Fullerite C60 Research Articles

Low temperature heat capacity and sound velocity in fullerite C60 orientational glasses

ABSTRACTThe nature of the linear term in the heat capacity of fullerite C60 has been investigated. The low-temperature dependence of the sound velocity has been determined from the data of the heat capacity at temperatures below 4 K. A model of the dynamic configuration excitations (DCE) is proposed to describe the contribution of the linear term in heat capacity and calculate the dependence of sound velocity. It is shown that this model, apparently, adequately describes the dynamics of cluster formations of the short-range order in fullerite C60 by taking into account excitations of both the atomic and electronic subsystems. In the framework of this model, it is shown that low-energy tunnel states that are located at the boundaries of C60 domains make a dominant contribution to the low-temperature effects in the heat capacity and sound velocity of C60.

Equation of state for fullerite C60

A new equation of state for fullerite C60 is derived in the framework of the quantum-statistical method. This equation includes two Gruneisen parameters responsible for vibration–rotational and intramolecular contributions of fullerene molecules, which are represented in the form of isotropic quantum oscillators. The intramolecular vibrations of carbon atoms are described by the Debye heat capacity theory, and the cold contribution to the free energy is calculated using the Lennard–Jones pair potential for fullerene molecules. The theory is in a very good agreement with the experiment.

Neutron diffraction study of metal-matrix composite with fullerite

Interaction of amorphous fullerite C60 with austenitic Fe-33.2 wt. % Ni alloy at pressures 0-8 GPa and temperatures 600-1100 °C was studied by neutron diffraction. The amorphous fullerite was obtained by ball milling and mixed with the powder of the crystalline alloy. The interaction at sintering led to the dissolution of carbon in fcc Fe-Ni solid solution and the formation of carbide (Fe, Ni)3C, but the Fe-Ni-C alloy did not undergo phase transformations and preserved the original fcc structure. As a result, the alloy hardened, we could also witness a clear barometric effect: at the pressure of 2 GPa the amount of the dissolved carbon and the microhardness turned out to be significantly higher than those at 8 GPa. During sintering amorphous fullerite is undergoing phase transitions and its microhardness is higher than the microhardness of the metal component. At high temperatures of interaction graphite appears. The presence of Fe-Ni alloy in the composite reduces the temperature of graphite formation in comparison with transformations in the pure amorphous fullerene.

Raman study of the temperature-induced decomposition of the fullerene dimers C120

Raman spectra of the C120 crystalline fullerene dimers, synthesized by high-pressure/high-temperature (HPHT) treatment of the C60 fullerite, were measured at ambient and elevated temperatures (T>130°C) in order to study the kinetics of their thermal decomposition. Measurements exhibit an intensity decrease of the dimer-related pentagon pinch (PP) mode and an increase of the monomer’s one. The relative intensity of the dimer’s PP mode decreases exponentially with the thermal treatment time and becomes faster at elevated temperatures. The activation energy EA of the dimer decomposition, obtained from the Arrhenius dependence of the exponential decay time constant on temperature, is (1.71±0.06)eV.

Magnetic phases and relaxation effects in fullerite C60

A highly sensitive torsional vibration technique is used to study the magnetic properties of fullerite C60 (99.98%) at temperatures of 77–300 K in dynamic and static experiments. Vibrational energy absorption peaks associated with phase transitions and realignment of the magnetic structure of the fullerite are detected at T = 152, 195, 230, and 260 K. Relaxation magnetic processes in fullerite C60 at room temperature are studied. “Spontaneous” rotation of a motionless sample of fullerite freely suspended on an elastic filament is observed when external longitudinal or transverse magnetic fields are switched on. The direction of the “spontaneous” rotation changes with time. It is proposed that these phenomena are related to relaxation processes in the rotational subsystem of C60 molecular rotators, as well as to magnetic flux trapped in the fullerite and weakly damped eddy currents induced in the sample by the applied field.

Influence of CO molecular impurity on the structural and thermodynamic properties of fullerite C60, in a broad range of sorption temperatures

An x-ray diffraction study of how sorption of CO gas at a pressure of 30 atm in the temperature range of 150–600 °C influences the structural characteristics of polycrystalline and single crystal fullerite C60. The sorption kinetics are studied by constructing a dependence of the lattice parameter on the time it takes for fullerite to be saturated by CO molecules. At temperatures Tsorb > 300 °C there is an observed dissociation of carbon monoxide, accompanied by the precipitation of carbon powder and the chemical interaction of atomic oxygen with C60 and CO molecules, and possibly with the carbon condensate. These processes have a strong influence on the structural characteristics of fullerite, thus creating, in part, a nonmonotonic dependence of the parameter and lattice matrix volume on the impurity saturation temperature. The concentrations of solid solutions C60(CO)x poly- and single crystal samples are determined in the physisorption range for two modes (150 and 250 °C). It is found that the CO impurity has a linear effect on the lattice parameter and the temperature of the orientational transition of fullerite C60.

Specific features of the glass transition in C60 fullerite saturated with carbon monoxide molecules: Photoluminescence studies

Low temperature (20–230 K) spectral-luminescence studies were conducted on C60 fullerite saturated with carbon monoxide in a physisorption regime. Substantial changes in the photoluminescence characteristics of C60–CO solutions with different impurity concentrations were found already for short intercalation times. Strong dependence of the CO solubility on the saturation temperature was revealed by analyzing the contribution of “deep X-traps” to the luminescence. Furthermore, it was found that filling of the octahedral voids by CO molecules occurs with a lower gradient of the impurity distribution into the bulk C60 crystals as compared with N2. The temperature dependences of the integral emission intensity for the samples with different concentrations of carbon monoxide were studied. For the first time, using the spectral-luminescence method, it was revealed that CO molecules, in contrast to H2 and N2, exhibit a significant effect on the formation of the orientational glass and the rotational dynamics of ...

Interaction of amorphous fullerene C60 with austenite Fe–Ni alloy at high temperatures and pressures

Interaction of amorphous fullerite C60 with austenitic Fe-33.2 wt.% Ni alloy at pressures 0–8 GPa and temperatures 600–1100 °C was studied by neutron diffraction. The amorphous fullerite was obtained by ball milling and mixed with the powder of the crystalline alloy. The interaction at sintering led to the dissolution of carbon in fcc Fe–Ni solid solution and the formation of carbide (Fe, Ni)3C, but the Fe–Ni–C alloy did not undergo phase transformations and preserved the original fcc structure. As a result, the alloy hardened, we could also witness a clear barometric effect: at the pressure of 2 GPa the amount of the dissolved carbon, hardness at micro- and nanoindentation turned out to be significantly higher than those at 8 GPa.During sintering amorphous fullerite is undergoing phase transitions and its microhardness is higher than the microhardness of the metal component. At high temperatures of interaction graphite appears. The presence of Fe–Ni alloy in the composite reduces the temperature of graphite formation in comparison with transformations in the pure amorphous fullerene.

C60 and C70 pressure-and-temperature transformations into fullerene-related forms

ABSTRACTA two-stage pressure-and-temperature treatment of the C60 and C70 fullerites was carried out. C60 and C70 molecules collapsed at the first-stage hot-isostatic-pressing (HIP; 220 MPa, argon) and transformed into some fullerene-related form in the 900–1750°C temperature range. These materials were used at the second stage of the high-pressure-high-temperature (HPHT; 7.7 GPa/1400°C) treatment to produce the bulk samples that had: a specific weight of about 2.0 g/cm3, 40/110 GPa Young modulus, 6.0/12.5 GPa, and elastic recovery above 81%. Transformations under the treatments were investigated with the X-ray and transmission electron microscopy techniques. The mechanism of the pressure-and-temperature transformations is discussed.

The low-temperature heat capacity of fullerite C60

The heat capacity at constant pressure of fullerite C60 has been investigated using an adiabatic calorimeter in a temperature range from 1.2 to 120 K. Our results and literature data have been analyzed in a temperature interval from 0.2 to 300 K. The contributions of the intramolecular and lattice vibrations into the heat capacity of C60 have been separated. The contribution of the intramolecular vibration becomes significant above 50 K. Below 2.3 K the experimental temperature dependence of the heat capacity of C60 is described by the linear and cubic terms. The limiting Debye temperature at T→0 K has been estimated (Θ0 = 84.4 K). In the interval from 1.2 to 30 K the experimental curve of the heat capacity of C60 describes the contributions of rotational tunnel levels, translational vibrations (in the Debye model with Θ0 = 84.4 K), and librations (in the Einstein model with ΘE,lib = 32.5 K). It is shown that the experimental temperature dependences of heat capacity and thermal expansion are proportional in the region from 5 to 60 K. The contribution of the cooperative processes of orientational disordering becomes appreciable above 180 K. In the high-temperature phase the lattice heat capacity at constant volume is close to 4.5 R, which corresponds to the high-temperature limit of translational vibrations (3 R) and the near-free rotational motion of C60 molecules (1.5 R).

The analytical model of the C60 equilibrium intercalation by impurity molecules

We developed an analytical approach to estimate the equilibrium intercalation degree of fullerite C60 by impurity molecules. This model is based on the pair interaction potential of the impurity and fullerene molecule. The potential consists of two parts. The first term describes the short-range repulsion calculated by DFT. The second term describes the long-range dispersion attraction calculated on the basis of experimental data. The approach was tested for CH4, N2 and H2 molecules and gives good agreement with experiment.

Phase transformations in amorphous fullerite C60 under high pressure and high temperature

First phase transformations of amorphous fullerite C60 at high temperatures (up to 1800K) and high pressures (up to 8GPa) have been investigated and compared with the previous studies on the crystalline fullerite. The study was conducted using neutron diffraction and Raman spectroscopy. The amorphous fullerite was obtained by ball-milling. We have shown that under thermobaric treatment no crystallization of amorphous fullerite into С60 molecular modification is observed, and it transforms into amorphous-like or crystalline graphite. A kinetic diagram of phase transformation of amorphous fullerite in temperature–pressure coordinates was constructed for the first time. Unlike in crystalline fullerite, no crystalline polymerized phases were formed under thermobaric treatment on amorphous fullerite. We found that amorphous fullerite turned out to be less resistant to thermobaric treatment, and amorphous-like or crystalline graphite were formed at lower temperatures than in crystalline fullerite.

Effect of molecular nitrogen impurity on the photoluminescence of fullerite C60

Polycrystalline samples of fullerite C60 with different concentrations of molecular nitrogen impurity are studied by luminescence spectroscopy at temperatures of 20–230 K. The photoluminescence characteristics of the C60-N2 solutions vary significantly. At T = 20 K there is a significant contribution to the luminescence from emission centers (“deep X-traps”) determined by the concentration of N2 molecules in octahedral voids of the fcc C60 lattice. The experimentally observed differences in the effect of nitrogen impurities on the structural and luminescence characteristics of C60-N2 solutions show that the filling of the lattice voids into the depth (from the surface) of the samples is substantially inhomogeneous. The temperature dependences of the integrated emission intensity from samples with different nitrogen concentrations are studied. N2 molecules are found to have little effect on the formation of fullerite orientational glass. A tendency for the vitrification temperature of these C60-N2 solutions to fall with increasing impurity concentration is observed experimentally.

Low-temperature dynamics of matrix isolated methane molecules in fullerite C60: The heat capacity, isotope effects

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.

Effect of nitrogen sorption mechanisms on the properties of fullerite C60 over a wide range of temperatures

X-ray diffractometry is used to study the effect of the adsorption of nitrogen at a pressure of 30 atm and temperatures of 200–550 °C on the structural and thermodynamic properties of fullerite C60. The sorption kinetics of nitrogen at different temperatures are studied, and the lattice parameter is plotted as a function of the time for fullerite to saturate with nitrogen. The sorption mechanism is found to change with increasing saturation temperature. The diffusive filling of lattice octahedral voids by nitrogen atoms at temperatures below 450 °C is supplanted at higher temperatures by a chemical interaction of nitrogen with fullerite molecules leading to the formation of a new molecular compound, fullerite nitride C60Nx. The transition from physisorption to chemisorption of nitrogen by fullerite (the adsorption crossover) takes place at saturation temperatures of 450 > T > 400 °C. When C60 molecules are nitrogenated, the volume of the cubic cell increases dramatically, while the intensity of the x-ray reflections decreases and their width increases rapidly. The limiting distortions of the fcc lattice are determined, as well as the characteristic times for diffusional filling of the lattice voids and for nitrogenation, respectively, during physical and chemical sorption of nitrogen. Nitrogenation of fullerite molecules leads to a significant reduction in thermal expansion of the crystalline material, and suppresses both the orientational phase transition and formation of the glassy state.

X-Ray diffraction study of the microstructure of C60 fullerite powder after grinding in a planetary ball mill

The profiles of diffraction peaks of C60 fullerite were used to follow changes in the microstructure of C60 powder during grinding in a planetary ball mill at a rotation rate of 500 rpm and milling time of 8 h. The results demonstrate that grinding for just 1 h leads to a significant decrease in crystallite size. With increasing milling time, the average crystallite size varies little, whereas the lattice strain increases considerably.

Experimental study of the Grüneisen parameter of a fullerite C60 single crystal near phase transitions at 90 and 260 K by the photoacoustic method

The behavior of the Gruneisen parameter of single-crystal fullerite C60 has been studied experimentally near orientational phase transitions at 90 and 260 K. The measurements have been performed by the photoacoustic method using an automated photoacoustic device with laser excitation (the intensity was modulated by a harmonic law) and shadow piezoelectric detection. The temperature dependence of the relative change in the Gruneisen parameter in the fullerite C60 single crystal has been measured near the phase transitions at 90 and 260 K. The results have been analyzed.

Neutron diffraction study of the interaction of iron with amorphous fullerite

The amorphous fullerite C60 has been prepared by mechanical activation (grinding in a ball mill), and its interaction with iron during sintering of powders with 0–95 at % Fe has been studied. After sintering in the range 800–1200°C under a pressure of 70 MPa, the samples have nonequilibrium structures different from the structures of both annealed and quenched steels. In this case, the carbon phase, i.e., amorphous fullerite, undergoes a polyamorphous transition to amorphous graphite. It has also been shown that the interaction of amorphous fullerite with iron is weaker compared to crystalline fullerite or crystalline graphite.

Neutron studies of structural changes in amorphous C60 fullerite under temperature, pressure, and thermobaric effects

The structural behavior of amorphous fullerites obtained as a result of mechanical activation and thermal, baric, and thermobaric effects is studied via neutron diffraction. It is shown that the phase transition between the molecular crystal (fullerite) and atomic crystal (graphite) phases in the nanoscale state occurs through intermediate amorphous phases.

Oxidation of C60 and C70 fullerites in air

The oxidation of C60 and C70 fullerites and C60/C70 mixture is accompanied by the decomposition of the carbon frameworks of the molecules at annealing temperatures of 250 and 445°C. Fourier-IR and HPLC studies show that the oxidation/decomposition of C70 molecules is more active than that of C60. The stages of fullerite interaction with oxygen while heating in air were determined.