnρ)T, were 5.5 and 9.5 for the fcc and sc phases, which are values typical for an orientationally disordered phase and a normal crystal phase, respectively.
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| 15. |
- Persson, Per-Axel, et al.
(author)
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High-pressure polymerized C60
- 1996
-
In: Extended Abstracts of Carbon´96, European Carbon Conference, Newcastle-upon-Tyne 1996, volume 2. - : The British Carbon Group. ; , s. 746-747, s. 746-747
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Conference paper (other academic/artistic)
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| 16. |
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| 17. |
- Winter, Johannes, et al.
(author)
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Charge transfer in alkali-metal-doped polymeric fullerenes
- 1996
-
In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 54, s. 17486-17492
-
Journal article (peer-reviewed)abstract
- We present room-temperature Raman measurements of pressure-polymerized C60 and compare them with the spectra of RbC60 in the orthorhombic phase. Although both materials were prepared according to two completely different routes the spectra show a surprising similarity with respect to mode positions and line splitting. We concluded from this that both materials, the uncharged pressure-polymerized C60 and the rubidium-doped orthorhombic compound, have the same overall structure and the AC60 compounds can be considered as the doped species of the C60, polymerized using moderate low pressure and high temperatures. From a detailed comparison between both spectra mode shifting and line broadening as a consequence of the charge transfer was determined and electron-phonon coupling constants were estimated for the high-frequency Hg(7) and Hg(8) modes. The low values for the coupling constants compared to the ones in the K3C60 can explain the lack of superconductivity in the AC60 compounds.
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| 18. |
- Aksenova, N.A., et al.
(author)
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Structure studies of C60 polymerized at low pressures
- 1997
-
In: Fullerenes. - Pennington, NJ : The Electrochemical Society. - 1566771927 ; , s. 687-694
-
Conference paper (peer-reviewed)abstract
- The structure and lattice properties of a polymerized C60 sample were investigated by X-ray powder diffraction over the range from liquid-nitrogen to room temperature. The aim of these studies was to establish the effect of low-pressure compression (up to 1.1 GPa) on the phase composition of the sample, to determine the structure of the phases involved, to obtain the variation of the lattice parameters with temperature, to observe eventual phase transformation in polymerized C60, and to elucidate the evolution of the orientational order with varying temperature. Analysis of the room-temperature diffraction patterns indicates that the sample contained at least two major phases, namely rhombohedral and and tetragonal with the polymerization array close to two-dimensional. Our data obtained on samples annealed at 300 C in the air allow us to conclude that polymerized C60 converts to a mixture of the fcc structure plus a new phase, presumably C60O.
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| 19. |
- Andersson, Britt M., et al.
(author)
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Defects and the thermal conductivity of YBa2Cu3O7-d and YBa2Cu4O8
- 1993
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In: Journal of Alloys and Compounds, volume 195. - : Elsevier Sequoia S.A.. - 0 444 81529 5 ; , s. 655-658
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Conference paper (peer-reviewed)abstract
- We present below the first measured data for the thermal diffusivity and the thermal conductivity, λ, of dense, sintered ceramic YBa2Cu4O8 (1-2-4) as a function of temperature T between 40 and 310 K. At 300 K, λ is 6.3 Wm−1K−1, increasing with decreasing T to about 9.5 Wm−1K−1 near Tc. This T dependence is much stronger than in YBa2Cu3O7−δ (1-2-3). Only a very small enhancement is observed in λ below Tc. As in 1-2-3, λ is mainly limited by phonon scattering by defects.
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| 20. |
- Andersson, Britt M., et al.
(author)
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Electrical resistivity and critical temperature of Bi-based high-Tc superconductors to 1 GPa
- 1990
-
In: High Pressure Research, volumes 3 to 5. - London : Gordon and Breach. - 2881247466 ; 3:1-6, s. 120-122
-
Conference paper (peer-reviewed)abstract
- We have measured the electrical resistance R of a sintered, two-phase, high-TC superconductor with the nominal composition BiSrCaCu2Ox, as a function of T and p. We find d(lnR)/dp ≃ -0.06 GPa-1 at 295 K, while dTC/dp is 2.5 K/GPa for the phase with Tc ≃ 76 and 2 K/GPa for that with TC ≃ 106 K.
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| 21. |
- Andersson, Britt M., et al.
(author)
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Electrical transport in dense, bulk YBa2Cu4O8 produced by hot isostatic pressing
- 1990
-
In: Physica B Volumes 165 & 166, Part 2. - : Elsevier B.V.. ; , s. 1699-1700
-
Conference paper (peer-reviewed)abstract
- Dense (98.5%) bulk sintered specimens of YBa2Cu4O8 have been produced by hot isostatic pressing. Data for the electrical resistivity π versus temperature T and pressure p in the range 70–300 K and 0–0.7 GPa are reported and discussed. The critical current density of the material studied exceeds 100 Acm−2 below 60 K.
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| 22. |
- Andersson, Britt M., et al.
(author)
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Electrical transport properties of dense bulk YBa2Cu4O8 produced by hot isostatic pressing
- 1990
-
In: Physica. C, Superconductivity. - 0921-4534 .- 1873-2143. ; 170:5-6, s. 521-531
-
Journal article (peer-reviewed)abstract
- Highly dense sintered YBa2Cu4O8 has been produced by hot isostatic pressing (HIP). The electrical resistivity varrho of this material has been measured as a function of temperature T and pressure varrho in the range 40–650 K and 0–0.7 GPa. Both the temperature dependence and the pressure dependence of varrho are found to be well described by a model based on the standard Bloch-Grüneisen theory. It is pointed out that varrho is liner in T only under isobaric conditions, while varrho is strongly nonlinear in all high-Tc superconductors under isochoric (constant volume) conditions. The critical current density of the material is 900 A/cm2 at 4 K, while the resistivity is 630 μΩ cm at 294 K.
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| 23. |
- Andersson, Britt M., et al.
(author)
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High-pressure properties of high-Tc superconductor samples produced by hot isostatic pressing
- 1990
-
In: High Pressure Research, volumes 3 to 5. - London : Gordon and Breach. - 2881247466 ; 3:1-6, s. 123-125
-
Conference paper (peer-reviewed)abstract
- The electrical resistance of dense YBa2Cu3Ox and YBa2Cu4Oy produced by hot isostatic pressing has been measured vs. T and p. At 295 K we find d (ln R)/dp ≃ -0.12 and -0.09 GPa-1, respectively, with no systematic dependence on initial density. For 1-2-4, dTC/dp ≃ 5.1 K/GPa, which is ten times that of 1-2-3.
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| 24. |
- Andersson, Britt M., et al.
(author)
-
Thermal conductivity and thermal diffusivity of dense, bulk YBa2Cu4O8
- 1994
-
In: High Temperatures - High Pressures volume 26, issue 1. - London : Pion Press. ; , s. 53-57
-
Conference paper (peer-reviewed)abstract
- We have measured the thermal conductivity, lambda, and the thermal diffusivity, a, of dense bulk ceramic YBa2Cu4O8 (1-2-4) between 30 and 300 K. For this material lambda is about 10 W/Km near 100 K, comparable with the in-plane thermal conductivity for single crystals of the more common high-Tc material YBa2Cu3O7 (1-2-3). Also, lambda has a much stronger T dependence in 1-2-4 than in 1-2-3. We find that, for 1-2-4, lambda can be very well described by a semiclassical phonon theory, which includes phonon, electron, and boundary scattering. The difference between the present results and those for 1-2-3 can be explained mainly in terms of a smaller amount of point-defect scattering in 1-2-4 because of the stable oxygen stoichiometry of this material.
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| 25. |
- Andersson, Britt M., et al.
(author)
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Thermal conductivity of polycrystalline YBa2Cu4O8
- 1994
-
In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 49:6, s. 4189-4198
-
Journal article (peer-reviewed)abstract
- We have measured the thermal conductivity κ and the thermal diffusivity a of a dense bulk ceramic polycrystalline sample of YBa2Cu4O8 (1:2:4) in the temperature range 30–300 K. We find κ≊10 W m-1 K-1 at 100 K, significantly higher than in ceramic YBa2Cu3O7-δ (1:2:3) and approaching the in-plane value for single-crystal 1:2:3, and decreasing to 7.6 W m-1 K-1 at 300 K. The data for this sample can be described by standard theories for phonon thermal conductivity of crystalline materials with boundary, phonon, and electron scattering. The higher κ in 1:2:4 as compared to 1:2:3 is, in this model, due to the smaller point defect scattering in the former. The fitted parameters for the three scattering mechanisms all agree with independent estimates based on simple models; inserting data for electric resistivity, grain size, carrier density, and lattice properties we can predict κ and its T dependence to within about 20%. We also discuss models for the phonon and electron thermal conductivities in some detail, including some second-order effects such as inelastic electron scattering and a T-dependent carrier density.
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