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• 151. McRae, Edward
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
Transport study of (T,p) phase diagram in PdAl2Cl8 intercalated graphite1996In: Journal of Physics and Chemistry of Solids, volume 57, issues 6-8: Proceedings of the 8th International Symposium on Intercalation Compounds (ISIC-8), Vancouver 1995, Elsevier Ltd. , 1996, p. 827-831Conference paper (Refereed)

We have examined the c-direction variations of resistance, Rc (or resistivity, varrhoc) over the 4.2–295 K. temperature range at ambient pressure, and over the 160–295 K range for pressures up to 1.5 GPa on first stage PdAl2Cl8 graphite intercalation compounds. All samples examined manifest a rapid, nonlinear decrease in Rc with pressure (maximum ¦dln Rc/dp¦ > 300 % GPa−1 at 0.4 GPa. The overall Rc decrease is around 90% by 1GPa, a value greater than that observed in any other GIC to date. Upon slow pressure release, the resistance increases linearly but to only about 20–25% of its initial (1 bar) value for HOPG-based materials: on the contrary, it returns along its initial R-p trajectory for GICs based on single crystal graphite. We discuss these data with reference to the X-ray diffraction results, to the significance of the residual resistivity and to studies on other similar lamellar solids.

• 152. McRae, Edward
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
C-axis transport studies and in-plane intercalate layer structure in sodium halide intercalated graphite1998In: Extended Abstracts of Eurocarbon´98, European Conference Science and Technology of Carbon, Strasbourg 1998, volume 2, Deutsche Keramische Gesellschaft, Arbeitskreis Kohlenstoff , 1998, p. 767-768Conference paper (Other academic)
• 153. McRae, Edward
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
On the relevance of certain transport-structure correlations in SbCl5-intercalated graphite to our overall understanding of GIC c axis conductivity1994In: Molecular Crystals and Liquid Crystals Section A, volumes 244-245: Proceedings of the 7th International Symposium on Intercalation Compounds (ISIC-7), Louvain-la-Neuve, Belgium 1993, Yverdon: Gordon and Breach , 1994, p. 61--66Conference paper (Refereed)

This paper presents some new data on the SbCl5-intercalated graphite family as concerns the relationships between c axis resistivity, ρc(T,p), and intercalate layer structure. Results on the influence of intercalate layer crystallization and the nature of the host graphite are discussed and compared with those on other GIC families. Certain data are examined in the light of available theories and such an analysis raises a number of questions.

• 154. McRae, Edward
Umeå University, Faculty of Science and Technology, Physics.
Twenty years of transport studies in intercalated graphite2000In: Molecular Crystals and Liquid Crystals vol. 340: Proceedings of the 10th International Symposium on Intercalation Compounds (ISIC 10), Okazaki 1999, Overseas Publishers Association , 2000, p. 325-330Conference paper (Refereed)

Upon the occasion of the 10th ISIC conference, it seems appropriate to give a brief overview of some of the major advances made in our understanding of charge transport in intercalated graphite since the 1st Franco-American Conference on Graphite Intercalation Compounds held in 1977.

• 155. McRae, Edward
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
Studies of intercalated graphite containing ionically and covalently bonded sodium1996In: Extended Abstracts of Carbon´96, European Carbon Conference, Newcastle-upon-Tyne 1996, volume 1, The British Carbon Group , 1996, p. 120-121Conference paper (Other academic)
• 156. McRae, Edward
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
Raman light scattering and c-axis resistivity evidence for a pressure-induced stage transformation in PdAl2Cl8 intercalated graphite.2000In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 62, no 20, p. 13757-13766Article in journal (Refereed)

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.

• 157. Meletov, Kostas P.
Umeå University, Faculty of Science and Technology, Physics.
Pressure-induced transformations and optical properties of the 2D-tetragonal polymer of C60 at pressures up to 30 GPa.2002In: Journal of Experimental and Theoretical Physics, ISSN 1063-7761, E-ISSN 1090-6509, Vol. 95, no 4, p. 736-747Article in journal (Refereed)

The Raman spectra of the two-dimensional tetragonal (2D(T)) polymeric phase of C60 have been studied in situ at pressures up to 30 GPa and room temperature. The pressure dependence of the phonon modes shows an irreversible transformation of the material near 20 GPa into a new phase, most probably associated with the covalent bonding between the 2D polymeric sheets. The Raman spectrum of the high-pressure phase is intense and well resolved, and the majority of modes are related to the fullerene molecular cage. The sample recovered at ambient conditions is in a metastable phase and transforms violently under laser irradiation: the transformed material contains mainly dimers and monomers of C60 and small inclusions of the diamond-like carbon phase. The photoluminescence spectra of the 2D(T) polymer of C60 were measured at room temperature and pressure up to 4 GPa. The intensity distribution and the pressure-induced shift of the photoluminescence spectrum drastically differ from those of the C60 monomer. The deformation potential and the Grüneisen parameters of the 2D(T) polymeric phase of C60 have been determined and compared with those of the pristine material.

• 158. Meletov, Kostas P.
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
High-pressure induced metastable phase in tetragonal 2D polymeric C60.2001In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 341, no 5-6, p. 435-441Article in journal (Refereed)

The structural stability of the tetragonal two-dimensional (2D) polymeric phase of C60 has been studied under pressure up to 24 GPa and room temperature by means of in situ Raman scattering. An irreversible transformation of the material to a new phase was observed at pressure 20 GPa. The phonon spectrum of the high-pressure phase provides a strong indication that the fullerene molecular cage is retained and therefore this phase may be related to a three-dimensional (3D) network of C60 cages. The new phase remains stable upon slow release of pressure to ambient conditions. The recovered material is metastable and transforms in air by detonation under laser irradiation to partially dimerized C60.,

• 159. Moret, Roger
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
First X-ray diffraction analysis of pressure polymerized C60 single crystals1997In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 40, no 1, p. 55-60Article in journal (Refereed)

It is shown that C60 single crystals can be polymerized under relatively modest pressure-temperature conditions. The resulting single crystals exhibit long-range order and they are made up of 12 orientation variants. The structure is orthorhombic with a short intermolecular distance along the chains (9.14 Å), characteristic of covalent bonding. We propose a structural mechanism for the polymerization of fullerene-based compounds; it involves a shift of one of the (111) cubic planes together with a shortening of the (111) spacing. The configuration of the polymer chains presents interesting relations with that found in the A1C60 polymer compounds.

• 160. Moret, Roger
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
Chain orientation and layer stacking in the high-pressure polymers of C60: Single crystal studies2000In: Electronic Properties of Novel Materials: Science and Technology of Molecular nanostructures: Proceedings of the XIV Winter School on Electronic Properties of Novel Materials, Kirchberg 2000, American Institute of Physics (AIP) , 2000, p. 81-84Conference paper (Refereed)

High-pressure polymerisation of C^o leads to a variety of new crystalline or amorphous phases which display interesting physical properties. We have prepared onedimensional (ID, C6o chains) and two-dimensional (2D, C60 layers) polymers from C6o single crystals. The resulting multi-domain crystals have been studied using x-ray diffraction and Raman spectroscopy. The relative orientations of the chains in the "low-pressure" ID orthorhombic polymer had been characterized previously [1]. We have now determined the specific stacking of the Ceo layers in the 2D tetragonal and rhombohedral polymers. Using these results we analyze the relations between the different polymers and the intermolecular environments which may play a role in stabilizing the observed polymer structures.

• 161. Moret, Roger
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
High-pressure synthesis, structural and Raman studies of a two-dimensional polymer crystal of C60.2000In: European Physical Journal B: Condensed Matter Physics, ISSN 1434-6028, E-ISSN 1434-6036, Vol. 15, no 2, p. 253-263Article in journal (Refereed)

Two-dimensional polymerisation of a C60 single crystal has been obtained under high-pressure high temperature conditions (700 K - 2 GPa). Crystalline order is preserved but the crystal splits into variants (orientational domains). The analysis of X-ray diffraction and Raman spectroscopy data reveals that the polymer crystal is primarily tetragonal with some admixture of rhombohedral phase. Furthermore, Raman spectroscopy gives evidence for additional C60-C60 dimers, which are probably disordered. For the tetragonal phase, it is shown that successive polymer layers are rotated by about the stacking axis, according to the P42/mmc space group symmetry. The structure of the rhombohedral phase is also clarified. The role of the interlayer interactions in stabilising the two-dimensional polymer phases of C60 is discussed.

• 162. Moret, Roger
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
Single-crystal structural study of the pressure-temperature induced dimerization of C602004In: European Physics Journal B, ISSN 1434-6028 (Print) 1434-6036 (Online), Vol. 37, no 1, p. 25-37Article in journal (Refereed)

We present a study by Raman spectroscopy and X-ray diffraction/diffuse scattering of C60 single-crystals treated at high-pressure and high-temperature. This allowed us to obtain structural information on the C60 dimer state which can be considered as an intermediate state in the polymerization process. In the 1-6 GPa pressure range the crystals are primarily formed of dimers with additional minor fractions of monomers, 1D and 2D polymers, as shown by the analysis of the Raman spectra. The dimers are disordered within an average cubic lattice derived from that of the monomer. Single-crystal diffraction patterns reveal a characteristic diffuse scattering intensity distribution which has been simulated by calculating the diffuse scattering produced by dimer and trimer model structures. Satisfactory agreement is obtained for random positional and orientational disorder of the C60-C60 dimers although a small concentration of similarly disordered trimers is likely. In a first approximation the dimer/trimer disorder can be considered as random but various inter-dimer correlations are probably present, as discussed.

• 163. Moret, Roger
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
Influence of the pressure-temperature treatment on the polymerization of C60 single crystals at 2 GPa - 700 K2005In: Carbon, ISSN 0008-6223, Vol. 43, no 4, p. 709-16Article in journal (Refereed)

X-ray diffraction and Raman spectroscopy have been used to characterize the structures obtained when C-60 single crystals are treated at 2GPa-700K. Two different experimental procedures have been applied: the temperature is raised before the pressure is applied, or the opposite. The "heating-then-pressing" path leads to the tetragonal polymer structure (P4(2)/mmc) together with a minor fraction of rhombohedral structure, which confirms previous results. In contrast, the "pressing-then-heating" path leads to a different state presenting similarities with both the rhombohedral and the disordered dimer structures. The results are discussed in light of the orientational and dynamical aspects of the C60 polymerization.

• 164. Nagel, Peter
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
High-resolution thermal expansion of high-pressure polymerized and 'normal' C60 from 10-500 K: Kinetics of depolymerization1998In: Molecular Nanostructures: Proceedings of the International Winterschool on Electronic Properties of Novel Materials, Kirchberg 1997, Singapore: World Scientific , 1998, p. 365-368Conference paper (Refereed)

High-resolution capacitance dilatometry was used to study the thermal expansion of both 'normal' and polymer phases of C60. The expansivity, alpha(T), of 'normal' C60 exhibits an unusual temperature dependence above the orientational disordering transition at 260 K; alpha(T) decreases by about 30% from 260 to 500 K and appears to reach a minimum near 500 K. Polymerized C60 has a much smaller expansivity than normal C60 due to the stronger covalent bonding between molecules. The polymerized state converts back to 'normal' C60 at temperatures between 450 and 500 K. We show that this is an activated process with a well-defined activation energy of 1.92 eV and a volume increase of about 2%.

• 165. Nagel, Peter
Umeå University, Faculty of Science and Technology, Physics.
Intermolecular bond stability of C60 dimers and 2D pressure-polymerized C601998In: AIP Conference Proceedings Vol. 442: Electronic Properties of Novel Materials - Progress in Molecular nanostructures. Proceedings of the XII International Winter School on Electronic Properties of Novel Materials, Kirchberg 1998, Woodbury, NY: American Institute of Physics (AIP) , 1998, p. 194-197Conference paper (Refereed)

The thermal stability of C60 dimers and 2D pressure-polymerized C60 is studied using high-resolution capacitance dilatometry. The transformation of both the dimer and the polymer phases back to 'normal' C60 is excellently described by a simple thermally activated process with activation energies of 1.75 +/- 0.1 eV (dimer) and 1.9 +/- 0.2 eV (polymer). These results are compared to previous data for 1D-polymerized C60 and photo-polymerized C60. The thermal expansivity of the 2D-polymer phase is as much as a factor of ten smaller than that of pure C60 and approaches values for diamond.

• 166. Nagel, Peter
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
C60 one- and two-dimensional polymers, dimers, and hard fullerite: Thermal expansion, anharmonicity, and kinetics of depolymerization1999In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 60, no 24, p. 16920-16927Article in journal (Refereed)

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.

• 167. Neve, Jesper
Umeå University, Faculty of Science and Technology, Physics.
Electron band structure, resistivity, and the electron-phonon interaction for niobium under pressure1983In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 28, no 2, p. 629-637Article in journal (Refereed)

Accurate measurements of the electrical resistance of Nb are presented as a function of temperature and pressure in the region 0-40°C and 0-1 GPa. From these measurements and published results for the compressibility and the pressure dependence of the superconducting transition temperature Tc we evaluate the pressure dependence of the electron-gas plasma frequency ω(p). The electronic structure of Nb is calculated from a self-consistent linear muffin-tin orbital method and results are obtained for the pressure dependence of the density of states at the Fermi surface, the root mean square over the Fermi surface of the Fermi velocity, the optical mass, and the plasma frequency. The experimental and calculated results for the pressure dependence of ω(p) are both close to dlnω2 / dlnV=-1.9. This agreement suggests that measurements of the electrical resistance as a function of temperature and pressure provide a new test of band-structure calculations. From our measurements of the resistance and calculations of ω(p) and from published results for the compressibility we obtain the pressure dependence of the electron-phonon interaction λ(p). With p given in gigapascals, the result is dlnλ / dp=-0.0047.

• 168. Niska, John
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
Formation of the 124 phase superconductor (YBa2Cu4O8) by retaining oxygen in a reaction HIP sintering process1990In: Journal of materials science letters, ISSN 0261-8028, E-ISSN 1573-4811, Vol. 9, no 7, p. 770-771Article in journal (Refereed)

The ceramic superconductor YBa2Cu4O8 has been produced by high temperature sintering of a mixture of CuO and YBa2Cu3O7 in a glass capsule under high hydrostatic argon pressure. The resulting highly dense material is investigated by X-ray diffraction, optical and electron microscopy, resistance measurements and hardness measurements, and shown to be a homogeneous High transition temperature superconductor.

• 169. Pei, Cuiying
Umeå University, Faculty of Science and Technology, Department of Physics.
Quasi 3D polymerization in C60 bilayers in a fullerene solvate2017In: Carbon, ISSN 0008-6223, E-ISSN 1873-3891, Vol. 124, p. 499-505Article in journal (Refereed)

The polymerization of fullerenes has been an interesting topic for almost three decades. A rich polymeric phase diagram of C60 has been drawn under a variety of pressure-temperature conditions. However, only linear or perpendicular linkages of C60 are found in the ordered phases. Here we used a unique bilayer structural solvate, C60∙1,1,2-trichloroethane (C60∙1TCAN), to generate a novel quasi-3D C60 polymer under high pressure and/or high temperature. Using Raman, IR spectroscopy and X-ray diffraction, we observe that the solvent molecules play a crucial role in confining the [2+2] cycloaddition bonds of C60s forming in the upper and lower layers alternately. The relatively long distance between the two bilayers restricts the covalent linkage extended in a single individual bilayer. Our studies not only enrich the phase diagram of polymeric C60, but also facilitate targeted design and synthesis of unique C60 polymers.

The full text will be freely available from 2019-11-04 09:09
• 170.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Chemistry. Umeå University, Faculty of Science and Technology, Department of Chemistry. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics.
High-pressure polymerized C601996In: Extended Abstracts of Carbon´96, European Carbon Conference, Newcastle-upon-Tyne 1996, volume 2, The British Carbon Group , 1996, p. 746-747Conference paper (Other academic)
• 171.
Umeå University, Faculty of Science and Technology, Physics.
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
The physical properties of high-pressure polymerized C601997In: Journal of Physics and Chemistry of Solids, volume 58, issue 11: Proceedings of Fullerenes '96, Oxford 1996, Elsevier B.V. , 1997, p. 1881-1885Conference paper (Refereed)

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.

• 172.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Chemistry. Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Chemistry. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics.
Physical properties of pressure polymerized C601996In: High Pressure Science and Technology: Proceedings of the Joint XV AIRAPT and XXXIII EHPRG International Conference, Warsaw 1995, Singapore: World Scientific , 1996, p. 716-718Conference paper (Refereed)

The properties of C60 have been studied after treatment at high temperature and high pressure (1.1 GPa and 565 K for 2 h). The treated material is insoluble in organic solvents. We present results obtained in NMR and Raman studies and measured data for the specific heat and the thermal expansion. Our results show clearly that there are no covalent bonds and no molecular rotation, but suggest that the molecules are slightly deformed and held together by weak pi-type bonds.

• 173.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Chemistry. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Chemistry. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics.
NMR and Raman characterization of pressure polymerized C-601996In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 258, no 5-6, p. 540-546Article in journal (Refereed)

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.

• 174.
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Applied Physics and Electronics.
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
Enhanced thermal dissociation of optically excited C60 chains.2000In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 49, no 5, p. 631-636Article in journal (Refereed)

The thermal dissociation of C60 polymers has been studied using Raman scattering. The measured dissociation rate depends on the intensity of the 1064 nm NIR excitation, showing that i) the band gap is smaller than 1.17 eV and ii) a radiation-enhanced thermal breakdown path exists in addition to the "normal'' thermal breakdown mechanism. Quantum chemical calculations show that the energy barrier Ea for thermal breakdown is about 40% lower in the first (optically) excited state than in the ground state. This agrees well with the ratio between our radiation-enhanced value $E_{a} = (1.1 \pm 0.02)\,eV$ and values near 1.9 eV measured by purely thermal methods.

• 175. Prokhvatilov, A.I.
Umeå University, Faculty of Science and Technology, Physics.
Hysteretic phenomena in Xe-doped C60 from X-ray diffraction2005In: Low Temperature Physics, ISSN 1063-777X, Vol. 31, no 5, p. 585-589Article in journal (Refereed)

Polycrystalline fullerite C60 intercalated with Xe atoms at 575 K and a pressure of 200 MPa was studied by powder x-ray diffraction. The integrated intensities of a few brighter reflections have been utilized to evaluate the occupancy of the octahedral interstitial sites in C60 crystals, which turned out to be (34±4)%, and in good agreement with another independent estimate. It is found that reflections of the (h00) type become observable in Xe-doped C60. The presence of xenon in the octahedral sites affects both the orientational phase transition as well as the glassification process, decreasing both characteristic temperatures as well as smearing the phase transition over a greater temperature range. Considerable hysteretic phenomena have been observed close to the phase transition and the glassification temperature. The signs of the two hysteresis loops are opposite. There is reliable evidence that at the lowest temperatures studied the thermal expansion of the doped crystal is negative under cool-down.

• 176. Rapp, Östen
Umeå University, Faculty of Science and Technology, Physics.
The pressure dependence of of the electron-phonon interaction and the normal state resistivity1981In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 24, no 1, p. 144-154Article in journal (Refereed)

Accurate measurements of the electrical resistance as a function of temperature and pressure are reported for Sn, Zr, dhcp La, and V. These measurements cover a temperature region around room temperature and pressures up to 1.3 GPa. From these data, including also our previous measurements for Al and published results for Pb, the pressure dependence of d ρ / dT (the resistivity-temperature derivative) is obtained. This quantity is found to be a significant factor in the pressure dependence of the electron-phonon interaction parameter λ. For the nontransition metals the relative pressure dependence of d ρ / dT is much larger than the compressibility. Therefore the pressure dependence of the superconducting Tc is quantitatively well accounted for by the resistance data for these metals. For the transition metals the pressure dependence of d ρ / dT is relatively smaller and Tc(p) calculated from the resistance data is, at the best, only qualitatively correct. These differences are discussed. Estimates for the pressure dependence of the plasma frequency are obtained.

• 177. Rapp, Östen
Umeå University, Faculty of Science and Technology, Physics.
Possibility of superconductivity in palladium at high pressures1984In: LT-17: Contributed papers of the Proceedings of the 17th International Conference on Low Temperature Physics, Universität Karlsruhe and Kernforschungszentrum, Karlsruhe 1984, Amsterdam: North-Holland: Elsevier Science Publishers B.V. , 1984, p. 865-866Conference paper (Refereed)

On the basis of measurements of the resistivity and calculations of the density of states and the plasma frequency it is concluded that palladium may become superconducting at high pressures.

• 178. Renker, Burkhard
Umeå University, Faculty of Science and Technology, Physics.
Fingerprints of solid-state chemical reactions in the dynamics of fullerenes1998In: AIP Conference Proceedings Vol. 442: Electronic Properties of Novel Materials - Progress in Molecular nanostructures. Proceedings of the XII International Winter School on Electronic Properties of Novel Materials, Kirchberg 1998, Woodbury, NY: American Institute of Physics (AIP) , 1998, p. 322-326Conference paper (Refereed)

Excitation spectra of polymerized fullerides such as RbC60, Na4C60 and pressurized C60 are studied by inelastic neutron scattering and Raman spectroscopy in view of differences in the interfullerene bonding. Changes in the dynamics are followed by temperature dependent measurements. A detailed analysis is performed by model calculations.

• 179.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics.
Thermal conductivity of solids and liquids under pressure1984In: Reports on progress in physics (Print), ISSN 0034-4885, E-ISSN 1361-6633, Vol. 47, no 10, p. 1347-1402Article, review/survey (Other academic)

Thermal conductivity of solids and liquids under pressure is covered in this review. Experimental techniques are critically considered and compared, and an introduction to theory is provided. Results are presented and discussed for ionic crystals, normal molecular crystals, plastic crystal phases, clathrate hydrates, polymers and glass-formers, liquids, covalent and semiconducting crystals, rocks and metals. Special attention is given to isochoric conditions, change of crystal structure and molecular orientational disorder. Available reliable measurements at pressures up to a few GPa indicate the need for theoretical development, especially in connection with molecular crystals and ferromagnetic metals.

• 180.
Umeå University, Faculty of Science and Technology, Physics.
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Chemistry. Umeå University, Faculty of Science and Technology, Physics.
Structural and vibrational properties of Li- and Na-doped fullerene polymers2004In: Fullerenes, Nanotubes and Carbon Nanostructures vol. 12: Proceedings of 6th biannual International Workshop on Fullerenes and Atomic Clusters (IWFAC'2003), St. Petersburg 2003, London: Taylor & Francis , 2004, p. 319-25Conference paper (Refereed)

We have studied tetragonal C60 and C60-based polymers doped with Lithium and Sodium. We show that the intercalated phases Li4C60 and Na4C60 both form two-dimensional polymers. X-ray diffraction diagrams for Li4C60 and tetragonal C60 can be accurately indexed assuming tetragonal structures, but for Na4C60 a monoclinic quasi-tetragonal structure is found. We conclude that in Li4C60 the covalent bonds are formed by (2 + 2) cycloadditions, in the same way as in the tetragonal polymer produced by treating pure C60 at high temperature and high pressure, while single C-C bonds connect the fullerene molecules in Na4C60.

• 181.
Umeå University, Faculty of Science and Technology, Physics.
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
Raman spectroscopy and X-ray diffraction studies of the single- and double-bonded two-dimensional polymers NanLi4-nC60.2004In: Journal of Physics and Chemistry of Solids vol. 65, issues 2-3: Proceedings of the 12th International Symposium on Intercalation Compounds, ISIC-12, Poznan, Poland 2003, Elsevier B.V. , 2004, p. 355-357Conference paper (Refereed)

We have examined intercalated compounds of C60 containing four light alkali metal atoms per molecule. The single-metal compounds Na4C60 and Li4C60 form two-dimensional polymers with intermolecular links consisting of one and two C–C bonds, respectively. We have here studied the compounds NanLi(4-n)C60, with n between 0 and 4, to find out what parameters define the polymeric structure, in particular the type of intermolecular bonding. The materials have been studied by X-ray diffraction and Raman spectroscopy. Although the results are compatible with a charge transfer model with different charge transfers for Na and Li ions, other models cannot be ruled out because disorder and mixed phases complicate the analysis.

• 182.
Umeå University, Faculty of Science and Technology, Physics.
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
Structural properties of the alkali-doped polymeric fullerene compounds LixNa(4-x)C602005In: Chemical Physics Letters, ISSN 0009-2614, Vol. 413, no 1-3, p. 157-161Article in journal (Refereed)

We have synthesized the series of nominally isoelectronic intercalated fullerene compounds LixNa(4 − x)C60 and investigated these by X-ray diffraction and Raman spectroscopy. All compounds are two-dimensional polymers, with the Li4C60 structure dominating for x > 1 and the Na4C60 structure for x less-than-or-equals, slant 1. We find almost no shift of the Ag Raman modes with x, indicating that the charge transfer is also practically independent of composition. We conclude that the Li4C60 structure is the lowest energy structure for all these compounds, and that the Na-rich compounds choose the Na4C60 structure because of geometrical factors connected with effective ion radii.

• 183.
Umeå University, Faculty of Science and Technology, Physics.
Umeå University, Faculty of Science and Technology, Physics.
Thermal properties of two low viscosity silicone oils as functions of temperature and pressure1982In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 53, no 12, p. 8751-8755Article in journal (Refereed)

The thermal conductivity and the specific heat capacity per unit volume have been measured for two low viscosity grades of Dow Corning 200 fluid (polydimethyl siloxane) in the range 110 to 350 K and under pressures up to 2.0 GPa (20 kbar). Both the quantities studied are found to increase with increasing pressure in the liquid phase. From the measured data the phase diagrams are obtained. The 5 mm2/s (5 cSt) grade fluid does not crystallize, but undergoes a glass transition at 1.0 GPa at room temperature. The 1 mm2/s (1 cSt) grade has a more complicated phase diagram with a partly crystalline phase at low temperatures and pressures and two glass transitions at high temperatures and pressures.

• 184. Sing, M.
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
Electronic structure studies of pressure-polymerized C601999In: Synthetic Metals vol. 103: Proceedings of the International Conference on Science and Technology of Synthetic Metals, ICSM'98, Montpellier 1998, Elsevier B.V. , 1999, p. 2454-2455Conference paper (Refereed)

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.

• 185.
Umeå University, Faculty of Science and Technology, Physics.
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
Transport and vibrational properties of pressure polymerized C601998In: Molecular Materials (Molecular Crystals and Liquid Crystals, Section C), vol. 11: Proceedings of the 3rd International Workshop on Fullerenes and Atomic Clusters (IWFAC'97), St. Petersburg 1997, Amsterdam: Overseas Publishers Association , 1998, p. 1-6Conference paper (Refereed)

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).

• 186.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Chemistry. Umeå University, Faculty of Science and Technology, Department of Chemistry.
Pressure polymerized C60: Raman and NMR studies1996In: Fullerenes and Fullerene Nanostructures: Proceedings of the International Winterschool on Electronic Properties of Novel Materials, Kirchberg, Tyrol, Austria 1996, Singapore: World Scientific , 1996, p. 344-348Conference paper (Refereed)

The properties of bulk C60 have been studied after treatment at 1.1 GPa and 550-585 K. The treated material is insoluble in both toluene and 1,2-dichlorobenzene. Raman spectroscopy on modified aterial shows a large number of new lines, and the Raman pentagon pinch mode (Ag2) shifts from 1469 to 1458 /cm 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. The results verify previous suggested polymeric structures where the fullerene cages are connected by four-membered rings.

• 187.
Umeå University, Faculty of Science and Technology, Physics.
Umeå University, Faculty of Science and Technology, Physics.
Polymeric fullerenes: from C60 to C701999In: AIP Conference Proceedings vol. 486: Electronic Properties of Novel materials - Science and Technology of Molecular Nanostructures: Proceedings of the XIII Winter School on Electronic Properties of Novel Materials, Kirchberg 1999, Melville, NY: American Institute of Physics (AIP) , 1999, p. 12-15Conference paper (Refereed)

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.

• 188.
Umeå University, Faculty of Science and Technology, Physics.
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
Vibrational and thermal properties of pressure polymerized C601998In: Fullerenes: Recent Advances in the Chemistry and Physics of Fullerenes and Related Materials, vol. 6: Proceedings of the 193rd Meeting of the Electrochemical Society, Symposium Fullerenes: Chemistry, Physics, and New Directions XI, San Diego 1998, Pennington, NJ: The Electrochemical Society , 1998, p. 769-779Conference paper (Refereed)

The kinetics of C60 polymerization were studied in the temperature range 450-500 K at pressures below 1 GPa by measurements of the time dependence of the thermal conductivity. The vibrational spectrum of polymerized C60 was studied by inelastic neutron scattering. We find drastic changes in the spectrum compared with the vibrational spectrum of pristine C60: hardening of intermolecular modes, appearance ofa new broad vibrational band at low energies, splitting and significant changes in the peak positions of intramolecular modes. The thermal conductivity, lambda, of polymerized C60 was measured in the temperature range 150-320 K and was found to increase with rising temperature, that reflects strong phonon scattering. The presence of polymeric chains of different length and a high degree of structural disorder in the crystalline lattice of the polymeric phase are the most probable factors responsible for the glass-lika behaviour of lambda(T).

• 189.
Umeå University, Faculty of Science and Technology, Physics.
Umeå University, Faculty of Science and Technology, Physics.
Molecular rotation in C70 at high pressures: a thermal conductivity study1996In: Journal of Physics and Chemistry of Solids, ISSN 0022-3697, E-ISSN 1879-2553, Vol. 57, no 9, p. 1371-1375Article in journal (Refereed)

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.

• 190.
Umeå University, Faculty of Science and Technology, Physics.
Umeå University, Faculty of Science and Technology, Physics.
Phase transitions in C70 at high pressure: A thermal conductivity study1995In: Fullerenes: Recent Advances in the Chemistry and Physics of Fullerenes and Related Materials, volume 2: Proceedings of the 187th ECS Meeting, Symposium Fullerenes: Chemistry, Physics, and New Directions VII, Reno 1995, Pennington, NJ: The Electrochemical Society , 1995, p. 881-890Conference paper (Refereed)

Measurements of the thermal conductivity lambda and the specific heat capacity per unit volume rhocv of polycrystalline C70 were carried out in the temperature range 100 - 450 K under pressures up to 1 GPa. Anomalies have been observed in lambda and rhocv near 300 K on cooling (increasing pressure) indicating a phase transition from a rotationally disordered phase into an ordered phase. The phase boundary for this transition was found to have a slope dT/dp = 70 K/GPa. The temperature dependence of lambda indicates a substantial amount of structural defects in the sample and strong hysteresis effects. We propose a phase diagram describing the probable evolution of orientational structure in T-p space.

• 191. Soldatov, Alexander V.
Umeå University, Faculty of Science and Technology, Chemistry. Umeå University, Faculty of Science and Technology, Physics.
Topochemical polymerization of C70 controlled by monomer crystal packing2001In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 293, no 5530, p. 680-683Article in journal (Refereed)

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.

• 192. Staun Olsen, J.
Umeå University, Faculty of Science and Technology, Physics.
High pressure studies up to 50 GPa of Bi-based high-Tc superconductors1991In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 44, no 2, p. 211-213Article in journal (Refereed)

The high-Tc superconductor with nominal composition BiSrCaCu2Ox has been studied at high pressure, i.e. up to 50 GPa. A tetragonal structure was compatible with the measurements at all pressures, and no phase change was observed. The bulk modulus, B0 = 62.5 GPa, obtained has a somewhat smaller value than the one estimated earlier.

• 193.
Umeå University, Faculty of Science and Technology, Physics.
Buckyballs under pressure2001In: Physica Status Solidi B, vol. 223, issue 2: Proceedings of the 9th International Conference on High Pressure Semiconductor Physics, Sapporo 2000, Wiley Interscience , 2001, p. 469-477Conference paper (Refereed)

A general overview is given over fullerenes under pressure, with an emphasis on polymeric phases produced by treatment of C60 and C70 under high pressure. The pressure-temperature phase diagrams of these materials are briefly reviewed. The discussion centers on recent advances regarding the details of the crystal structures and the electron structures and electronic transport properties, in particular for the one- and two-dimensional phases of C60. Several polymerized phases of C70 are also discussed.

• 194. Sundqvist, Bertil
Carbon nanostructures under high pressure2002In: Journal of Physics Condensed Matter vol. 14: Proceedings of the Joint 18th International Conference on High Pressure Science and technology (AIRAPT 18) and the 11th High Pressure Conference of China (HPCC-11), Beijing 2001., Bristol: Institute of Physics , 2002, p. 10449-10452Conference paper (Refereed)

Results from recent high-pressure experiments in the field of fullerenes are briefly reviewed. In particular, new results on one-, two- and three-dimensional polymerized C60 and C70 are discussed. Results discussed include the first synthesis of a well defined, one-dimensional polymer based on C70, transformations from two-dimensional (2D) to three-dimensional phases in C60, and doping of 2D C60 polymers.

• 195.
Umeå University, Faculty of Science and Technology, Physics.
C-axis resistance of stage 2 SbCl5 intercalated graphite under pressure1988In: Synthetic Metals, volume 23: Proceedings of the 4th International Symposium on Graphite Intercalation Compounds (ISGIC-4), Jerusalem 1987, Elsevier B.V. , 1988, p. 345-350Conference paper (Refereed)

The c-axis resistance R of stage 2 SbCl5 intercalated graphite has been measured in the range 80–300 K and 0–4 GPa. The pressure dependence of R is similar to that of HOPG in the ordered low-T phase below 200 K but significantly larger at 300 K, where the initial pressure coefficient of R is −1.37 GPa−1. Two phase transitions are always observed under pressure, one at 0.3–0.5 GPa and one above 1 GPa. The latter is not completed until above 2.4 GPa and both are slow, with large time-dependent effects in R. R decreases by over 80% to 4 GPa.

• 196.
Umeå University, Faculty of Science and Technology, Physics.
Chapter 5: The effect of pressure on the thermal transport properties of pure metals and alloys1991In: Landolt-Börnstein: Numerical Data and Functional Relationships in Science and Technology, New Series: Group III: Crystal and Solid State Physics; Vol. 15: Metals, subvolume c: Thermal Conductivity of Pure Metals and Alloys, Berlin: Springer-Verlag , 1991, p. 449-460Chapter in book (Other academic)

Overview and complete collection of all available data for the pressure dependence of the thermal conductivity of metals and alloys.

• 197.
Umeå University, Faculty of Science and Technology, Physics.
Comment on “Characteristics of silicone fluid as a pressure transmitting medium in diamond anvil cells” [Rev. Sci. Instrum. 75, 4450 (2004)]2005In: Review of Scientific Instruments, ISSN 0034-6748, Vol. 76, no 5, p. 057101-Article in journal (Refereed)

A recent article [Y. Shen, R. S. Kumar, M. Pravica, and M. F. Nicol, Rev. Sci. Instrum. 75, 4450 (2004)] argues that silicone fluids are almost ideal pressure transmitting media at room temperature. However, silicone fluids solidify (vitrify) at pressures near 1 GPa, much lower than the solidification pressures of commonly used pressure media, and crystallization may occur under some conditions. Other fluids may thus be a better choice for transmitting pressure to strain sensitive samples in the range 1-10 GPa.

• 198.
Umeå University, Faculty of Science and Technology, Physics.
Comment on "Pressure and temperature diagram of polymerized fullerite"1998In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 57, no 5, p. 3164-3165Article in journal (Refereed)