Viatcheslav Agafonov

'Low-pressure' orthorhombic phase formed from pressure-treated C60

Abstract X-ray and electron diffraction and Raman spectroscopy have shown that the solid formed by a pressure-temperature treatment of C 60 at 1.5 GPa-723 K has an orthorhombic structure O′. It may be considered that phases O′ and O, the previously known orthorhombic phase formed at 8 GPa-573 K, are the same so that our pressure-temperature treatment provides “the optimal way of obtaining the linear-chain orthorhombic fullerene phase”. However, differences in the respective spectra of these two phases led us to re-examine the structure of the previously described phase O: it is found that a rhombohedral structure fits the experimental X-ray data at least as well as an orthorhombic one does. In any case, phase O′ may be an intermediary for the formation of the tetragonal high-pressure modification.

A new hexagonal phase of fullerene C60

Abstract A new phase of fullerene C 60 with a simple hexagonal unit cell with 6/mmm symmetry was grown by slowly evaporating solutions of C 60 in dichloromethane. X-ray measurements reveal that the c / a ratio is 1.616 at 298 K and increases as temperature decreases. At low temperature, it seems to extrapolate to 1.633, the ideal ratio for a close-packed hexagonal lattice. The unit cell volume which is higher than that of the usual cubic C 60 phases at high temperature decreases near 90 K. A structural model is proposed according to which a 3-fold molecular axis is parallel to the 6-fold crystallographic z -axis. This suggests that the molecules could reorient around this axis at high temperature.

Pressure-induced dimerization of fullerene C60: a kinetic study

Abstract The kinetics of pressure-induced dimerization of fullerite C60 at 1.5 GPa in the 373–473 K temperature interval was studied by X-ray diffraction, infrared and Raman spectroscopy. Kinetic curves of the dimerization reaction in the fcc and sc phases of C60 were obtained by monitoring the dimeric (C60)2 IR line at 796 cm−1. The value of the dimerization activation energy was determined to be E a ( dim ) =134±6 kJ mol −1 , assuming the second order irreversible reaction. The peculiarities of the dimerization processes in the fcc and sc phases of C60 fullerite are also discussed.

Fullerene C60, 2CCl4 solvate. A solid-state study

By slowly evaporating solutions of fullerene C60 in CCl4 at room temperature, a stable solvate (C60, 2CCl4) crystallizes in a simple hexagonal system (Laue class 6/mmm) with a=10.10(5) A and c = 10.75(5) A. This solvate undergoes a phase transition at 210–220 K and decomposes at 397 K. Atomic force microscopy of the (001) face shows a C60 molecular packing analogous to that which exists in the {111} planes of face-centered cubic C60. It seems likely that C60 and CCl4 molecules are orientationally disordered at room temperature.

The crystal structure of the 2D polymerized tetragonal phase of C60

Abstract Single crystals of the 2D polymerized tetragonal phase of C 60 without orientational domains were obtained under high pressure and high temperature. The crystal structure of this phase was resolved using single crystal X-ray diffraction data. The cell parameters are a =9.064(3) and c =15.039(8) A with the space group P4 2 / mmc and Z =2. 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 P4 2 / mmc layers (84%) and of the Immm layers (16%), along the c -axis.

Decagonal C60 crystals grown from n-hexane solutions: solid-state and aging studies

Abstract Decagonal C60 crystals grown from n-hexane solutions correspond to an orthorhombic 1:1 solvate (a=10.249 A, b=31.308 A, c=10.164 A). It forms with negative excess volume ( −55.5 A 3 per formula unit) and transforms on heating into fcc C60 (desolvation enthalpy of +50.6 kJ per solvate mole, close to the sublimation enthalpy for pure n-hexane) while n-hexane desorption from fcc C60 is accompanied by an enthalpy of +48.6 kJ per solvent mole. Thus solvate formation is preferred to solvent adsorption. Orthorhombic C 60 ·1 n -hexane undergoes no degradation when stored in air for 9 years at room temperature in the dark.

Spectroscopic properties of individual pressure-polymerized phases of C60

Abstract The Raman and IR spectra of pure polymerized phases of C 60 obtained at high pressure and temperature were investigated. Our data showed that some previous spectra were obtained from a mixture of different polymeric states. Determination of the characteristic bands of dimer, chain and layer polymers permit the use of the IR and Raman spectra in the molecular fractional analysis of different polymerized states of C 60 .

The Influence of C60 Powders On Cultured Human Leukocytes

Abstract In order to check its possible acute toxicity, C60 was incorporated into living human phagocytes. It was observed that C60 has no influence on the survival of human leukocytes.

Molecular packing of fullerene C60 at room temperature

A single crystal of C60 grown from a toluene solution has been studied by X-ray diffraction at room temperature. The lattice is face-centered cubic, with space group Fm3m and Z = 4, which agrees with previous powder diffraction measurements. It is shown that, contrary to what is obtained in other plastic crystals, the Pauling-Fowler model (the so-called free rotation one), which implies an isotropic molecular disorder, gives a better description of the molecular packing than the site model does. It is concluded that the molecules undergo a rotational diffusion as previous molecular dynamics simulations have described.

Solid-state studies on C60 solvates grown from n-heptane

Crystallographic and thermodynamic experiments show that crystalline C60 obtained by slow evaporation of solutions in n-heptane is a C60, n-heptane 1:1 solvate. Its lattice is hexagonal, Laue class 6/mmm, with a = 10.00(4) A and c = 10.16(1) A. No transition is observed at temperatures higher than 100 K, and desolvation into fcc C60 occurs at about 360 K with ΔH = +43.5 J g−1, close to the sublimation enthalpy for pure n-heptane. Another binary compound, presumably a polymorph of the former, is sometimes obtained by rapid evaporation of toluene + n-heptane mixtures. Its lattice is orthorhombic, Immm, with a = 10.07 A, b = 10.22 A and c = 48.9 A.