Fullerite C70 Research Articles

Thermodynamic properties of fullerite C70

A new expression for the isochoric heat capacity and the equation of state of fullerite C70 are obtained in the framework of a quantum-statistical method. Analogs of the Debye law and Dulong–Petit law for this fullerite are formulated. Fullerene C70 molecules are modeled by isotropic quantum oscillators under the assumption that their nonsphericity weakly influences the thermodynamic properties of the condensed phase. The intramolecular oscillations of carbon atoms are described using the Debye theory and the cold contribution to the free energy of fullerite is calculated using the Lennard-Jones pair potential for fullerene molecules. A comparison of the proposed theory to experiment shows good agreement.

Polyamorphous transition in amorphous fullerites C70

Samples of amorphous fullerites C70 have been obtained by mechanical activation (grinding in a ball mill). The structure of the samples has been investigated by neutron and X-ray diffraction. The high-temperature (up to 1200°C) annealing of amorphous fullerites revealed a polyamorphous transition from molecular to atomic glass, which is accompanied by the disappearance of fullerene halos at small scattering angles. Possible structural versions of the high-temperature amorphous phase are discussed.

Anisotropic Carbon Phases Prepared from Fullerite C70 under High Pressure

We present the structural and ultrasonic study of carbon phases prepared by quenching after heating of fullerite C70 in the temperature range 300–1100°C at pressures 4 and 7.5 GPa. The main aspect of the work concerns the structural and elastic anisotropy of samples resulted as consequence of quasi hydrostatic pressure with an additional pressure component along the axis of pressure load. Structural anisotropy correlates with anisotropy of the tensor of effective elastic constants taken for the medium with single axis of anisotropy. Comparison of the data obtained with anisotropy effects, earlier observed in the phases synthesized from C60 in the similar quasi hydrostatic conditions, clarifies the role of non‐spherical symmetry of C70 molecule.

Permutation-Inversion Group of Fullerite C70 in the Intermediate Phase

The permutation-inversion symmetry group of fullerite C 70 in the intermediate phase (at temperatures 270 K < T < 340 K) and the permutation-inversion site symmetry group of a rotating molecule C 70 in a nonrigid crystal are generated. Their irreducible representations compatible with the principle of symmetry of wave functions with respect to the permutations of identical nuclei are found. Group theoretical classification of quantum-mechanical states of a rotating molecule and a crystal in the intermediate phase of fullerite C70 is made. The selection rules for electronic and IR spectra in terms of the irreducible representations of the permutation-inversion group are found.

Energy spectrum and phase transitions in C70 fullerite crystals at high pressure

Measurements have been made of the Raman, optical absorption, and luminescence spectra of single crystals and pellets of the fullerite C70 at T=300 K and at pressures up to 12 GPa. The baric shift dω/dP and the Gruneisen parameters of the Raman-active intramolecular phonon modes have been determined. It has been established that the dω/dP value for certain phonon modes abruptly changes at pressures of P1≈2 GPa and P2≈5.5 GPa, as do the half-widths of the Raman lines. These features in the Raman spectrum are associated with phase transitions at high pressure. The baric shifts of the absorption and luminescence edges of C70 crystals have been determined and are −0.12 eV/GPa and −0.11 eV/GPa, respectively, for absorption and luminescence. The baric shift of the absorption edge decreases significantly with increasing pressure and is −0.03 eV/GPa at 10 GPa. These data have been used to determine the deformation potential of the fullerite C70, which is about 2.1±0.1 eV.

Electronic transitions and excitations in solid C70 studied by reflection electron energy loss spectroscopy

The electronic transition and excitation properties of highly ordered C70 films have been studied by reflection electron energy loss spectroscopy (EELS). The EELS study revealed a total of eleven features in the energy loss range 1–40 eV. These include a prominent peak at 2.55 eV revealing the first dipole allowed transition across the band gap in C70 fullerite; a feature at 2.20 eV correlated with the electron transition between the highest-occupied-molecular-orbital- and lowest- unoccupied-molecular-orbital-derived bands; two excitonic levels corresponding to a tight-binding Frenkel exciton in C70 fullerite at 1.53 and 1.88 eV; and a doublet at 5.2 and 5.9 eV showing evidence of the π-plasmon splitting in fullerite C70. A broad hump at ∼28 eV corresponds to the excitation of the (σ+π) plasmon.

Absorption spectra of fullerite C70 at pressures up to 10 GPa

The absorption spectra of thin polycrystalline C70 specimens have been measured in the energy range 1–3 eV at T = 300 K and pressures up to 10 GPa. The pressure dependence of the fundamental absorption edge position has been defined. A mean rate of the pressure shift is found to be −(0.1 ± 0.005) eV GPa in the starting range and −(0.029 ± 0.005) eV GPa −1 at 10 GPa.

Diamond recovered from Shocked Fullerites.

Fullerite C60 powders (with a small amount of fullerite C70) in copper matrix are subjected to shock loadings in the pressure-temperature range of 12 to 37GPa and 1100 to 2400K, and are investigated by X-ray diffraction, electron microscopy and Raman spectroscopy. Diamond is recovered in specimens quenched above 20GPa, and the typical grain size ranges 5 to 25nm. The C60 and C70 fullerites remain stable under shock conditions up to 19GPa. The high reactivity of C60 crystals is shown by formation of carbide, which indicates reaction with steel containers. Graphitic carbon and amorphous solid are detected as well.