Superparamagnetism of magnetite particles in C<Subscript>60</Subscript> fullerite powder

A new magnetic material, C₆₀ fullerite powder doped by magnetite (Fe₃O₄) nanoparticles, is obtained by heating a mixture of fullerite and iron(III) acetylacetonate. It is shown that the material offers superparamagnetic properties. Surface bonding between the nanoparticles and the fullerite is established.     

Thermodynamic properties of C<Subscript>60</Subscript> polyfullerites

This paper discusses the results of calorimetric studies of the 1D C₆₀ (orthorhombic) and 2D C₆₀ (tetragonal and rhombohedral) fullerites, as well as of the graphite-like polyfullerite, which are produced from a starting C₆₀ fullerite subjected to a pressure of 1–8 GPa at temperatures ranging from 300 to 1270 K. The analysis is made primarily of the C _p ⁰ heat capacity measurements performed in adiabatic calorimeters in the 5-to 350-K range.     

The structure of fullerite C<Subscript>60</Subscript> intercalated with molecular oxygen

A (O₂) _x C₆₀ sample with a high content of oxygen (x ≥ 0.4) and free of technological solvent impurities was obtained by precipitation from solution. For the first time, the results of the determination of the x coefficients using ¹³C NMR and elemental analysis were compared. It was shown by Raman spectroscopy, mass spectrometry, and NMR that the inclusion of oxygen into fullerite was accompanied by a decrease in the frequency of O=O stretching vibrations by no less than 12 cm⁻¹ compared with gaseous O₂. Nevertheless, oxygen exists in the molecular form in (O₂)_0.4C₆₀ and is released in the form of O₂ as the sample is heated to 373 K. The number of oxygen molecules occupying octahedral pores closets to the fullerene molecule takes on all the possible values, from 0 to 6. At room temperature, the (O₂) _x C₆₀ sample lost oxygen much more slowly than similar products prepared by diffusion saturation of pure fullerite with oxygen.     

Kinetics of the sorption of <Superscript>3</Superscript>He by C<Subscript>60</Subscript> fullerite. The quantum diffusion of <Superscript>3</Superscript>He and <Superscript>4</Superscript>He in fullerite

The kinetics of the sorption and subsequent desorption of gaseous ³He in a C₆₀ fullerite powder has been studied in the temperature range of 2–292 K. The temperature dependences of the diffusion coefficients of ³He and ⁴He impurities in fullerite have been plotted using the measured characteristic times of filling of octahedral and tetrahedral interstices, as well as previous data. These temperature dependences of the diffusion coefficients of ³He and ⁴He impurities in fullerite are qualitatively similar. A decrease in the temperature from 292 to 79 K is accompanied by a decrease in the diffusion coefficients, which corresponds to the dominance of the thermally activated diffusion of helium isotopes in fullerite. A further decrease in the temperature to 8–10 K leads to an increase in the diffusion coefficients by more than an order of magnitude. The diffusion coefficients of ³He and ⁴He are independent of the temperature below 8 K, indicating the tunnel character of the diffusion of helium in C₆₀ fullerite. The isotope effect is manifested in the difference between the absolute values of the diffusion coefficients of ³He and ⁴He atoms at the same temperatures.     

Cleavage of C<Subscript>60</Subscript> fullerite crystals

It is found that, under certain conditions, C₆₀ fullerite crystals can be cleaved along cleavage planes that are close-packed planes of the {111} type. Rigid gas-phase grown crystals exhibit good cleavage properties. In experiments with active compressive deformation, these crystals showed a high yield point τ_y = 2.65 MPa, a “parabolic” stress-strain curve, and brittle fracture after attaining a shear strain of about 8%. The fracture surface was clearly seen to have fragments parallel to the (111) plane. Typical microstructures observed in the cleavage plane are discussed: crystallographic cleavage steps, an indentation pattern, and a dislocation prick rosette. The fact that the activation volume V ? 60b³ is small (b is the Burgers vector of a dislocation) and strain-independent indicates the Peierls character of fullerite deformation or dislocation drag in a dense network of local defects.     

Structure of C<Subscript>60</Subscript> polymer coatings deposited via electron-beam dispersion of fullerite

The structure and surface of thin coatings deposited via electron-beam dispersion of the C₆₀ fullerite have been investigated using IR and Raman spectroscopy, mass spectroscopy, and atomic-force microscopy. It has been demonstrated that layers with different contents of the polymerized phase, crystals of the tetragonal polymer phase, and three-dimensional polymeric forms of the C₆₀ fullerene are formed under the conditions providing for irradiation by secondary electrons in vacuum at a substrate temperature of 300 K.     

Change in the C<Subscript>60</Subscript> fullerite film growth mechanism on a sulfur-doped Nb(100) surface observed under changes in the chemical state of the surface

It is shown that the growth of a nanosized fullerite film in the C₆₀ molecules-Nb(100) surface adsorption system depends essentially on the chemical state of adsorbed sulfur. In particular, sulfur as the surface sulfide NbS with a concentration of (9±0.2) × 10¹⁴ cm^?2 has almost no effect on the adsorption: as on the pure metal, fullerene molecules from the first and, partially, second layers undergo considerable degradation and do not desorb at any temperatures upon the subsequent heating. On the contrary, C₆₀ molecules retain their structure on a valence-saturated NbS₂ monolayer with almost the same surface concentration of S atoms, build a fullerite film as crystallites without the formation of an intermediate monolayer (Volmer-Weber mechanism), and completely leave the surface at 800 K, which remains unchanged and uncontaminated.     

Thermodynamic and dilatometric properties of the dimerized phase of a C<Subscript>60</Subscript> fullerene

This paper reports on the results of complex investigations into the structural, thermodynamic, and dilatometric properties of the C₆₀ dimerized phase prepared under compression of a C₆₀ fullerite at a pressure up to 8 GPa and a temperature of 290 K. It is demonstrated that the dimerized phase has a face-centered cubic structure with a lattice parameter a=14.02±0.05 Å. The dimeric structure of the studied sample is confirmed by x-ray diffraction analysis. According to the dilatometric data, the volume jump observed in the vicinity of the orientational transition for the dimerized phase is estimated to be approximately 30 times less than that for the C₆₀ fullerite. The temperature dependence of the heat capacity of the (C₆₀)₂ crystalline dimer is examined using precision adiabatic vacuum calorimetry under normal pressure in the temperature range from T → 0 K to 340 K. The results obtained are used in the calculations of thermodynamic functions, namely, the heat capacity C _p ⁰ (T), the enthalpy H⁰(T)-H⁰(0), the entropy S⁰(T), and the Gibbs function G⁰(T)-H⁰(0). The fractal dimension D is determined as a function of the heat capacity. The standard entropy of the formation of the (C₆₀)₂ crystalline dimer from a simple compound (graphite) at T=298.15 K and normal pressure is calculated.     

Electroconductivity and pressure–temperature states of step shocked C60 fullerite

A study of electrophysical and thermodynamic properties of C₆₀ single crystals under step shock loading has been carried out. The increase and the following reduction in specific electroconductivity of C₆₀ fullerite single crystals at step shock compression up to pressure 30 GPa have been measured. The equations of state for face centred cubic (fcc) C₆₀ fullerite as well as for two-dimensional polymer C₆₀ and for three-dimensional polymer C₆₀ (3D-C₆₀) were constructed. The pressure–temperature states of C₆₀ fullerite were calculated at step shock compression up to pressure 30 GPa and temperature 550 K. The X-ray diffraction studies of shock-recovered samples reveal a mixture of fcc C₆₀ and a X-ray amorphous component of fullerite C₆₀. The start of the formation of the X-ray amorphous component occurs at a pressure P _m≈ 19.8 GPa and a temperature T _m≈ 520 K. At pressures exceeding P _m and temperatures exceeding T _m, the shock compressed fullerite consist of a two-phase mixture of fcc C₆₀ fullerite and an X-ray amorphous component presumably consisting of the nucleators of polymer 3D-C₆₀ fullerite. The decrease in electroconductivity of fullerite can be explained by the percolation effect caused by the change of pressure, size and number of polymeric phase nuclei.     

Effect of oxygen and iodine on the optical and magnetic properties of fullerite C<Subscript>60</Subscript>

The effect of oxygen and iodine on the optical and magnetic properties of fullerite C₆₀ is studied by luminescence and EPR spectroscopy within widely varied experimental conditions (temperature of the medium, oxygen or buffer gas pressure, concentration of iodine vapor). It is demonstrated that the efficiency of the singlet oxygen formation when a fullerene sample is irradiated by a neodymium laser at a wavelength of 532 nm and the amplitude of the EPR signal emitted from the unirradiated sample are strongly affected by the concentrations of both oxygen and iodine vapor sorbed by the fullerene sample, as well as by its surface temperature. The spin-spin and spin-lattice relaxation times of paramagnetic centers in fullerite samples studied in the presence of molecular oxygen are determined by the method of microwave radiation absorption saturation.     

The influence of dimerization on the orientational phase transition in the C<Subscript>60</Subscript> fullerite

The influence of dimerization of a C₆₀ fullerite subjected to thermobaric treatment on the change in linear dimensions upon the orientational phase transition is investigated. It is demonstrated that the effects associated with the dimerization of fullerites substantially affect the thermal expansion coefficient only for samples synthesized under the conditions P_syn≥8 GPa and T_syn≥70°C (where P_syn and T_syn are the pressure and temperature of the synthesis, respectively). These effects bring about a smearing of the phase transition, a shift of the transition toward low temperatures, and a decrease in the volume jump.     

Dislocation mobility in C<Subscript>60</Subscript> fullerite crystals

The dependences of the path of leading dislocations in indentation rosette rays on the load, the loading time, and the indentation temperature in the range 260 < T ≤ 373 K were studied for C₆₀ fullerite crystals. The dislocation mobility parameters are estimated: the exponent m characterizing the stress dependence of the dislocation velocity depends on the structural perfection of the crystal and ranges from 2.3 to 24.5, the activation energy for dislocation motion ΔH ₀ ? (0.4–0.5) eV, and the velocity of leading dislocations in indentation rosette rays v _l ? 10^?5?10^?4 cm/s. The data from micro-and macromechanical experiments are shown to agree with each other. The dislocation mobility is assumed to be controlled by the dislocation interaction with local barriers.     

Fine band structure of the vibrational spectra of fullerite C<Subscript>60</Subscript> and enhancement of intermolecular interaction in high-temperature phase

The fine structure of the fundamental vibrational bands and some combination tones of fullerite C₆₀ in its IR absorption and reflection spectra, as well as in Raman spectra, has been studied. This structure is due to the overlapping components of Davydov and isotopic splittings and the removal of vibrational degeneracy with symmetry lowering. It is shown that for IR F _u (i) bands (i = 1–4) and low-frequency H _g (1) and A _g (1) bands in the Raman spectrum the splittings at room temperature exceed those for the low-temperature phase. The enhancement of intermolecular interaction at elevated temperatures is explained by the nonequilibrium vibrational excitation of the medium as a result of nonlinear interaction of vibrational modes and by the change in the electronic states.     

Cluster <Emphasis Type="Italic">ab initio</Emphasis> calculations for the C<Subscript>60</Subscript>F<Subscript>24</Subscript>, C<Subscript>60</Subscript>Cl<Subscript>24</Subscript>, and C<Subscript>60</Subscript>Br<Subscript>24</Subscript> halofullerenes

This paper reports on ab initio cluster calculations of the equilibrium geometry, electronic structure, and vibrational properties of the C₆₀, C₆₀F₂₄, C₆₀Cl₂₄, C₆₀Br₂₄ molecules.     

Low-frequency dispersion of the negative dielectric permittivity in C<Subscript>70</Subscript> films

Measurements performed at frequencies of 0.1–10 kHz on films of C₇₀ fullerite revealed a negative dielectric permittivity ?’相似文献   

Collision dynamics of He@C<Subscript>60</Subscript>+He@C<Subscript>60</Subscript> at low energies

A semi-empirical molecular dynamics model is developed. The central collisions of C₆₀+C₆₀ and He@C₆₀+He@C₆₀ at different incident energies are investigated based on this model. It is found that the dimer structures have been produced at proper incident energies and these fullerene dimers could be formed by a self-assembly of C₆₀ fullerene and He@C₆₀. The He atom has a significant effect at higher incident energy and this embedded He atom can enhance the stability of the dimer structure.     

On the thermal decomposition of the C60D19 deuterium fullerite

The gas-phase products of thermal decomposition of the C₆₀D₁₉ deuterium fullerite are studied by mass spectrometry. It is found that, in addition to D₂ molecules, the gas phase over the deuterium fullerite sample heated to a temperature of 773 K contains CD₄ methane and C₆D₆ benzene molecules. The deuterocarbon molecules are revealed in the gas phase even at 673 K.     

High-pressure hydrogenated fullerenes: Optical spectra and stability of C<Subscript>60</Subscript>H<Subscript>36</Subscript> at high pressure

The optical Raman and photoluminescence (PL) spectra of the high-pressure hydrogenated fullerene C₆₀ are studied at normal conditions and at high pressure. The Raman spectrum of the most stable hydrofullerene C₆₀H₃₆ contains a large number of peaks related to various isomers of this molecule. Comparison of the experimental data with the results of calculations shows that the most abundant isomers have the symmetries S₆, T, and D_3d. The Raman spectrum of deuterofullerene C₆₀H₃₆ is similar to that of C₆₀H₃₆, but the frequencies of the C-H stretching and bending modes are shifted due to the isotopic effect. The PL spectrum of hydrofullerene C₆₀H₃₆ is shifted to higher energies by approximately 1 eV with respect to that of pristine C₆₀. The effect of hydrostatic pressure on the Raman and PL spectra of C₆₀H₃₆ has been investigated up to 12 GPa. The pressure dependence of the phonon frequencies exhibits peculiarities at approximately 0.6 and 6 GPa. The changes observed at approximately 0.6 GPa are probably related to a phase transition from the initial orientationally disordered body-centered cubic structure to an orientationally ordered structure. The peculiarity at approximately 6 GPa may be related to a pressure-driven enhancement of the C-H interaction between the hydrogen and carbon atoms belonging to neighboring molecular cages. The pressure-induced shift of the photoluminescence spectrum of C₆₀H₃₆ is very small up to 6 GPa, and a negative pressure shift was observed at higher pressure. All the observed pressure effects are reversible with pressure.     

On the effective Debye temperatures of the C60 fullerite

The effective Debye temperatures Θ_eff determined for solids by different physical methods have been analyzed and compared. Attention has been focused on the original parameter of the Debye theory of heat capacity, i.e., the translational calorimetric Debye temperature Θ _c ^t (0), and the X-ray Debye temperature Θ _x in the framework of the Debye-Waller theory for the C₆₀ fullerite. It has been established that the true Debye law T ³ is satisfied for the C₆₀ fullerite over a very narrow range of temperatures: 0.4 K ≤ T ≤ 1.8 K. For this reason, the experimental Debye temperatures Θ _c ^t (0) obtained for the C₆₀ fullerite by different authors in the range T > 4.2 K are characterized by a large scatter (by a factor of ∼5). It has been revealed that the value Θ _c ^t (0) = 77.12 K calculated in this paper with the use of the six-term Betts formula from the harmonic elastic constants $ \tilde C_{ijkl} $ \tilde C_{ijkl} of the C₆₀ single crystal in the limit T = 0 K is closest to the true Debye temperature. It has been demonstrated using the method of equivalent moments that the real spectral frequency distribution of translational lattice vibrations g(ω) for the C₆₀ fullerite deviates from a parabolic distribution. The effective Debye temperatures Θ_eff involved in applied problems of thermodynamics of crystals and elastic scattering of different radiations from lattice vibrations have been determined. The quantitative measure of anharmonicity of translational and librational lattice vibrations of the C₆₀ fullerite has been determined. This has made it possible to empirically evaluate the lattice thermal conductivity κ of the C₆₀ fullerite at T ≈ 300 K: κ(300) = 0.80 W (m/K), which is in good agreement with the experimental thermal conductivity κ_exp = 0.78 W (m/K) at T ≈ 250 K.     

Effect of heating of the electronic subsystem on the thermal stability of the C<Subscript>60</Subscript> and C<Subscript>20</Subscript> fullerenes and (C<Subscript>20</Subscript>)<Subscript>2</Subscript> cluster molecule

The effect of heating of the electronic subsystem on the thermal stability of C₆₀ and C₂₀ fullerenes and a (C₂₀)₂ cluster molecule is investigated theoretically. It is demonstrated that the excitation of electrons to upper energy levels in accordance with the Fermi-Dirac distribution function does not lead to a substantial change in the activation energy E _a for decay of the C₂₀ fullerene. The stability of the C₆₀ fullerene and the (C₂₀)₂ cluster molecule likewise does not change radically. However, the inclusion of corrections associated with the finite sizes of the heat bath leads to the activation energy E _a which is in better agreement with the calculated height of the potential barrier preventing the cluster decay.