On the prospects of preparing fullerites from small and large fullerenes

The evolution of the properties of face-centered cubic fullerites with a variation in the number (nc) of carbon atoms in a C_nc fullerene molecule (15 ≤ nc ≤ 147) is investigated using the dependence of the parameters of the interfullerene interaction in face-centered cubic fullerites on the mass of the C_nc fullerene molecule. It is demonstrated that, for nc < 20, the face-centered cubic fullerites become unstable because such light small fullerene molecules cannot be kept by weak van der Waals forces. For nc ≥ 110, the fullerites have anomalously low surface energies, which should lead to fragmentation of nanoclusters composed of large hollow spherical molecules C_nc. The inference is made that the range 30 < nc < 100 is optimum for the formation of stable face-centered cubic fullerites.     

Specific features of the stone-wales transformation in the C20 and C36 fullerenes

The initial stage of the defect formation in the C₂₀ and C₃₆ fullerenes has been investigated by the ab initio method and in terms of tight-binding models. A comparison with the Stone-Wales transformation in the C₆₀ fullerene has revealed the presence of two independent stages in this process, the first of which is an “incomplete” Stone-Wales transformation. At this stage, the C₂₀ and C₃₆ fullerenes transform into metastable defect configurations with two adjacent “windows” on their surface, whereas a similar configuration of the C₆₀ fullerene is unstable and corresponds to a saddle stationary point of the potential energy of the cluster. A new mechanism of plastic deformation due to the Stone-Wales transformation has been predicted for the (C₃₆) _n fullerites.     

Modeling of the structure and electronic structure of condensed phases of small fullerenes C<Subscript>28</Subscript> and Zn@C<Subscript>28</Subscript>

A comparative analysis of the stability factors and electronic structure of two possible crystalline forms of small fullerene C₂₈ and endohedral fullerene Zn@C₂₈ with diamond and lonsdaleite structures is performed using a cluster model. Atoms of elements that, when placed inside C₂₈ cages, have no significant effect on the stability of free small-fullerene molecules are shown to be able to dramatically change the electronic properties and reactivity of the C₂₈ skeleton and to be favorable for forming small-fullerene crystalline modifications, which are covalent crystals. In contrast, if the presence of foreign atoms inside C₂₈ cages stabilizes the isolated nanoparticles, then molecular crystals (such as C₆₀ fullerites) are formed due to weak van der Waals forces.     

Superparamagnetic properties of C60Co3 complexes

Preparation of fullerites containing cobalt and analyses of reactions based on semiempirical quantum calculations are described. The magnetic properties of thermally treated C₆₀Co₃ samples: Curie constant (C≈3500 emu K/mol Oe) temperature and field dependencies of magnetization and nonequilibrium effects of magnetization are interpreted in terms of superparamagnetic blocking model of the compound.     

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.     

Molecular interactions in fullerenes and equilibrium of higher fullerites C76 and C84 with their vapors

From simple topological considerations on the molecular shapes, a new method for calculating the coefficients of the Girifalco intermolecular potential for various fullerenes is proposed. This eliminates the necessity for fitting the coefficients to data of measurements for each specific fullerene. We calculate them for C₇₆ and C₈₄ and apply this potential to perform research on the equilibrium of these fullerites with their vapors. The temperature dependence of the lattice parameters, the saturated vapor pressures and the enthalpies of sublimation is studied. Results are in good agreement with available experimental data. Received 13 January 2000 and Received in final form 18 June 2000     

Elastic properties of ultrahard fullerites

The elastic properties of C₆₀ fullerite samples synthesized under pressure P=13.0 GPa at high temperatures were investigated using acoustic microscopy. The velocities of longitudinal (c _L=17–26 km/s) and transverse (c _T=7.2–9.6 km/s) elastic waves in the samples were measured. It was established that the longitudinal sound velocity of ultrahard fullerites is higher than that of any other known solid. The bulk modulus of these ultrahard samples is higher than that of diamond and reaches a value greater than 1 TPa. The high bulk modulus, the relatively large shear moduli, and the substantial Poisson ratio indicate that the structure of the ultrahard fullerites is fundamentally different from that of diamond. Zh. éksp. Teor. Fiz. 114, 1365–1374 (October 1998)     

<Superscript>2</Superscript>H and <Superscript>13</Superscript>C NMR investigation of deuterofullerites C<Subscript>60</Subscript>D<Subscript>x</Subscript>

Deuterofullerites C₆₀D_x have been studied by ²H and ¹³C NMR. These fullerites have two types of carbon–deuterium bonds: C–D terminal bonds, characterized by the quadrupole coupling constant (QCC) of 171 kHz, and –C ··· D ··· C– bridging bonds with a QCC of 56 kHz. The latter is responsible for the rigid lattice found in these fullerites, which is untypical of fullerenes. PACS 81.05.Tp; 82.56.Fk; 61.48.+c; 61.18.Fs; 61.10.Nz     

Elastic and electronic properties of YNi2B2C under pressure from first principles

The elastic and electronic structure properties of YNi₂B₂C under pressure are investigated by performing the generalized gradient approximation (GGA) and local density approximation (LDA) correction scheme in the frame of density functional theory (DFT). The pressure dependences of the normalized lattice parameters a/a₀ and c/c₀, the ratio c/a, and the normalized primitive volume V/V₀ of YNi₂B₂C are also obtained. The lattice constants and bulk modulus obtained are in agreement with the available experimental and other theoretical data. We have also studied the pressure dependences of elastic properties. It is found that, as pressure increases, the elastic constants C₁₁, C₃₃, C₆₆, C₁₂, and C₁₃ increase, the variation of elastic constant C₄₄ is not obvious. Moreover, our compressional and shear wave velocities V_L=6.99 km/s and V_S=3.67 km/s as well as the Debye temperature Θ=549.7 K at 0 GPa compare favorably with the available experimental data. The pressure dependences of band structures, energy gap and density of states are also investigated.     

Paramagnetic relaxation kinetics of the cation radical C 60 + in C60 powder

The kinetics of spin-lattice and phase relaxation of C ₆₀ ⁺ radicals in C₆₀ powder has been studied at room temperature by pulsed EPR. It is found that the kinetics can be described by the relation exp $( - a\sqrt t )$ characteristic of the case in which the paramagnetic centers are distributed over relaxation times. It is concluded that the observed kinetics are due to the presence of oxygen molecules, which act as a fast-relaxing impurity and accelerate the relaxation of the C ₆₀ ⁺ radicals. The results obtained offer an explanation for a number of features of C ₆₀ ⁺ relaxation in fullerites discovered earlier.     

Study of elastic properties of CeO2 by statistical moment method

The purpose of the present paper is to investigate the temperature and pressure dependences of the elastic properties of cerium dioxide using the statistical moment method (SMM). The equation of states of bulk CeO₂ is derived from the Helmholtz free energy, and the pressure dependences of the elastic moduli like the bulk modulus, B_T, shear modulus, G, Young’s modulus, E, and elastic constants (C11, C12, and C44) are presented taking into account the anharmonicity effects of the thermal lattice vibrations. In the present study, the influence of temperature and pressure on the elastic moduli and elastic constants of CeO₂ has also been studied, using three different interatomic potentials. We compare the results of the present calculations with those of the previous theoretical calculations as well as with the available experiments.     

Microphotoluminescence of C<Subscript>60</Subscript> Fullerites Synthesized under a Pressure of 13 GPa and Temperatures of 770–2100 K

C₆₀ fullerite samples synthesized under a pressure of P = 13 GPa and temperatures of T_s = 770 ? 2100 K were studied by scanning microphotoluminescence (MPL) at room temperature. The MPL of cleaved chip surfaces indicates the presence of emitting areas with linear dimensions from 35 to 350 μ. A band at 700 nm, which is characteristic for linear and planar C₆₀ polymers, was revealed in all 3D-polymerized fullerites too. New bands at 1100?2300 nm are observed in PL spectra of substances synthesized at 870 ≤ T_s ≤ 1270 K. Separated areas of several optically active moieties responsible for luminescence in the range 1100?2300 nm were detected by MPL cartography. The origin of this band is tentatively attributed to traps located on C₆₀-oxygen and nitrogen inclusions associated with defects of the crystal lattice.     

Deformation-induced changes in the structure of fullerites C60/70 during their mechanical activation

Structural changes occurring during the mechanical activation of fullerites C_60/70 have been investigated using X-ray diffraction, IR and UV spectroscopy, and scanning electron microscopy. The complete destruction modes of fullerite have been determined (3.5 h at the intensity of the mill of 4.3 W/g and 28 h at 2.2 W/g). The destruction of the crystal structure of fullerites is accompanied by the destruction of fullerene molecules. The residual solvent, which enters into the composition of C_60/70, is retained during the entire time of mechanical activation. In this case, the low-frequency shift of absorption bands of toluene (729 → 725 cm^?1), which is caused by the deformation of the solvent molecule in the composition of crystal solvates, has been observed. It has been shown that the deformation stability of graphite is substantially lower than in the case of fullerite.     

Electron energy-loss spectroscopy studies on C60 and C70 fullerite

The electronic structure of the crystalline fullerites C₆₀ and C₇₀ has been investigated by high-energy electron energy-loss spectroscopy in transmission. From valence band excitations and from core-level excitations of the C 1s level information on occupied and unoccupied and bands has been obtained.     

Elastic and thermal properties of Zr z Nb1 − z C x N y solid solutions

The temperature and concentration dependences of the elastic moduli and the thermal linear expansion coefficient of Zr _z Nb_{1 ? z} C _x N _y solid solutions containing from 3 to 8 at % of structural vacancies in a nonmetallic sublattice have been found. The temperature dependences of the Debye temperature Θ_D(T) have been calculated using the elastic data and the data on the heat capacity. It has been shown, using carbide NbC_0.97 as an example, that the Θ_D(T) dependences found from the elastic properties and the heat capacity coincide in the temperature range ～220–300 K. By analogy with the niobium carbide, the heat capacity C _p (300) of Zr _z Nb_{1 ? z} C _x N _y solid solutions of various compositions is calculated based on the values of Θ_D(300) determined from the elastic properties.     

Features of structure and properties of Na<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>C<Subscript>60</Subscript> (<Emphasis Type="Italic">n</Emphasis> = 2, 3) fullerides synthesized in toluene

The structure and electron properties of Na _n C₆₀ (n = 2, 3) sodium fullerides synthesized from simple compounds in toluene were studied. It was shown that Na₂C₆₀ fulleride forms a face-centered cubic lattice at temperatures above 300 K. As the temperature is lowered, the phase transition to a structure with a simple cubic lattice takes place. The temperature dependences of the properties of Na₃C₆₀ with a more complex structure exhibit features that are presumably due to sodium atom redistribution in the Na₃C₆₀ fulleride lattice and the formation of sodium ion clusters.     

Magnetic susceptibility and magnetic resonance study of acceptor doped fullerenes C60(MF6)2 (M=As, Sb, P)

In order to check whether superconductivity occurs in the acceptor doped fullerenes C₆₀(MF₆)₂ (M=As, P, Sb) and to study their magnetic and structural properties, we have carried out magnetic, EPR and NMR measurements of these compounds. Temperature dependences of magnetic susceptibility down to 5 K and field dependences of magnetic moment at 5 K show no ‘bulk’ transition in superconducting state. Some reasons of the absence of superconductivity, such as insufficient charge transfer between C₆₀ and intercalated species and inhomogeneity of the compounds under study, are discussed.     

Formation of higher-order fullerene dianions in collisions with Na atoms

We have measured electron capture cross sections in collisions between higher order fullerene anions C_n ^- (n=76, 78, 82, 84, 86, 90 and 96) and Na atoms. The ions were produced in an electrospray ion source (ESI) and accelerated to an energy of 50 keV. The measured cross section for dianion formation is three times larger for C₉₆ than that for C₆₀. The latter cross section was earlier found to be 36 ?². The dramatic increase of the cross section with fullerene size is explained by means of the curve crossing model for electron transfer.     

Structural and vibrational properties of C36 and its oligomers (C36) M = 2, 3, 4 by tight-binding molecular dynamics

Non-orthogonal tight-binding molecular-dynamics is employed to calculate structural and vibrational properties of C₃₆ and its oligomers (C₃₆) _{M = 2, 3, 4} . The lowest energy configuration of the C ₃₆ cage is confirmed to have D _6h symmetry. For the dimer, too, the D _2h structure reported in the literature is found. The vibrational spectrum is identified with the power spectrum of the displacement autocorrelation function. Additional vibrational properties are extracted from the dynamical matrix. For the monomer, fair agreement with available ab initio calculations is achieved, with comparatively smaller deviations in the Raman-frequencies than for published semi-empirical calculations. The features of the vibrational modes are correlated with the structural properties of the oligomers. Received 24 November 2000 and Received in final form 24 August 2001