A. V. Rakhmanina

Thermal studies of C60 transformed by temperature and pressure treatments

Raman spectroscopy, X-ray and electron diffractions have been applied to study samples of fullerene C60 after they have undergone pressure and temperature treatments up to 8 GPa and 1073 K. It is confirmed that mixtures of rhombohedral and tetragonal structures are formed in the 2–4 GPa and 673–1073 K pressure-temperature domain. Traces of a hexagonal phase are also observed in the same range. Only the rhombohedral one is obtained in the 4–8 GPa range in the same temperature interval. The material obtained at temperatures between 473 and 673 K in the whole pressure range which has previously been described in terms of a cubic structure can be understood in terms of an orthorhombic one. It may also contain mixtures of different phases including a cubic one and even a cubic superstructure. The widened poorly resolved X-ray profiles indicate the existence of disorder within the materials formed and the increasing complexity of Raman spectra as temperature increases at fixed pressures (from 2 to 8 GPa) suggests that this disorder is related to phase mixtures in almost all samples. A rhombohedral sample formed at 6 GPa-873 K reverted to a mixture of rhombohedral and tetragonal phases at 2.5 GPa-873 K. Thus thermodynamic equilibrium between these two kinds of systems could exist.

High‐Resolution X‐Ray Powder Diffraction Structure Determination of C60F48

Abstract Whereas previously reported X‐ray powder diffraction experiments could be interpreted using an isotropic spherical two‐shell model, our recently obtained data cannot. Rather, they were Rietveld‐analyzed using three distinct anisotropic models for the C60F48 molecule featuring the S6, D3 and Th molecular symmetries, respectively. The best fit was obtained for the D3 model, which is consistent with the 19 F NMR characterization of our sample. The χ2 dependence upon the Euler angles of the C60F48 molecule is found to be substantial. The unit cell is very nearly (if not) tetragonal and P 21/n turns out to be a satisfactory space group.

Fluorination of Crystalline Polymerized Phases of C60 Fullerene

Abstract The reactivity of the orthorhombic (O), tetragonal (T), and rhombohedral (R) polymerized phases of C60 towards gaseous fluorine was studied in the 50–250°C temperature range. It was shown that the reactivity of the polymerized phases greatly surpasses that of C60 and increases during a transition from the O phase to the T‐ and further to R phases. Direct fluorination of the polymerized C60 phases provides an opportunity for production of fluoropolymers of C60 representing a new class of fluorocarbon materials.

Single Crystals Synthesis and Refinement of the Crystal Structure of the Polymerized Tetragonal Phase of C60

Abstract The possibilities of single crystals synthesis in conditions of nonisotropic one‐axis contraction were studied. As a result of the work the single crystals of the tetragonal polymerized phase of C60 without orientational domains were obtained. The crystal structure of this phase was resolved using single crystal x‐ray diffraction data. A structural model of this phase proposed early was confirmed and refined to final R = 0.075. The structure is slightly disordered. It consists of a random combination of the P 4 2/mmc layers (84%) and of the Immm layers (16%), along the c axis.

Study of C60 Peapods After a High-Pressure–High-Temperature Treatment

The present work is devoted to studying the products formed from C60 peapods after treatment at 8 GPa, from 200°C to 1200°C by SEM, TEM, XRD, and Raman techniques. It is confirmed that at temperatures up to 600°C the C60 molecules polymerize in forming the known 1D polymeric crystal with a C60 – C60 distance of 9 Å. In addition, we also find that at 1000°C the peapods begin to transform into different products including nanodiamond particles, amorphous carbon, and graphitic ribbons. At 1200°C only well-formed graphite is observed.

Nano-sized carbon structures in the thermal conversions of hydrocarbons at high pressures

Abstract As a consequence of high resolution transmission electron microscopic (HRTEM) investigation, it was established that different nanometric‐sized carbon structures (spherical onion‐like and coalesced two‐core onion‐like carbon particles, faceted polyhedral graphitic particles, graphitic ribbons, arched graphene sheets, and nanocrystalline diamonds) can be obtained as by‐products of the treatment of hydrocarbons at high pressures and temperatures.

Study of the Orthorhombic Polymeric Phase of C60 Under High Pressure Using Synchrotron X-Ray Powder Diffraction

As an extension of our previously published work (2007) on the orthorhombic phase of polymeric C60, high-resolution powder diffraction experiments under high pressure were recently carried out at the ESRF/SNBL/BM01A beamline. The acquisition times were very short, of the order of 10 minutes. In contrast to our first laboratory experiment, which involved much longer exposures (50–150 hours), no photo-induced transition to a crystalline state of lower symmetry could be observed up to 6 GPa. The obtained powder diffractograms are all consistent with an orthorhombic unit cell. A Birch-Murnaghan equation of state is fitted to the resulting pressure-volume data for the orthorhombic phase.

Probing the Dynamics of C60 Encaged Inside Single-Walled Carbon Nanotubes by Inelastic Neutron Scattering

In this paper, the important role of neutron spectroscopy for the study of the dynamics of C60 molecules hosted inside single‐walled carbon nanotubes (SWNT) is described. We find that the intramolecular vibrations of the fullerene are very sensitive to temperature and that an excess of low frequency modes is observed in the density of states below T = 50K. These are strong indications of some coupling between vibrational modes and whole molecule mobility of the confined fullerene. However, no libration modes of the C60 inside the nanotubes are observed, down to a temperature of 2K, indicating that the order‐disorder transition, if it exists, does not show up in the same way that it does for cubic C60. The lack of such ordering of the C60 chains inside the nanotubes could be explained by the low dimensionality of this system added to a smooth nanotubes field potential.