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  • 1. Aksenova, N.A.
    et al.
    Isakina, A.P.
    Prokhvatilov, A.I.
    Strzhemechny, M.A.
    Soldatov, Alexander V.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Structure studies of C60 polymerized at low pressures1997In: Fullerenes: Recent Advances in the Chemistry and Physics of Fullerenes and Related Materials, vol. 5, Pennington, NJ: The Electrochemical Society , 1997, 687-694 p.Conference paper (Refereed)
    Abstract [en]

    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.

  • 2. Aleksandrovskii, A. N.
    et al.
    Dolbin, A.V.
    Esel'son, V.B.
    Gavrilko, V.G.
    Manzhelii, V.G.
    Udovidchenko, B.G.
    Bakai, A.S.
    Gadd, G.E.
    Moricca, S.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Low-temperature thermal expansion of pure and inert gas-doped fullerite C602003In: Fizika Nizkikh Temperatur (Kharkov), ISSN 0132-6414 (Print) 1816-0328 (On-line), Vol. 29, no 4, 432-442 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 3. Aleksandrovskii, A. N.
    et al.
    Esel'son, V.B.
    Manzhelii, Vadim Grigorovich
    Udovidchenko, B.G.
    Soldatov, Alexander
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Negative thermal expansion of fullerite C60 at helium temperatures1997In: Low temperature physics (Woodbury, N.Y., Print), ISSN 1063-777X, E-ISSN 1090-6517, Vol. 23, no 11, 943-946 p.Article in journal (Refereed)
    Abstract [en]

    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

  • 4. Aleksandrovskii, A. N.
    et al.
    Esel'son, V.B.
    Manzhelii, V.G.
    Udovidchenko, B.G.
    Soldatov, Alexander
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Thermal expansion of single-crystal fullerite C60 at helium temperatures2000In: Low temperature physics (Woodbury, N.Y., Print), ISSN 1063-777X, E-ISSN 1090-6517, Vol. 26, no 1, 75-80 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 5. Aleksandrovskii, A. N.
    et al.
    Gavrilko, V.G.
    Esel'son, V.B.
    Manzhelii, V. G.
    Udovidchenko, B.G.
    Maletskiy, V.P.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Effect of argon on the thermal expansion of fullerite C60 at helium temperatures2001In: Low temperature physics (Woodbury, N.Y., Print), ISSN 1063-777X, E-ISSN 1090-6517, Vol. 27, no 3, 245-246 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 6. Aleksandrovskii, A.N.
    et al.
    Bakai, A.S.
    Cassidy, D.
    Dolbin, A.V.
    Esel'son, V.B.
    Gadd, G.E.
    Gavrilko, V.G.
    Manzhelii, V.G.
    Moricca, S.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    On the polyamorphism of fullerite-based orientational glasses.2005In: Low Temperature Physics, ISSN 1063-777X, Vol. 31, no 5, 429-444 p.Article in journal (Refereed)
    Abstract [en]

    The dilatometric investigation in the temperature range of 2–28 K shows that a first-order

    polyamorphous transition occurs in the orientational glasses based on C60 doped with H2, D2 and

    Xe. A polyamorphous transition was also detected in C60 doped with Kr and He. It is observed that

    the hysteresis of thermal expansion caused by the polyamorphous transition (and, hence, the transition

    temperature) is essentially dependent on the type of doping gas. Both positive and negative

    contributions to the thermal expansion were observed in the low-temperature phase of the glasses.

    The relaxation time of the negative contribution occurs to be much longer than that of the positive

    contribution. The positive contribution is found to be due to phonon and libron modes, whilst the

    negative contribution is attributed to tunneling states of the C60 molecules. The characteristic

    time of the phase transformation from the low-T phase to the high-T phase has been found for the

    C60–H2 system at 12 K. A theoretical model is proposed to interpret these observed phenomena.

    The theoretical model proposed, includes a consideration of the nature of polyamorphism in

    glasses, as well as the thermodynamics and kinetics of the transition. A model of noninteracting

    tunneling states is used to explain the negative contribution to the thermal expansion. The experimental

    data obtained is considered within the framework of the theoretical model. From the theoretical

    model the order of magnitude of the polyamorphous transition temperature has been estimated.

    It is found that the late stage of the polyamorphous transformation is described well by the

    Kolmogorov law with an exponent of n = 1. At this stage of the transformation, the two-dimensional

    phase boundary moves along the normal, and the nucleation is not important.

  • 7. Aleksandrovskii, A.N.
    et al.
    Gavrilko, V.G.
    Esel'son, V.B.
    Manzhelii, V. G.
    Udovidchenko, B.G.
    Maletskiy, V.P.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Thermal expansion of fullerite C60 alloyed with argon and neon2001In: Low temperature physics (Woodbury, N.Y., Print), ISSN 1063-777X, E-ISSN 1090-6517, Vol. 27, no 12, 1033-1036 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 8.
    Andersson, Britt M.
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Easterling, Kenneth E.
    Guo, Shi-ju
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Electrical resistivity and critical temperature of Bi-based high-Tc superconductors to 1 GPa1990In: High Pressure Research, volumes 3 to 5: High Pressure Science and Technology: Proceedings of the 12. AIRAPT and 27. EHPRG Conference, University Paderborn 1989, London: Gordon and Breach , 1990, 120-122 p.Conference paper (Refereed)
    Abstract [en]

    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.

  • 9.
    Andersson, Britt M.
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Easterling, Kenneth E.
    Loberg, Bengt
    Niska, John
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    High-pressure properties of high-Tc superconductor samples produced by hot isostatic pressing1990In: High Pressure Research, volumes 3 to 5: High Pressure Science and Technology: Proceedings of the 12. AIRAPT and 27. EHPRG Conference, University Paderborn 1989, London: Gordon and Breach , 1990, 123-126 p.Conference paper (Refereed)
    Abstract [en]

    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.

  • 10.
    Andersson, Britt M.
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Defects and the thermal conductivity of YBa2Cu3O7-d and YBa2Cu4O81993In: Journal of Alloys and Compounds, volume 195: Proceedings of Symposium A on High Tc Superconductors of the 1992 E-MRS Fall Conference, Strasbourg 1992, Elsevier Sequoia S.A. , 1993, 655-658 p.Conference paper (Refereed)
    Abstract [en]

    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.

  • 11.
    Andersson, Britt M.
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Thermal conductivity and thermal diffusivity of dense, bulk YBa2Cu4O81994In: High Temperatures - High Pressures volume 26, issue 1: Proceedings of the 13th European Conference on Thermophysical Properties (13 ECTP), Lisboa 1993, London: Pion Press , 1994, 53-57 p.Conference paper (Refereed)
    Abstract [en]

    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.

  • 12.
    Andersson, Britt M.
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Thermal conductivity of YBa2Cu4O8 dominated by phonon-phonon interactions1993In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 48, no 5, 3575-3578 p.Article in journal (Refereed)
    Abstract [en]

    The thermal conductivity κ of dense sintered ceramic YBa2Cu4O8 in the range 30–310 K has been measured. At 100 K, κ is 10 W m-1 K-1, approaching the in-plane κ of single crystals of other high-Tc materials. κ decreases rapidly with increasing T to 7.4 W m-1 K-1 at 300 K. Fitting standard models for κ(T) to the data we find that κ is limited mainly by phonon-phonon interactions. Depending on the model used, the best fit is found for effective values of FTHETADebye between 155 and 185 K, characteristic for the acoustic phonon branches, indicating that such phonons carry most of the heat. Finally, we suggest a possible way to test the electron-phonon model for the electrical and thermal conductivities in high-Tc materials.

  • 13.
    Andersson, Britt M.
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Thermal conductivity of YBa2Cu4O8 under high pressure1993In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 216, no 1-2, 187-194 p.Article in journal (Refereed)
    Abstract [en]

    The thermal diffusivity a of YBa2Cu4O8 (1-2-4) has been measured as a function of pressure p and temperature T over the range 85<T<310 K and up to 1 GPa (10 kbar). From the data obtained we have calculated the pressure dependence of the thermal conductivity κ as a function of T. κ increases slowly with increasing p above 150 K but decreases below this value. Most of the high-T increase with p is caused by an increase in the electronic thermal conductivity κe. For the phonon thermal conductivity, which dominates κ in 1-2-4, we find that charge transfer with p causes a strong increase in electron-phonon scattering with p, while the p dependence of the boundary scattering and phonon-phonon scattering is in reasonable agreement with the behaviour predicted by simple models.

  • 14.
    Andersson, Britt M.
    et al.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Thermal diffusivity and thermal conductivity of YBa2Cu3O7-d and YBa2Cu4O8 between 20 and 300 K1992In: Proceedings of the Joint Nordic Spring Meeting '92 / [ed] Per-Anker Lindgård, Roskilde: Risö National Laboratory/Grafisk Service, Risö , 1992, 251- p.Conference paper (Other academic)
  • 15.
    Andersson, Britt M.
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Niska, John
    Loberg, Bengt
    Thermal conductivity of polycrystalline YBa2Cu4O81994In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 49, no 6, 4189-4198 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 16.
    Andersson, Britt M.
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Niska, John
    Loberg, Bengt
    Easterling, Kenneth
    Electrical transport in dense, bulk YBa2Cu4O8 produced by hot isostatic pressing1990In: Physica B Volumes 165 & 166, Part 2: LT-19 Part II: Contributed Papers, Proceedings of the 19th International Conference on Low Temperature Physics, Brighton 1990, Elsevier B.V. , 1990, 1699-1700 p.Conference paper (Refereed)
    Abstract [en]

    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.

  • 17.
    Andersson, Britt M.
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Niska, John
    Loberg, Bengt
    Easterling, Kenneth
    Electrical transport properties of dense bulk YBa2Cu4O8 produced by hot isostatic pressing1990In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 170, no 5-6, 521-531 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 18.
    Andersson, Göran
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Pressure dependence of the c-axis resistance of stage 2 SbCl5 intercalated graphite to 4 GPa1988In: Solid State Communications, ISSN 0038-1098, E-ISSN 1879-2766, Vol. 65, no 7, 735-738 p.Article in journal (Refereed)
    Abstract [en]

    The c-axis electrical resistance R has been measured under pressure up to 4 GPa (40 kbar) at room temperature for stage 2, HOPG-based SbCl5 intercalated graphite. In addition to the well known ordering transition at 0.3–0.5 GPa a second anomaly in R is observed above 1 GPa, corresponding to a further phase transition which is not completed until above 2.4 GPa. In all phases R depends more strongly on p for the intercalated material than for HOPG, and the total decrease in R is over 80% to 4 GPa.

  • 19.
    Andersson, Göran
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Bäckström, Gunnar
    Umeå University, Faculty of Science and Technology, Physics.
    A high-pressure cell for electrical resistance measurements at hydrostatic pressures up to 8 GPa: Results for Bi, Ba, Ni, and Si1989In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 65, no 10, 3943-3950 p.Article in journal (Refereed)
    Abstract [en]

    A modified Bridgman anvil high-pressure device, capable of producing hydrostatic pressures up to 8 GPa (80 kbar), was designed and built. The size of the pressure chamber (10 mm in diameter) allows the use of large specimens and simple experimental procedures. Experimental results show that hydrostatic conditions are necessary if accurate quantitative information is desired about the electrical properties of materials under pressure. Accurate data on resistance (and resistivity) versus pressure at 294 K are given for Bi, Ba, Ni, and Si. The initial pressure coefficients of R were d(ln R)dP=0.13, −7.6×10−2, −2.0×10−2, and −0.26 GPa−1, respectively. Barium has a resistance minimum near 0.9 GPa. For Bi we observe sharp transitions at 2.55, 2.7, and 7.7 GPa, and for Ba at 5.55 GPa, but we cannot verify the existence of a transition in Ba near 7 GPa. Neither do we confirm the phase transformation in Ni recently reported to occur above 2.5 GPa. For Si, R(P) agrees very well with a theoretical function calculated from the change in band gap and electron mobility with pressure.

  • 20.
    Andersson, Göran
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Bäckström, Gunnar
    Umeå University, Faculty of Science and Technology, Physics.
    Electrical resistivity measurements under hydrostatic conditions up to 10 GPa1984In: Journal de Physique Colloques vol. 45: Supplément C8, Physics and Physicochemistry of Highly Condensed Matter; Proceedings of the XXIInd Meeting of the EHPRG, Aussois, Sept. 1984, 1984, 379-382 p.Conference paper (Refereed)
    Abstract [en]

    A large scale Bridgman anvil system has been designed. Steel gaskets permit compression of a methanol-ethanol medium over a volume of 500-250 mm3. Up to 12 wires have been used to contact specimens. The system has so far served up to 7.5 GPa, with anvils made of ASP tool steel. The transition Bi III-V was found to occur at a lower pressure than the recommended average. The electrical resistance of copper was measured up to 6 GPa under hydrostatic conditions.

  • 21.
    Andersson, Odd E.
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Behrens, Peter
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    C-axis resistance of HgCl2 intercalated graphite as a function of temperature and pressure1989In: Synthetic Metals volume 34: Proceedings of the Fifth International Symposium on Intercalated Graphite, Berlin (West) 1989, Lausanne: Elsevier Sequoia SA , 1989, 485-490 p.Conference paper (Refereed)
    Abstract [en]

    The c-axis resistance of stage 3 HgCl2 intercalated graphite has been measured as a function of temperature T and pressure p in the range 40 to 300 K and up to 1 GPa (10 kbar). The absolute resistivity and its T- and p- dependence are all in good agreement with data for other acceptor compounds. The measurements show that a previosly unknown phase transformation occurs above 0.4 GPa. Below 250 K the high pressure phase is metastable down to zero pressure, and an x-ray diffraction analysis of metastable material shows that no stage change occurs. The transition thus probably leads to a different in-plane structure characterized by a more densely packed intercalate layer.

  • 22.
    Andersson, Odd E.
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Lelaurain, Michelle
    Marêché, Jean-Francois
    McRae, Edward
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Pressure-induced modifications of structural and transport properties in SbCl5 intercalated graphite1992In: Materials Science Forum volumes 91-93, Part I: Intercalation Compounds ISIC-6: Proceedings of the Sixth International Symposium on Intercalation Compounds, Orléans 1991., Zürich: Trans Tech Publications , 1992, 301-306 p.Conference paper (Refereed)
    Abstract [en]

    The structure and the c-axis resistivity of SbCl5 intercalated graphite have been studied as functions of T and p, over the ranges 40 to 295 K and 0 to 0.7 GPa. Slow cooling or slow application of pressure p > 0.4 GPa lead to states of complete but different crystallization of the intercalate, but after rapid cooling or pressurization any crystallization is incomplete. The resistivity of the high pressure crystallized phase is about 50% lower than that of the low T, low p phase under identical p-T conditions. No significant difference is seen between the behaviour of HOPG-based and single crystal based materials.

  • 23.
    Andersson, Odd E.
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Lelaurain, Michelle
    Marêché, Jean-Francois
    McRae, Edward
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Structure, c-axis resistivity, and p-T phase diagram of SbCl5 intercalated graphite1989In: Synthetic Metals volume 34: Proceedings of the Fifth International Symposium on Intercalated Graphite, Berlin (West) 1989, Lausanne: Elsevier Sequoia SA , 1989, 187-192 p.Conference paper (Refereed)
    Abstract [en]

    Experimental data are reported for the c-axis resistivity varrhoc of stages n = 3,4,5 and 8 SbCl5 intercalated graphite as a function of T and p, in the range 40 – 300 K and 0-0.5 GPa (5 kbar). varrhoc varied in a systematic way with p, T, and n. Well ordered low-T phases were obtained for all stages; disordering at 200 – 235 K gave rise to well defined multiple anomalies in varrhoc. We also report x-ray diffraction data on the stage 2 compound: evidence was found for large variations in the in-plane structure even for nominally identical samples.

  • 24. Andersson, Odd E.
    et al.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Low temperature calibration of Manganin pressure gauges1997In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 68, no 2, 1344-1345 p.Article in journal (Refereed)
    Abstract [en]

    High pressures are often measured using the resistance of Manganin wires. However, the pressure coefficient of resistance is known to depend on temperature. We have measured this temperature dependence by comparing the output from a Manganin gauge with that of a well calibrated Zeranin gauge and determined a correction factor which enables us to measure the pressure at any temperature between 150 and 300 K with a temperature dependent error well below 0.5%.

  • 25.
    Andersson, Odd E.
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    McRae, Edward
    Lelaurain, Michelle
    Marêché, Jean-Francois
    A study of temperature and pressure induced structural and electronic changes in SbCl5 intercalated graphite: Part IV: Basal plane resistivity1995In: Journal of Materials Research, ISSN 0884-2914, E-ISSN 2044-5326, Vol. 10, no 7, 1653-1660 p.Article in journal (Refereed)
    Abstract [en]

    Using an inductive technique, we have measured the in-plane resistivity rhoa of stages 2, 4, 5, and 8 SbCl5-GIC's versus temperature T and pressure p in the ranges 130–300 K and 0–0.85 GPa. The room temperature values of rhoa range from 4.0 µOcm for the stages 5 sample to 7.7 µOcm for the stage 8 sample. At all pressures, rhoa shows a metallic temperature dependence rhoa ~ Ta, with 1 < a < 2, but in contrast to the c-axis resistivity rhoc, it depends only very weakly on pressure and/or intercalate structural order. We show that the behavior observed is consistent with a band conduction model.

  • 26.
    Andersson, Odd E.
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    McRae, Edward
    Marêché, Jean-Francois
    Lelaurain, Michelle
    A study of temperature and pressure induced structural and electronic changes in SbCl5 intercalated graphite: Part II. Experimental data for c-axis resistivity1992In: Journal of Materials Research, ISSN 0884-2914, E-ISSN 2044-5326, Vol. 7, no 11, 2989-3000 p.Article in journal (Refereed)
    Abstract [en]

    We present experimental data for the c-axis resistivity ?c of stage s = 2, 3, 4, 5, and 8, SbCl5 intercalated graphite, over the temperature range 40 to 300 K and the pressure range 0 to 0.8 GPa (0–8 kbar). For most specimens studied, resistance anomalies are observed below 230 K at atmospheric pressure and at pressures up to 0.5 GPa at 293 K. These anomalies are explained in terms of structural changes from a disordered or partly crystallized in-plane intercalate structure at atmospheric pressure and T > 230 K to an almost completely crystallized structure below this temperature, or at elevated pressures, as discussed in a companion paper. In the crystallized phases ?c is approximately linear in T except for stage 8, for which a nonlinear behavior, with a negative temperature coefficient of resistivity below 200 K, is observed. The results are compared with previously available literature data, and the p-T phase diagram is briefly discussed.

  • 27.
    Andersson, Odd E.
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Polo, V.
    Vangelisti, René
    Marêché, Jean Francois
    McRae, Edward
    The c-axis resistivity for two HOPG-based, first stage CuAl2Cl8 and second stage CuCl2 - graphite intercalation compounds as a function of both pressure and temperature1996In: 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, 719-723 p.Conference paper (Refereed)
    Abstract [en]

    In this paper we present data for the c-axis resistivity of first stage CuAl2Cl8 GIC and second stage CuCl2 GIC as functions of pressure, p, (0.0 < p < 1.5 GPa) and of temperature, T, (4 < T < 300 K). We show evidence of a possible new high-pressure phase above 1.0 GPa for the CuAl2Cl8 GIC and we show that the CuCl2 GIC is very stable at all pressures and temperature investigated. We make a comparison with previous results on SbCl5 GICs and other chloride intercalation compounds and with theories treating the temperature and pressure coefficients of transport parameters in layered solids.

  • 28.
    Andersson, Ove
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Filinchuk, Yaroslav
    Dmitriev, Vladimir
    Quwar, Issam
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Talyzin, Alexandr
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Phase coexistence and hysteresis effects in the pressure-temperature phase diagram of NH3BH32011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 2, 024115- p.Article in journal (Refereed)
    Abstract [en]

    The potential hydrogen storage compound NH3BH3 has three known structural phases in the temperature and pressure ranges 110–300 K and 0–1.5 GPa, respectively. We report here the boundaries between, and the ranges of stability of, these phases. The phase boundaries were located by in situ measurements of the thermal conductivity, while the actual structures in selected areas were identified by in situ Raman spectroscopy and x-ray diffraction. Below 0.6 GPa, reversible transitions involving only small hysteresis effects occur between the room-temperature tetragonal plastic crystal I4mm phase and the low-temperature orthorhombic Pmn21 phase. Transformations of the I4mm phase into the high-pressure orthorhombic Cmc21 phase, occurring above 0.8 GPa, are associated with very large hysteresis effects, such that the reverse transition may occur at up to 0.5 GPa lower pressures. Below 230 K, a fraction of the Cmc21 phase is metastable to atmospheric pressure, suggesting the possibility that dense structural phases of NH3BH3, stable at room temperature, could possibly be created and stabilized by alloying or by other methods. Mixed orthorhombic Pmn21/Cmc21 phases were observed in an intermediate pressure-temperature range, but a fourth structural phase predicted by Filinchuk et al. [ Phys. Rev. B 79 214111 (2009)] was not observed in the pressure-temperature ranges of this experiment. The thermal conductivity of the plastic crystal I4mm phase is about 0.6 W m−1 K−1 and only weakly dependent on temperature, while the ordered orthorhombic phases have higher thermal conductivities limited by phonon-phonon scattering.

  • 29.
    Andersson, Ove
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Soldatov, Alexander
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Reorientational relaxation in C60 following a pressure induced change in the pentagon/hexagon equilibrium ratio1995In: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 206, no 3-4, 260-264 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 30.
    Andersson, Ove
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Soldatov, Alexander
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Thermal conductivity of C60 at pressures up to 1 GPa and temperatures in the range 50-300 K1996In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 54, no 5, 3093-3100 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 31.
    Andersson, Ove
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Soldatov, Alexander
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Thermal conductivity of C60 under high pressure1995In: Science and Technology of Fullerene Materials: Materials Research Society Symposium Proceedings vol. 359, Pittsburgh, PA: Materials Research Society , 1995, 549-554 p.Conference paper (Refereed)
    Abstract [en]

    We have measured the thermal conductivity lambda of highly pure polycrystalline C60 in the range 50 to 300 K under pressures up to 1 GPa. The results are discussed in terms of the lattice structure and dynamics. In particular, we discuss the phase diagram as delineated by anomalies observed in lambda and cp at the f.c.c.-to-s.c. transition at 260 K and the glass transition at Tg = 90 K, and also the effect on lambda of the orientational motion in the s.c. phase. The results are found to be compatible with a p/T phase diagram recently suggested by us.

  • 32.
    Andersson, Ove
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Bäckström, Gunnar
    Umeå University, Faculty of Science and Technology, Physics.
    A low-temperature high-pressure apparatus with a temperature control system1992In: High Pressure Research, ISSN 0895-7959, E-ISSN 1477-2299, Vol. 10, no 4, 599-606 p.Article in journal (Refereed)
    Abstract [en]

    A low-temperature high-pressure apparatus was designed using commercial cryogenic equipment. Pressures up to 1 GPa and temperatures down to 40 K can be obtained in a volume of up to 30 cm3. The apparatus is of the piston-cylinder type with a piston diameter of 45 mm, and the pressure can be varied at all temperatures, An adaptive temperature control system keeps the temperature inside the pressure cylinder constant to within ±0.1 K.

  • 33.
    Andersson, Per
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Pressure dependence of the thermal conductivity, thermal diffusivity, and specific heat of some polymers1975In: Journal of Polymer Science Polymer Physics Edition, ISSN 1542-9385, Vol. 13, no 2, 243-251 p.Article in journal (Refereed)
    Abstract [en]

    The pressure dependence of the specific heat of poly(methyl methacrylate), polystyrene, and atactic and isotactic polypropylene was determined from simultaneous measurements of thermal conductivity and diffusivity in a cylindrical geometry at 300°K and in the pressure range 0-37 kbar. The thermal conductivity and the diffusivity both increase strongly with pressure, while the specific heat decreases. The pressure dependencies are most pronounced at low pressures. The results are compared with other experimental results and with theoretical calculations.

  • 34. Araújo, C. Moysés
    et al.
    Ahuja, Rajeev
    Talyzin, Alexandr V
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Pressure-induced structural phase transition in NaBH42005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 72, no 5, 054125- p.Article in journal (Refereed)
    Abstract [en]

    We present a combined experimental and theoretical study of the technologically important NaBH4 compound under high pressure. Using Raman spectroscopy at room temperature, we have found that NaBH4 undergoes a structural phase transformation starting at 10.0 GPa with the pure high-pressure phase being established above 15.0 GPa. In order to compare the Raman data recorded under high pressure with the low-temperature tetragonal phase of NaBH4, we have also performed a cooling experiment. The known order-disorder transition from the fcc to the tetragonal structure was then observed. However, the new high pressure phase does not correspond to this low-temperature structure. Using first-principle calculations based on the density functional theory, we show that the high-pressure phase corresponds to the alpha-LiAlH4–type structure. We have found a good agreement between the measured and calculated transition pressures. Additionally, we present the electronic structure of both the fcc and the high-pressure phases.

  • 35. Bagatskii, M. I.
    et al.
    Sumarokov, V. V.
    Dolbin, Alexander V.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Low-temperature heat capacity of fullerite C-60 doped with deuteromethane2012In: Low temperature physics (Woodbury, N.Y., Print), ISSN 1063-777X, E-ISSN 1090-6517, Vol. 38, no 1, 67-73 p.Article in journal (Refereed)
    Abstract [en]

    The heat capacity C of fullerite doped with deuteromethane (CD4)(0.4)(C-60) has been investigated in the temperature interval 1.2-120K. The contribution Delta C-CD4 of the CD4 molecules to the heat capacity C has been isolated. It is shown that at T approximate to 120K the rotational motion of CD4 molecules in the octahedral voids of the C-60 lattice is weakly hindered. When the temperature is lowered to 80K, the rotational motion of the CD4 molecules changes from weakly hindered rotation to libration. In the range T = 1.2-30 K, Delta C-CD4 is described quite accurately by the sum of contributions from the translational and librational vibrations and tunneling rotation of CD4 molecules. The contribution of tunneling rotation to the heat capacity Delta C-CD4(T) is dominant below 5K. The effect of nuclear-spin conversion of the CD4 molecules on the heat capacity has been observed and the characteristic times for nuclear spin conversion between the lowest levels of the A- and T-species of CD4 molecules at T < 5K have been estimated. A feature observed in Delta C-CD4(T) near T = 5.5K is most likely a manifestation of a first-order phase transition in the orientational glass form of the solution. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3677237]

  • 36. Bagatskii, M.I.
    et al.
    Barabashko, M.S.
    Dolbin, Alexander V.
    Sumarokov, V.V.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    The specific heat and the radial thermal expansion of bundles of single-walled carbon nanotubes2012In: Low temperature physics (Woodbury, N.Y., Print), ISSN 1063-777X, E-ISSN 1090-6517, Vol. 38, no 6, 523-528 p.Article in journal (Refereed)
    Abstract [en]

    The specific heat at constant pressure C(T) of bundles of single-walled carbon nanotubes (SWNTs) closed at their ends has been investigated in the temperature interval of 2–120 K. It is found that the curve C(T) has features near 5, 36, 80, and 100 K. The experimental results on the C(T) and the radial thermal expansion coefficient αR(T) of bundles of SWNTs oriented perpendicular to the sample axis have been compared. It is found that the curves C(T) and αR(T) exhibit a similar temperature behavior at T > 10 K. The temperature dependence of the Grüneisen coefficient γ(T) has been calculated. The curve γ(T) also has a feature near 36 K. Above 36 K the Grüneisen coefficient is practically independent of temperature (γ ≈ 4). Below 36 K, γ(T) decreases monotonically with lowering temperature and becomes negative at T < 6 K.

  • 37. Bagatskii, M.I.
    et al.
    Manzhelii, V.G.
    Sumarokov, V.V.
    Dolbin, Alexander V.
    Barabashko, M.S.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Low-temperature dynamics of matrix isolated methane molecules in fullerite C60: the heat capacity, isotope effects2014In: Low temperature physics (Woodbury, N.Y., Print), ISSN 1063-777X, E-ISSN 1090-6517, Vol. 40, no 8, 678-684 p.Article in journal (Refereed)
    Abstract [en]

    The heat capacity of the interstitial solid solution (CH4)0.4C60 has been investigated in the temperature interval 1.4–120 K. The contribution of CH4 molecules to the heat capacity of the solution has been separated. The contributions of CH4 and CD4 molecules to the heat capacity of the solutions (CH4)0.40C60 and (CD4)0.40C60 have been compared. It is found that above 90 K the character of the rotational motion of CH4 and CD4 molecules changes from libration to hindered rotation. In the interval 14–35 K the heat capacities of CH4 and CD4 molecules are satisfactorily described by contributions of the translational and libration vibrations, as well as the tunnel rotation for the equilibrium distribution of the nuclear spin species. The isotope effect is due to mainly the difference in the frequencies of local translational and libration vibrations of molecules CH4 and CD4. The contribution of the tunnel rotation of the CH4 and CD4 molecules to the heat capacity is dominant below 8 K. The isotopic effect is caused by the difference between both the conversion rates and the rotational spectra of the nuclear spin species of CH4 and CD4 molecules. The conversion rate of CH4 molecules is several times lower than that of CD4 ones. Weak features observed in the curves of temperature dependencies of the heat capacity of CH4 and CD4 molecules near 6 and 8 K, respectively, are most likely a manifestation of first-order polyamorphic phase transitions in the orientational glasses of these solutions.

  • 38. Blank, Vladimir D.
    et al.
    Buga, Sergei G.
    Dubitsky, Gennadii A.
    Zhu, Xiao-Mei
    Umeå University, Faculty of Science and Technology, Physics.
    Nyeanchi, Emmanuel
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Electric resistivity and  magnetoresistance of some superhard and ultrahard fullerites in the range 300-2K1999In: 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, 416-420 p.Conference paper (Refereed)
    Abstract [en]

    Electric resistivity and magnetoresistance were measured on samples with disordered structure synthesized from pure C60 and C70 at pressures in the range 8 - 12.5 GPa and temperature 900 - 1500 K. Different type of behaviour were observed: semimetallic and semiconducting, depending on the particular short-range order of the structure.

  • 39. Blank, Vladimir D.
    et al.
    Buga, Sergey G.
    Dubitsky, Gennadii A.
    Serebryanaya, Nadejda R.
    Popov, M. Yu.
    Kulnitskiy, B.A.
    Prokhorov, V.M.
    Levin, V.M.
    Zhu, Xiaomei
    Nyeanchi, Emmanuel
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Fransson, Åke
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    High-pressure synthesis, structure and properties of superhard and ultrahard fullerites2000In: High Pressure - 2000: Proceedings of the VI International Conference on High Pressure, Donetsk, Ukraina: Donetsk Intitute for Physics and Engineering , 2000, 127-132 p.Conference paper (Refereed)
  • 40.
    Blank, Vladimir D.
    et al.
    Research Center for Superhard Materials, Centralnaya 7a, Troitsk, Moscow Region 142092, Russian Federation.
    Buga, Sergey G.
    Research Center for Superhard Materials, Centralnaya 7a, Troitsk, Moscow Region 142092, Russian Federation.
    Dubitsky, Gennadii A.
    Institute of Spectroscopy of the Russian Academy of Sciences, Troitsk, Moscow Region 142092, Russian Federation.
    Serebryanaya, Nadejda R.
    Research Center for Superhard Materials, Centralnaya 7a, Troitsk, Moscow Region 142092, Russian Federation.
    Popov, M. Yu.
    Research Center for Superhard Materials, Centralnaya 7a, Troitsk, Moscow Region 142092, Russian Federation.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    High-pressure polymerized phases of C601998In: Carbon, ISSN 0008-6223, E-ISSN 1873-3891, Vol. 36, no 4, 319-343 p.Article, review/survey (Other academic)
    Abstract [en]

    Data from recent experimental studies of C60 under high pressures are collected and analyzed, concentrating on the polymerized states where covalent intermolecular bonds have been formed through treatment of molecular C60 at high pressures and temperatures at or above room temperature. We give an overview of the observed phase transformations and the structures of metastable polymeric phases, both crystalline and disordered, as analyzed under ambient conditions, and we present and discuss a pressure-temperature diagram showing which synthesis conditions result in which final phases in the pressure range up to 20 GPa (200 kbar) and at temperatures up to 2300 K. The physical properties of the various phases are discussed whenever enough data are available. The pressure-temperature stability limits for C60 and the conditions for the transformation of C60 into graphite, diamond and chaoite-type carbon are derived.

  • 41. Buga, Sergei
    et al.
    Blank, Vladimir
    Bagramov, Rustem
    Dubitsky, Gennadii
    Fransson, Åke
    Umeå University, Faculty of Science and Technology, Applied Physics and Electronics.
    Serebryanaya, Nadejda
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Thermal stability of superhard and ultrahard fullerites2000In: Proceedings of International Conference on Engineering and Technological Sciences 2000 (ICETS-2000), vol. 1, Beijng: Chinese Academy of Engineering: New World Press , 2000, 505-511 p.Conference paper (Refereed)
    Abstract [en]

    The thermal stability of superhard and ultrahard carbon phases synthesized from C60 and C70 fullerenes under pressure in the range 9.5-13 GPa and temperature between 670 and 1850 K was investigated by DSC in the range 260-640 K and TGA up to 1200 K. Endothermal heat effects were 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.

  • 42. Buga, Sergei G.
    et al.
    Blank, Vladimir D.
    Dubitsky, Gennadii A.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Zhu, Xiaomei
    Nyeanchi, Emmanuel B.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Semimetallic and semiconductor properties of some superhard and ultrahard fullerites in the range 300-2 K2000In: Proceedings of the 5th IUMRS International Conference on Advanced Materials, Beijing 1999, Elsevier B.V. , 2000, 1009-1015 p.Conference paper (Refereed)
    Abstract [en]

    Electrical resistivity and magnetoresistance were measured on samples with disordered structures synthesized from pure C60 and C70 at pressures in the range 8–12.5 GPa and temperatures of 900–1500 K. Different types of behaviour were observed: semimetallic, VRH and semiconducting, depending on the degree of disorder and the particular short-range order of the samples. A negative magnetoresistance was observed at T<10 K on samples with a semimetallic type of conductivity synthesized at 8 GPa pressure. The temperature dependence of resistivity in the sample with a disordered crystalline structure based on 3D-polymerized C60 molecules fits Mott's law for hopping conductivity. T3/2, T2 and T4 dependencies of conductivity are observed for samples with densities of 2.8 and 3.05 g/cm3 synthesized at a pressure of 12.5 GPa. The effect of hydrostatic pressure on the resistivity of cross-linked layered carbon structures obtained from C60 at P=8 GPa, T=1600 K was investigated up to 0.6 GPa at room temperature. An approximately linear decrease of resistivity was observed with a very small value of the derivative d ln ρ/dp=0.06 /GPa, which correlates with a very low compressibility of the material.

  • 43. Buga, Sergei G.
    et al.
    Blank, Vladimir D.
    Dubitsky, Gennadii A.
    Serebryanaya, Nadejda R.
    Fransson, Åke
    Umeå University, Faculty of Science and Technology, Applied Physics and Electronics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Synthesis of superhard 3D-polymeric C60 fullerites from rhombohedral 2D-polymer by high-pressure-high-temperature treatment.2003In: High Pressure Research vol. 23, issue 3: Proceedings of the 40th European High Pressure Research Group Meeting on Advances in High Pressure Research (EHPRG'40), London: Taylor & Francis , 2003, 259-264 p.Conference paper (Refereed)
    Abstract [en]

    Rhombohedral C60 polymer was subjected to high-pressure-high-temperature treatment at P =13 GPa, T =620-1620 K. After quenching, crystalline and disordered structures with densities in the range of 2.1-2.9 g cm-1 were obtained. The structures of the samples have been investigated by powder X-ray diffraction and Raman scattering. DSC analysis showed a transformation of the polymeric structure into monomeric on annealing in the range 400-640 K. The temperature dependence of the electrical resistance of samples with disordered structure was measured in the range 2.5-300K. For different samples, the conductivity was proportional to T1/2, T3/2, T4 and exp(-1/T1/4).

  • 44. Buga, Sergei G.
    et al.
    Blank, Vladimir D.
    Fransson, Åke
    Umeå University, Faculty of Science and Technology, Applied Physics and Electronics.
    Serebryanaya, Nadejda R.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    DSC study of annealing and phase transformations of C60 and C70 polymerized under pressures in the range 9.5 - 13 GPa.2002In: Journal of Physics and Chemistry of Solids, ISSN 0022-3697, E-ISSN 1879-2553, Vol. 63, no 2, 331-343 p.Article in journal (Refereed)
    Abstract [en]

    C60 and C70 fullerenes polymerized under pressures between 9.5 and 13 GPa and temperatures between 670 and 1850 K were investigated by differential scanning calorimetry (DSC) in the range 240–640 K. Endothermal heat effects were 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.

  • 45. Buga, Sergei G.
    et al.
    Blank, Vladimir D.
    Serebryanaya, Nadejda R.
    Dzwilewski, Andrzej
    Umeå University, Faculty of Science and Technology, Physics.
    Makarova, Tatiana
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Electrical properties of 3D-polymeric crystalline and disordered C60 and C70 fullerites.2005In: Diamond and Related Materials vol. 14, special Issue (3-7): Proceedings of Diamond'04, 15th European Conference on Diamond and Diamond-Like Materials, Riva del Garda, Italy 2004., Elsevier B.V. , 2005, 896-901 p.Conference paper (Refereed)
    Abstract [en]

    Crystalline and disordered 3D-polymeric C60 and C70 with different lattice parameters and with densities in the range of 2.2÷3.25 g cm−1 were obtained by high-pressure-high-temperature treatment at P=11.5÷15 GPa, T=670÷1800 K. Their d.c. electrical conductivity was investigated at temperatures 30÷350 K. The variable range hopping mechanism dominates in all samples up to room temperature. A temperature activated conductivity via delocalized states was observed at T>160÷260 K in crystalline C60 samples and in some samples with disordered structure. The evaluated band gap value is Eg=0.28÷0.54 eV for different crystalline structures and 0.06÷0.25 eV for disordered ones

  • 46. Buga, Sergei G.
    et al.
    Blank, Vladimir D.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Dubitsky, Gennadii A.
    Prokhorov, V.M.
    Serebryanaya, Nadejda R.
    Kazarian, S.A.
    Oraevsky, A.N.
    Starodubtsev, N.F.
    Pressure effect on electrical properties and photoluminescence spectra of solid C60 and C70 fullerenes2001In: Frontiers of High Pressure Research II: Application of High Pressure to Low-Dimensional Novel Electronic Materials: Proceedings of the NATO Advanced Research Workshop, Pingree Park, CO, USA, from 10-15 June 2001, Dordrecht: Springer/Kluwer , 2001, 483-491 p.Conference paper (Refereed)
    Abstract [en]

    Electrical resistivity of crystalline and disordered fullerite samples obtained by static high-pressure-high-temperature treatment of C-60 and C-70 at P = 12.5 GPa and T = 820-1500 K was investigated in the temperature range of 2.4-300 K, Room-temperature activation energy of charge carriers was found to be in the range 40-200 meV. T-3/2 and T-4 dependencies of conductivity versus temperature were revealed both in crystalline and disordered structures. Photoluminescence spectra of C-60 samples treated at P = 13 GPa. T = 770-1470 K show 50 nm short-wave length shift of characteristic 750 nm PL band.

  • 47. Buga, Sergei G.
    et al.
    Fransson, Åke
    Umeå University, Faculty of Science and Technology, Applied Physics and Electronics.
    Serebryanaya, Nadejda R.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Blank, Vladimir D.
    Dubitsky, Gennadii A.
    Dissociation energy of 3D-polymeric C60: Calorimetric study and structural analysis2001In: Electronic Properties of Novel Materials - Science and Technology of Molecular Nanostructures: Proceedings of the XV International Winter School on Electronic Properties of Novel Materials, Kirchberg 2001, Melville, NY: American Institute of Physics , 2001, 49-53 p.Conference paper (Refereed)
    Abstract [en]

    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.

  • 48. Celzard, Alain
    et al.
    McRae, Edward
    Marêché, Jean-Francois
    Furdin, Guy
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Conduction mechanisms in some graphite-polymer composites: effects of temperature and pressure1998In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 83, no 3, 1410-1419 p.Article in journal (Refereed)
    Abstract [en]

    This article is devoted to the conduction mechanisms involved in some highly electrically anisotropic resin–graphite particle composites. These materials are known to show a percolation phenomenon as the filler content is varied; they are epoxy or polyurethane based, the conducting particles are oriented single-crystal platelets, and samples are in the form of thick films. Because of their strong anisotropy, two types of measurements were made, i.e., parallel to and perpendicular to the plane of the films. Study of the resistivity variations of samples containing various concentrations in conducting particles was carried out first as a function of temperature from 4.2 to 300 K at ambient pressure, and second as a function of hydrostatic pressure up to 1.2 GPa, at room temperature. As the temperature is varied, the changes in resistivity of all the samples studied (i.e., above percolation threshold) are weak; analysis leads to the conclusion that thermally activated tunneling plays a dominant role above but close to the percolation threshold phic. As a function of pressure, more samples were studied: when the filler content is above phic, resistivity changes are quantitatively in agreement with what is expected from both percolation theory and tunneling; below threshold, the observed behavior is partially attributable to an ionic conduction mechanism throughout the polymer.

  • 49. Chorro, Matthieu
    et al.
    Cambedouzou, Julien
    Iwasiewicz-Wabnig, Agnieszka
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Noé, Laure
    Rols, Stéphane
    Monthioux, Marc
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Launois, Pascale
    Discriminated structural behaviour of C60 and C70 peapods under extreme conditions2007In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 79, no 5, 56003- p.Article in journal (Refereed)
    Abstract [en]

    We studied structural changes in C60 and C70 peapods—i.e. C60 and C70 chains inserted inside single-walled carbon nanotubes—when submitted to high pressure and temperature conditions. X-ray diffraction experiments showed that while C60 molecules polymerize inside nanotubes at pressures and temperatures consistent with polymerization in bulk C60 phases, no polymerization is observed in C70 peapods, even at pressures and temperatures above the threshold for polymerization in bulk C70. Tubular confinement in the nanotube container prevents the monomers to rearrange in the way needed for polymerization. This result testifies for the strong influence of confinement on the behaviour of C70.

  • 50. Cui, Wen
    et al.
    Liu, Dedi
    Yao, Mingguang
    Li, Quanjun
    Liu, Ran
    Liu, Zhaodong
    Wu, Wei
    Zou, Bo
    Cui, Tian
    Liu, Bingbing
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Synthesis of alkali-metal-doped C60 nanotubes2011In: Diamond and Related Materials: Proceedings of New Diamond and Nano Carbon Conference (NDNC 2010), 2011, Vol. vol. 20, issue 2, 93-96 p.Conference paper (Refereed)
    Abstract [en]

    C60 nanotubes have been synthesized by a solution–solution method. After degassing in a dynamic vacuum, the C60 nanotubes were doped with alkali metals by means of vapor evaporation method. Different temperatures have been studied to evaporate the alkali metals for the doping experiments. Raman spectrum was further employed to analyze the doping concentration of the obtained samples. It was found that all three alkali metals (Li, Na and K) used can be efficiently doped into the C60 nanotubes, forming AxC60 nanotubes. The doping concentration of Li, Na changed from low to high level, depending on the experiment temperatures, while K doping always gave saturated doping. The melt points, the ionic sizes and vapor pressures of alkali metals were thought to affect the final doping results.

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