Polymerization of solid C<Subscript>60</Subscript> under C<Stack><Subscript>60</Subscript><Superscript>+</Superscript></Stack> cluster ion bombardment

The structure transformation occurring in fullerene film under bombardment by 50 keV C₆₀⁺ cluster ions is reported. The Raman spectra of the irradiated C₆₀ films reveal a new peak rising at 1458 cm⁻¹ with an increase in the ion fluence. This feature of the Raman spectra suggests linear polymerization of solid C₆₀ induced by the cluster ion impacts. The aligned C₆₀ polymeric chains composing about 5–10 fullerene molecules have been distinguished on the film surface after the high-fluence irradiation using atomic force microscopy (AFM). The surface profiling analysis of the irradiated films has revealed pronounced sputtering during the treatment. The obtained results indicate that the C₆₀ polymerization occurs in a deep layer situated more than 40 nm below the film surface. The deep location of the C₆₀ polymeric phase indirectly confirms the dominant role of shock waves in the detected C₆₀ phase transformation.     

Pressure-induced transformations in two-dimensional polymeric phases of C<Subscript>60</Subscript>

The structural stability of the tetragonal and rhombohedral two-dimensional (2D) polymeric phases of C₆₀ was studied under pressures up to 27 GPa at room temperature by means of in situ Raman scattering spectroscopy. The results show that the tetragonal 2D phase undergoes an irreversible transformation in the region of 20 GPa while no pressure-induced transitions were observed for the rhombohedral 2D phase. The obtained data are discussed within the framework of recent numerical calculations, which predict the pressure-induced transformation of the 2D polymeric phases of C₆₀ into three-dimensional (3D) polymers in the pressure range 14–20 GPa.     

Investigation of fluorination reactions of the C<Subscript>60</Subscript>Br<Subscript>24</Subscript> bromofullerene with xenon difluoride

The products of the reaction between the C₆₀Br₂₄ bromofullerene and xenon difluoride are studied by IR spectroscopy, laser desorption mass spectrometry, ¹⁹F NMR spectroscopy, and thermogravimetry. It is revealed that the degree of fluorination of the substrate is primarily determined by the duration of contact between the reactants. An attempt is made to perform nucleophilic substitution of fluorine for bromine in the C₆₀Br₂₄ bromofullerene in a solution with potassium fluoride.     

Magnetic and structural anomalies of Na<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>C<Subscript>60</Subscript> (n = 2, 3)

Sodium fullerides Na _n C60 (n = 2, 3) have been synthesized by a liquid phase reaction and investigated with X-ray diffraction (XRD), nuclear magnetic resonance (NMR), electron paramagnetic resonance, and differential thermal analysis. XRD data indicate that the crystal structure of Na₂C₆₀ at 300 K is face centered cubic (FCC). A phase transition from primitive cubic to FCC crystal structure has been observed in this work in Na₂C₆₀ fulleride at 290 K. The transition is accompanied by the step-like change of paramagnetic susceptibility. The crystal structure of Na₃C₆₀ is more complicated than, and different from, what has been reported in the literature. A nearly seven-fold increase of paramagnetic susceptibility with increasing temperature has been observed in the Na₃C₆₀ fulleride at 240–260 K. In the same temperature range, a new line at about 255 ppm appears in the ²³Na NMR spectrum, indicating a significant increase of electron density near the Na nucleus. The observed effect can be explained by a metal-insulator transition caused by a structural transition.     

Scanning tunneling spectroscopy and manipulation of C<Subscript>60</Subscript> on Cu(111)

The properties of C₆₀ adsorbed on Cu(111) have been studied using low temperature scanning tunnelling microscopy and spectroscopy. In the electronic spectrum of the molecule, we observe features that can be assigned to molecular orbitals. The LUMO level is split into two states, as a consequence of the charge transfer from the substrate to the carbon cage. The data from the inelastic electron tunnelling spectroscopy reveal two peaks that can be assigned to the intramolecular vibrational modes of the C₆₀ cage. We demonstrate also controlled manipulation of single molecules. The plot of the tip height, recorded during the manipulation process, indicates that the C₆₀ is pushed along the surface. PACS 68.37.Ef; 73.61.Wp; 68.43.Pq; 82.37.GK; 68.43.-h     

Photopolymerization of C<Subscript>60</Subscript> and Li@C<Subscript>60</Subscript> studied by second-harmonic generation and infrared spectroscopy

The photopolymerization of C₆₀ and Li@C₆₀ films was investigated by means of optical second-harmonic generation. The films were deposited under ultra-high-vacuum conditions and irradiated in situ with an Ar⁺ laser at 514 nm. The second harmonic generated by a Nd:YAG laser working at 1064 nm was monitored after different steps of irradiation. Photopolymerization was observed after very low irradiation doses, of the order of 10²⁰ photons/cm², and confirmed with infrared absorption spectroscopy. Similar kinetics for C₆₀ and Li@C₆₀ were observed. The measurements give evidence for photopolymerization of the endohedral fullerene Li@C₆₀. PACS 78.30.Na; 82.50.Hp; 81.05.Tp     

Glass and phase transitions in solid C<Subscript>60</Subscript> charged with Ar,Ne, He,and O<Subscript>2</Subscript>

C₆₀ pellets, charged respectively with He, Ne, Ar, and O₂ under high pressure, were studied with complex impedance spectroscopy. The latter two were found to depress the temperatures for both the phase change (T_c) and glass transition (T_g), although in time shifts, they stayed almost constant in the Ar, but decreased in the O₂, case. The effective barriers for the glass transition were determined as 218±15 meV in Ar_0.49C₆₀ and 182±20 meV in (O₂)_0.5C₆₀. In contrast, neither He nor Ne had a discernible influence on these material parameters. These observations could be explained as follows. (1) Interstitial Ar causes a reduction in the energy barrier and a weakening in the restorative force of the cage libration mode, thereby depressing T_c. (2) For (O₂)_0.5C₆₀, coupling between the cage libration and the tumbling of an O₂ diatomic molecule further weakens the restorative force, so that a larger temperature shift results. (3) Its tumbling motion makes O₂ more susceptible to the “paddle wheel” action of rotating C₆₀ and more easy to diffuse even under ambient conditions.     

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.     

Grain Boundary Diffusion in C<Subscript>60</Subscript> Thin Films

This paper focuses on the effect of grain boundaries on the diffusion processes in polycrystalline C₆₀ thin films. Electrically induced diffusion of Au was investigated by in situ measurements of the film conductivity. Electron Paramagnetic Resonance (EPR) spectroscopy was used to study diffusion of oxygen. Increase in grain sizes in polycrystalline C₆₀ thin films was found to result in the acceleration of gold and oxygen diffusion. The results are interpreted assuming that these impurities diffuse in C₆₀ films dominantly along grain boundaries.     

Ordering phenomena in C<Subscript>60</Subscript>-tetraphenylphosphonium bromide

The fulleride salt C₆₀-tetraphenylphosphonium bromide is investigated as a function of temperature by single crystal X-ray diffuse scattering and diffraction. At room temperature, the C₆₀ orientational disorder is found to be more complex than previously expected. Moreover, a structural phase transition, due to the C₆₀ orientational ordering, is evidenced around 120 K. Its relation with the stabilization of a static Jahn-Teller effect is discussed. Received 3 November 1999     

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.     

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.     

Epitaxial growth of C<Subscript>60</Subscript> thin films on the Bi(0001)/Si(111) surface

The morphology and atomic structure of C₆₀ fullerene films on the Bi(0001)/Si(111)?7 × 7 surface with different coverages have been studied by scanning tunneling microscopy and spectroscopy and low-energy electron microscopy in high vacuum. It is shown that the most favorable sites for nucleation of C₆₀ islands are double steps and domain boundaries on the surface of epitaxial Bi film.     

Superconductivity and spectroscopy of heterofullerides Rb<Subscript>2</Subscript>MC<Subscript>60</Subscript>, K<Subscript>2</Subscript>MC<Subscript>60</Subscript>, and KM<Subscript>2</Subscript>C<Subscript>60</Subscript> (M = Mg,Be)

A series of new heterofullerides with compositions Rb₂MC₆₀, K₂MC₆₀, and KM₂C₆₀ (M = Mg, Be) have been synthesized. Measurements of the temperature dependences of the magnetic susceptibility in the temperature interval from 4.2 to 300 K reveal a superconducting transition in heterofullerides K₂MgC₆₀, KMg₂C₆₀, K₂BeC₆₀, and Rb₂BeC₆₀ at temperatures T _c = 13–22 K. The electron states with uncompensated spin are studied by the electron paramagnetic resonance technique. The contributions of conduction electrons and localized electrons to the paramagnetic susceptibility are extracted.     

On the interaction of self-assembled C<Subscript>60</Subscript>F<Subscript>18</Subscript> polar molecules with the Ni(100) surface

The current work is dedicated to investigation of the interaction between self-assembled polar molecules of fullerene fluoride C₆₀F₁₈ with the chemically active surface Ni(100) under radiation and heat treatments. X-ray photoelectron spectroscopy is used in combination with quantum-chemical simulation. For the first time, the transformation of an as-deposited dielectric continuous 2D thin film to a 3D island-type assembly with molecular ordering within the islands is shown to take place. The degree of coverage of the Ni surface by C₆₀F₁₈ islands (0.6–0.7) and their height (~6 nm) are estimated. Quantum-chemical simulation shows that the chemisorption energy of the C₆₀F₁₈ molecule on the Ni surface equals ~6.6 eV and fluorine atoms are located at a distance of 1.9 Å above the Ni surface. The results of the investigation provide an opportunity to create nanoscale ordered structures with local changes in the work function.     

Comparative study of the spectral and structural properties of EuAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> single crystals with different morphologies

Europium alumoborate EuAl₃(BO₃)₄ microcrystals have been synthesized by the flux method at a temperature of 1050°C. The obtained crystals have different morphologies: both plane-faced and skeletal microcrystals have been observed. Infrared spectroscopy, cathodeluminescence, and transmission electron microscopy investigations of individual microcrystals showed that the spectral and structural characteristics of these morphological forms coincide. The obtained crystals are characterized by the rhombohedral symmetry (sp. gr. R32) with the inclusions of C2/c monoclinic phase domains.     

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.     

Photo-and pressure-induced transformations in the linear orthorhombic polymer of C<Subscript>60</Subscript>

Stability of the linear orthorhombic polymer of C₆₀ under pressure and laser irradiation is studied by Raman scattering and X-ray diffraction measurements. The Raman spectrum at ambient pressure remains unchanged, in the time scale of the experiment, up to an intensity of 3200 W/cm² of the 514.5 nm line of an Ar⁺ laser, but irreversible changes are observed at higher intensities. The Raman spectra recorded at increased pressure show similar irreversible changes even at the laser intensity as low as 470 W/cm². The X-ray diffraction and Raman measurements of the pressure-treated samples, performed after pressure release, show that the nonirradiated material does not exhibit any changes in the crystal structure and phonon spectra. This behavior indicates a pressure-enhanced photo-induced transformation to a new polymeric phase characterized by a Raman spectrum that differs from those of the other known polymeric phases of C₆₀. The Raman spectra of the phototransformed linear orthorhombic polymer of C₆₀ were measured at a pressure of up to 29 GPa. The pressure dependence of the Raman mode frequencies show singularities near 4 GPa and 15 GPa, respectively, related to a reversible phase transition and an irreversible transformation to a metastable disordered phase. The diffuse Raman spectrum of the disordered phase does not exhibit substantial changes with an increase in pressure up to 29 GPa. The high-pressure phase transforms to a mixture of pristine and dimerized C₆₀, after pressure release and exposure to ambient conditions for 30 h. The text was submitted by the authors in English.     

Single-crystal structural study of the pressure-temperature-induced dimerization of C<Subscript>60</Subscript>

We present a study by Raman spectroscopy and X-ray diffraction/diffuse scattering of C₆₀ single-crystals treated at high-pressure and high-temperature. This allowed us to obtain structural information on the C₆₀ dimer state which can be considered as an intermediate state in the polymerization process. In the 1-6 GPa pressure range the crystals are primarily formed of dimers with additional minor fractions of monomers, 1D and 2D polymers, as shown by the analysis of the Raman spectra. The dimers are disordered within an average cubic lattice derived from that of the monomer. Single-crystal diffraction patterns reveal a characteristic diffuse scattering intensity distribution which has been simulated by calculating the diffuse scattering produced by dimer and trimer model structures. Satisfactory agreement is obtained for random positional and orientational disorder of the C₆₀-C₆₀ dimers although a small concentration of similarly disordered trimers is likely. In a first approximation the dimer/trimer disorder can be considered as random but various inter-dimer correlations are probably present, as discussed.Received: 3 October 2003, Published online: 19 February 2004PACS: 61.48. + c Fullerenes and fullerene-related materials - 61.43.Bn Structural modeling: serial-addition models, computer simulation - 78.30.Na Fullerenes and related materialsT. Wågberg: Present address: Groupe de Dynamique des Phases Condensées, Université Montpellier II, Place E. Bataillon, CC026, 34095 Montpellier, France     

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.