Magnetic-field-induced slowing-down of molecular rotation in C<Subscript>60</Subscript> crystals

The molecular dynamics of C₆₀ crystals was studied by inelastic neutron scattering at T=290 K, i.e., above the first-order phase transition temperature (T_C≈260 K), in the region of free C₆₀-spheroid rotation in the lattice. The energy broadening of the original neutron spectrum 2Γ₀≈0.1 meV for a momentum transfer q=2 Å^?1 is in agreement with NMR data on the rotational relaxation time of the molecule τ～10^?11 s～ ?Γ₀. This effect was observed to decrease in magnetic fields H=2.5–4.5 kOe applied along the scattering vector: Γ_H=0.7Γ₀. The slowing-down of the molecular rotation is discussed in connection with the interaction of a magnetic field with the molecular currents, which fluctuate when the C₆₀ cage rotates.     

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.     

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.     

Withdrawal of Electrodynamical Forbiddance and Peculiarities of the SERS Spectra in Fullerene C<Subscript>70</Subscript>

It is demonstrated that in fullerene C₇₀, which can be considered as a deformed fullerene C₆₀ in some mean sense there is a withdrawal of an Electrodynamical forbiddance of a strong quadrupole light-molecule interaction, which is realized in the fullerene C₆₀. This situation occurs because of the reduction of symmetry of C₆₀ from the icosahedral symmetry group Y_h to the group D_5h. The withdrawal results in appearance of the lines in the SERS spectra of C₆₀, which are forbidden in usual Raman scattering and are active in the infrared absorption spectra. The experimentally measured SERS spectra of C₇₀ demonstrates existence of such lines that strongly confirms our ideas about the dipole-quadrupole SERS mechanism.     

Equilibrium state of C<Subscript>60</Subscript>, C<Subscript>70</Subscript>, and C<Subscript>72</Subscript> nanoclusters and local defects of the molecular skeleton

The stability of C₆₀ and C₇₀ fullerenes and C₆₀ and C₇₂ nanotubes devoid of 2–12 atoms of the cluster skeleton was theoretically studied. It was established that C_n molecules with an even number of atoms remain stable, which was confirmed by experimental studies of monomolecular decay of clusters with the number of atoms n≥30. The change in the internuclear distances and in the ionization potential of nanoclusters was determined depending on the number of eliminated atoms. Such defects were shown to decrease the ionization potential of nanoclusters by 0.5–0.8 eV. The electron spectrum was calculated within the Harrison semiempirical tight-binding model in the Goodwin modification. A new parametrization of interatomic matrix elements of the Hamiltonian and atomic terms for carbon nanoclusters was suggested.     

Study of third-and fifth-order nonlinear optical processes in thin C<Subscript>60</Subscript> films

The nonlinear refraction in thin films of fullerene C₆₀ (100 nm) is studied by the Z-and RZ-scan methods using the second harmonic of a picosecond Nd:YAG laser (λ = 532 nm, τ = 55 ps). The combined effect of n₂ (self-focusing of laser radiation) and n₄ (self-defocusing) is analyzed. Mechanisms responsible for the nonlinear refraction in films are discussed.     

Peculiarities of magnetic,galvanomagnetic, elastic,and magnetoelastic properties of Eu<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3</Subscript>

Magnetic, elastic, magnetoelastic, transport, and magnetotransport properties of the Eu_0.55Sr_0.45MnO₃ ceramics have been studied. A break was detected in the temperature dependence of electrical resistivity ρ(T) near the temperature of the magnetic phase transformation (41 K), with the material remaining an insulator down to the lowest measurement temperature reached (ρ=10⁶ Ω cm at 4.2 K). In the interval 4.2≤T≤50 K, the isotherms of the magnetization, volume magnetostriction, and ρ were observed to undergo jumps at the critical field H_C1, which decreases with increasing T. For 50≤T≤120 K, the jumps in the above curves persist, but the pattern of the curves changes and H_C1 grows with increasing T. The magnetoresistance Δρ/ρ = (ρ _H ?ρ_H=0)/ρ _H is positive for H<H_C1 and passes through a maximum at 41 K, where Δρ/ρ = 6%. For H>H_C1, the magnetoresistance is negative, passes through a minimum near 41 K, and reaches a colossal value of 3×10⁵ % at H=45 kOe. The volume magnetostriction is negative and attains a giant value of 4.5×10^?4atH=45 kOe. The observed properties are assigned to the existence of three phases in Eu_0.55Sr_0.45MnO₃, namely, a ferromagnetic (FM) phase, in which carriers are concentrated because of the gain in s-d exchange energy, and two antiferromagnetic (AFM) phases of the A and CE types. Their fractional volumes at low temperatures were estimated to be as follows: ～3% of the sample volume is occupied by the FM phase; ～67%, by the CE-type AFM phase; and ～30%, by the A-type AFM phase.     

Stability of C<Subscript>20</Subscript> fullerene chains

The stability of (C₂₀)_N metastable chains, where C₂₀ fullerenes are joined by tight covalent bonds, is analyzed by numerical simulation using a tight-binding potential. Various channels of losing the chain-cluster structure of the (C₂₀)_N complexes have been determined including the decay of the C₂₀ clusters, their coalescence, and the separation of one C₂₀ fullerene from a chain. The lifetimes of the (C₂₀)_N chains with N = 3–7 for T = 2000–3500 K are directly calculated by the molecular dynamics method. It has been shown that, although the stability of the chains decreases with an increase in N, it remains sufficiently high even for N ? 1. An interesting lateral result is the observation of new (C₂₀)_N isomers with the combination of various intercluster bonds with the maximum binding energy of fullerenes in the chain.     

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.     

C<Subscript>60</Subscript> heat capacity anomaly at the orientational phase transition

An orientational phase transition in C60 crystals was studied by differential scanning calorimetry with the highest resolution provided by this method. The temperature dependence of the specific heat ΔC _p (T) was found to have a double peak in the range 250–270 K. An analysis of the temperature dependences of heat capacity in the region of the peaks revealed that the lower temperature peak follows a power law of the type ΔC _p = A/(T?T₀)^1/2 characteristic of order-disorder second-order phase transitions, while the high-temperature peak can be identified with a diffuse Λ-shaped first-order phase transition.     

Spin-dimerization in rare-earth substituted La<Subscript>2</Subscript>RuO<Subscript>5</Subscript>

The Ru-Ru spin-singlet formation in La_2 ? x L n _x RuO₅ (Ln = Pr, Nd, Sm, Gd, Dy) was investigated by measurements of the specific heat and magnetic susceptibility. After subtraction of the lattice contribution from the specific heat (C _p ), similar excess entropy values were obtained for all compounds. These entropies can be explained by the formation of antiferromagnetic Ru-spin dimers at low temperatures and provide a lower estimate for the intradimer exchange strength. Pronounced changes in the transition temperatures and a broadening of the corresponding peak in C _p were observed. These changes depend on the rare-earth element and are due to local structural changes and heterogeneities caused by the substitution. The magnetic susceptibilities can be described by the sum of a rare-earth paramagnetic moment and the susceptibility of the unsubstituted La₂RuO₅. Density functional theory (DFT) calculations were performed for various compounds to investigate the origin of the magnetic transition and the relationship between structural changes and the spin-dimerization temperature. The combination of the present results with previous structural investigations supports the model of a spin-pairing of the Ru moments which occurs as a reason of the structural phase transition in La_2 ? x L n _x RuO₅.     

Mechanical and Dynamic Stability of Complete and Nonstoichiometric 3<Emphasis Type="Italic">C</Emphasis>-Si<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>C<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> from Ab Initio Calculations

The energies of formation of vacancies in the carbon and silicon sublattices, the independent elastic constants, the all-round compression, shear and Young’s moduli, and the anisotropy coefficients are determined for the complete and nonstoichiometric cubic phases of 3C-Si_xC_y (x, y = 1.0–0.75) by ab initio methods of the band theory. In the formalism of the density functional perturbation theory (DFPT), the phonon dispersion dependences are obtained for these phases (the comparison with the experiment is given for the complete phase). It is shown that the mechanical characteristics of the phases become strongly anisotropic upon the transition from 3C-SiC_0.875 to 3C-SiC_0.75. It is established from the analysis of the phonon dispersion curves that the 3C-SiC_0.875 and 3C-SiC_0.75 phases, in contrast to the complete 3C-SiC phase, are dynamically unstable at T = 0 K.     

X-ray emission and X-ray photoelectron spectroscopic studies of fullerene fluoride C<Subscript>60</Subscript>F<Subscript>24</Subscript>

The structure of the fullerence fluoride C₆₀F₂₄ of the T _h symmetry contains two types of chemically different carbon atoms, namely, atoms of isolated double bonds and atoms of CF groups. X-ray photoelectron and x-ray emission spectroscopic studies of C₆₀F₂₄ revealed a difference in the widths of the x-ray bands corresponding to these types of atoms. Nonempirical quantum-chemical calculations performed for C₅₉NF ₂₄ ⁺ ions with a hole in the C 1s core level of the fullerence fluoride showed that the difference in the bandwidths may be due to the fact that the vibrational states of the system are different when 1s electrons are removed from chemically nonequivalent atoms.     

Optical properties of mechanically compressed K<Subscript>2</Subscript>SO<Subscript>4</Subscript> crystals

The effect of a uniaxial mechanical compression (σ_m ≤ 100 bar) on the spectral dependences (300–800 nm) of the birefringence Δn _i and refractive indices n _i of K₂SO₄ crystals is studied. The electronic polarizabilities, refractions, and parameters (λ_0i , B _1i ) of ultraviolet oscillators of mechanically compressed crystals are calculated. It is shown that the dispersions of Δn _i(λ) and n _i(λ) are normal and sharply increase near the absorption edge. It is found that the uniaxial compression changes the value of the dispersions dΔn _i/dλ and dn _i/dλ rather than their character. It is ascertained that the simultaneous action of the compressions σ_x and σ_z, as well as of σ_y and σ_z, leads to the appearance of new isotropic states in the K₂SO₄ crystal, which manifests itself in the equality of corresponding birefringences. It is shown that the baric dependences n _i(σ) are determined by the change in the density of oscillators (～30%), by the shift of the absorption edge and effective band maximum and by the change in the oscillator strength (～70%).     

Sign inversion of the radiation-induced plastic effect in C<Subscript>60</Subscript> single crystals upon the SC-FCC phase transition

A reversible variation in the microhardness of C₆₀ single crystals induced by low-dose β irradiation is found to change sign in the vicinity of the phase transition at T_c=250–260 K. Radiation hardening is observed at T<T_c and radiation softening, at T>T_c. The kinetic characteristics of the β-induced variation in the microhardness of C₆₀ and its recovery to the initial value are studied in the temperature range 170–350 K. Both processes are found to be thermally activated. The activation energies are determined and are shown to be different for sc and fcc lattices.     

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.     

X-ray study of [N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>]<Subscript>2</Subscript>ZnCl<Subscript>4</Subscript> at low temperatures

The unit cell parameters a, b, and c of [N(CH₃)₄]₂ZnCl₄ have been measured by x-ray diffraction in the temperature range 80–293 K. Temperature dependences of the thermal expansion coefficients α_a, α_b, and α_c along the principal crystallographic axes and of the unit cell thermal expansion coefficient α_V were determined. It is shown that the a=f(T), b=f(T), and c=f(T) curves exhibit anomalies in the form of jumps at phase transition temperatures T₁=161 K and T₂=181 K and that the phase transition occurring at T₃=276 K manifests itself in the a=f(T) and b=f(T) curves as a break. A slight anisotropy in the coefficient of thermal expansion of the crystal was revealed. The phase transitions occurring at T₁=161 K and T₂=181 K in [N(CH₃)₄]₂ZnCl₄ were established to be first-order.     

Rotation of the inner shell in a C<Subscript>20</Subscript>@C<Subscript>80</Subscript> nanoparticle

The stability of a C₂₀@C₈₀ nanoparticle and the rotation of its inner shell are studied theoretically within the tight-binding approximation. It is found that the C₂₀ skeleton in the free state is described by space group D_3d; in the case where C₂₀ is placed into the C₈₀(I _h ) fullerene field, the space group of C₂₀ is raised to I _h due to isomerization. The total energy surface of the C₂₀@C₈₀ compound is scanned over two rotation angles. Based on an analysis of the surface relief and energy isoline map, orientational melting of the nanoparticle is predicted. A nanoparticle gyroscope—C₂₀ rotating in the field of C₈₀ at a certain relative orientation and energy supply—is also predicted to exist.     

The oxidation of fullerene [C70] with various oxidants by ultrasonication

The reaction of C₇₀ by ultrasonication with various oxidants such as 3-chloroperoxy benzoic acid (Fluka 99%), 4-methyl morpholine N-oxide (Aldrich 97%), chromium (VI) oxide (Aldrich 99.9%), and oxone^® monopersulfate compound, at room temperature causes the oxidation of fullerene [C₇₀(O)_n] (n=1–2 or n=1). The FAB-MS, UV–visible, FT-IR spectra, and HPLC analysis confirmed that products of fullerene oxidation are [C₇₀(O)_n] (n=1–2 or n=1).     

Pressure dependence of the superconducting state parameters of a binary Ca<Subscript>70</Subscript>Mg<Subscript>30</Subscript> metallic glass superconductor

Theoretical computation of the pressure dependence of superconducting state parameters of a binary Ca₇₀Mg₃₀ metallic glass has been performed using the model potential formalism. Explicit expressions have been derived for the volume dependence of the electron-phonon coupling strength λ and the Coulomb pseudopotential μ*, considering the variation of the Fermi momentum k _F and Debye temperature θ_D with volume. Well-known Ashcroft’s empty core model pseudopotential and five different types of the local-field correction functions, namely, Hartree, Taylor, Ichimaru-Utsumi, Farid et al. and Sarkar et al. have been used for obtaining pressure dependence of transition temperature T _C and the logarithmic volume derivative Φ of the effective interaction strength N ₀ V for the metallic glass superconductor. It has been obtained that T _C of Ca₇₀Mg₃₀ metallic glass decreases rapidly with increasing pressure up to 60% decrease in the volume, for which the μ* and Φ curves show a linear nature. The superconducting phase disappears at about 60% decrease in the volume.