Stability of hexagonal modification of fullerite C60

Stability of the hcp modification of fullerite C₆₀ under heating, compression, and mechanical grinding was studied. It was established that uniaxial load and mechanical grinding caused transition of the hcp modification into more stable fcc modification, while no transition was observed upon heating or hydrostatic pressure. The results were discussed in terms of the theory of the deformation of solids.     

Synthesis of a hexagonal modification of C60 using cryoextraction

Samples of the hexagonal close-packed phase (h.c.p.) of fullerite C₆₀ with a low (less then 7%) content of more stable face-centered cubic (f.c.c.) phase were synthesized using cryoextraction with n-hexane. X-Ray diffraction analysis and thermogravimetry were applied for characterization of the samples. The role of n-hexane in the formation of the h.c.p. structure is discussed.     

A study of the synthesis of endohedral metallofullerenes and empty fullerenes by the carbon-arc and RFICP technique

Fullerenes and endohedral metallofullerenes (M_x@C_2n; M=La, Mn, Nb, Au, and Li; x=1-3, n≥30) were synthesized by two different techniques: the standard carbon-arc technique and a modification involving an RF-inductive coupled plasma discharge. Both techniques employ a modified carbon-arc reactor which is durable, easy to operate, and capable of providing a high degree of control over many parameters of the reaction. Our results suggest that both fullerenes and endohedral metallofullerenes are formed by a radical pathway.     

Positron annihilation in C60 fullerites and fullerene-like nanovoids

C₆₀ fullerites, fatty-acid triglycerides containing nanovoids, and triglycerides with dissolved fullerenes were studied by means of positron annihilation spectroscopy. Two types of nanovoids with mean radii of 0.48 and 0.34 nm were found in the fatty-acid triglycerides. The nanovoids of the latter type change size in the course of structural relaxation and ordering at room temperature. The nanovoid radius is stabilized at the value of 0.355 nm, equal to the radius of C₆₀ fullerenes, when the latter are dissolved in triglycerides. Tetrahedral interstitials in the f.c.c. lattice are shown to be the most probable positron-annihilation sites in C₆₀ fullerites. The shell of the C₆₀ molecule is a potential barrier for positron penetration to the interior of the fullerene. Vice versa, the nanovoid in triglycerides is a potential well for positively charged particles: positrons, protons and cations.     

Solid state studies of the C60. 2(CH3)CCl3 solvate

At room temperature, the hexagonal C₆₀. 2(CH₃)CCl₃ solvate (a = 10.13(1) Å, c = 10.84(1) Å), made of alternating layers of C₆₀ and solvent molecules, forms with a negative excess volume, and its desolvation enthalpy is virtually the same as the sublimation enthalpy of the pure solvent. Crystallographic and calorimetric studies vs temperature indicate that hexagonal C₆₀. 2(CH₃)CCl₃ changes at 211.7 K (1.3 kJ mol⁻¹ of solvate) into an intermediate triclinic phase which transforms at 189.7 K (4.1 kJ mol⁻¹ of solvate) into another triclinic phase.A crystallographic analysis in the series of hexagonal C₆₀. 2 YCCl₃ solvates (Y = H, Cl, Br, CH₃) reveals that: (i) the change in the unit-cell volume values is due to a change in axis c whose value depends on the size of Y, (ii) the molar volume of the solvates depends linearly on the molar volume of the solvents.Ageing studies at room temperature show that C₆₀. 2(CH₃)CCl₃ loses its solvent molecules within a few days or a few months, depending on storage conditions.     

Polymeric fullerene oxide (fullerene ozopolymers) produced by prolonged ozonation of C60 and C70 fullerenes

The prolonged ozonation of C₆₀ and C₇₀ fullerene produces light brown to brown amorphous solids which are insoluble in chlorinated and hydrocarbon solvents but which are readily soluble in water, acetone and ethanol where they give dark-brown solutions. The polymeric and polyelectrolytic nature of these solids has been shown by viscosimetric and cryoscopic measurements. Due to the polymeric nature the solids have been called ‘ozopolymers’. The reactivity and the ozone uptake have been measured quantitatively during the ozonation of C₆₀ and C₇₀ fullerene in CCl₄. Three different C₆₀ ozopolymer samples have been produced at different degrees of ozonation: at O₃/C₆₀ molar ratio of 8, 14 and 26. The C₇₀ ozopolymer has been produced at an O₃/C₇₀ molar ratio of 30. All the samples have been studied by FT-IR spectroscopy and the C₆₀ ozopolymers have been easily reduced by the action of Zn dust and acetic acid or by the action of H₂S and studied by FT-IR. The action of oxidizing agents has been studied as well. C₆₀ ozopolymer is a polyelectrolyte rich in carboxyl groups and for this reason it possesses a high metal binding capacity similar to that of humic acids. The thermal stability of C₆₀ ozopolymer and its reduced derivatives as well as the thermal stability of C₇₀ ozopolymer has been checked by thermogravimetric analysis in nitrogen and air flow. Finally, the electrical conductivity of C₆₀ ozopolymer was found to be σ=2.8×10⁻⁸ S/cm which is three orders of magnitude lower than that of pure C₆₀.     

New solid solvates of C60 and C70 fullerenes: The relationship between structures and lattice energies

The goal of this study was to establish the relationship between the X-ray and the thermodynamic data on fullerene’s solvated crystals. X-ray diffraction study of 12 solid solvates of C₆₀ and C₇₀ with different aromatic solvents have been performed. It has been demonstrated that the solid solvates under consideration were typical van-der-Waals complexes with the negative excess volumes, packing coefficients from 0.72 to 0.78, stable due to the formation of the fullerene to solvent bonds, reasonably described by six to 12 Lennard-Jones potentials. The atom-atom potential method has been used to describe both the crystal structures and the thermodynamics of the solid solvates. The minimization of the lattice energy with respect to the cell and rigid body parameters (T=0 K) has led to the crystal structures very close to the experimental ones. The minimum energies found have reasonably reproduced the calorimetric lattice energies of the solvates. The theory has also demonstrated its ability to account for the trends in thermodynamic stability of solid solvates, e.g., has predicted correctly the low stability of the hypothetical monoclinic solvate C₆₀·2C₆H₁₄.     

Synthesis and structural investigation of new graphite intercalation compounds containing the perfluoroalkylsulfonate anions C10F21SO3, C2F5OC2F4SO3, and C2F5(C6F10)SO3

The preparation of graphite intercalation compounds (GIC’s) of three perfluorinated alkylsulfonate anions, C₁₀F₂₁SO₃⁻, C₂F₅OC₂F₄SO₃⁻ and C₂F₅(C₆F₁₀)SO₃⁻ is described for the first time. Pure stage 2 GIC’s are obtained by chemical oxidation of graphite with K₂MnF₆ in a solution containing hydrofluoric and nitric acids for 72 h. One-dimensional electron density maps derived from powder diffraction data are fit to obtain models for the intercalate interlayer regions (galleries) structures: the structure models provide details on anion concentrations, orientations, and conformations. In all cases, anion bilayers are observed with anion sulfonate headgroups oriented towards graphene sheets. Compared with structures calculated for the isolated anions, the intercalated anion conformations show changes in dihedral angles, involving rotations about C-C or C-O bonds. For the GIC containing C₂F₅(C₆F₁₀)SO₃⁻, the anion conformation change is related to the more efficient packing of anions in the intercalate gallery.     

High energy-storage properties of [(Bi1/2Na1/2)0.94 Ba0.06] La(1−x) ZrxTiO3 lead-free anti-ferroelectric ceramics

Energy-storage properties of [(Bi_1/2Na_1/2)_0.94Ba_0.06]La_(1−x)Zr_xTiO₃ (BNT-BLZT, x=0, 0.02, 0.04, and 0.06) lead-free anti-ferroelectric ceramics fabricated via the conventional sintering technique were first investigated. Calculation from the X-ray diffraction results reveals that BNT-BLZT ceramic possesses a single perovskite structure phase. In addition, the P–E hysteresis loops measured at room temperature show that the BNT-BLZT (x=0.02) ceramics obtain the maximum P value of 37.5 μC/cm² and the largest energy-storage density W_max is 1.58 J/cm³. The temperature dependence of dielectric permittivity ε_r and dielectric loss tanδ illustrate that the addition of Zr can improve the piezoelectric properties of BT-BLZT ceramics. These properties indicate that BNT-BLZT ceramics might be a promising lead-free anti-ferroelectric material for energy storage application.     

Direct conversion mechanism of fluorine-GIC into poly(carbon monofluoride), (CF)n

The reaction mechanisms of a stage-1 fluorine-graphite intercalation compound (GIC), C_2.5F, with 0.10 MPa of fluorine gas have been studied at 573-773 K. The original stage-1 structure of C_2.5F with semi-ionic C-F bonds and planar sp² carbon sheets is maintained in most part of the compound after the reaction at 573 K, although a large number of covalent C-F bonds are formed on the surface. This compound is partially or completely converted to poly(carbon monofluoride), (CF)_n, with covalent C-F bonds and puckered sp³ carbon sheets at 673 or 773 K, respectively. Single-phase (CF)_n obtained at 773 K possesses remarkably small BET specific surface area, 61 m²/g carbon, almost unchanged from the value of the precursor C_2.5F (69 m²/g carbon). In this reaction, the accommodation of fluorine atoms supplied from the atmosphere into the galleries of C_2.5F is facilitated by the rearrangement of originally intercalated fluorine atoms in the GIC, forming (CF)_n with fewer defects compared to those by the conventional direct fluorination of graphite.     

Influence of the pressure-temperature treatment on the polymerization of C60 single crystals at 2 GPa-700 K

X-ray diffraction and Raman spectroscopy have been used to characterize the structures obtained when C₆₀ single crystals are treated at 2 GPa-700 K. Two different experimental procedures have been applied: the temperature is raised before the pressure is applied, or the opposite. The “heating-then-pressing” path leads to the tetragonal polymer structure (P4₂/mmc) together with a minor fraction of rhombohedral structure, which confirms previous results. In contrast, the “pressing-then-heating” path leads to a different state presenting similarities with both the rhombohedral and the disordered dimer structures. The results are discussed in light of the orientational and dynamical aspects of the C₆₀ polymerization.     

The graphite arc-discharge in the presence of CCl4: Chlorinated carbon clusters in relation with fullerenes formation

The graphite arc-discharge in the atmosphere consisting of CCl₄ 40 Torr and increasing partial pressure of He from 0 to 300 Torr produced, in addition to carbon species such as graphite and fullerenes C₆₀ and C₇₀, numerous chlorinated carbon clusters (CCCs). The yields of some CCCs, including C₆Cl₆ (perchlorobenzene), C₁₀Cl₈ (perchloronaphthalene), C₁₂Cl₈ (perchloroacenaphthylene) and C₁₂Cl₈ (isomer II), C₁₄Cl₈ (isomer I), C₁₆Cl₁₀ (perchlorofluoranthene), C₁₈Cl₁₀ (isomer I and II), and C₂₀Cl₁₀ (isomer I), correspond well with C₆₀ yields, which may imply these CCCs share the same growth mechanism with fullerenes. Such a yield correlation can be used to distinguish the fullerene precursors from the irrelevant by-products, thus contributes to the mechanistic study of fullerene formation.     

High growth of SWNTs and MWNTs from C2H2 decomposition over Co-Mo/MgO catalysts

A series of MgO supported catalysts having Co and Mo metals 5-40 wt.% in a ratio of 1:1 was prepared by impregnation method. Carbon nanotubes (CNTs) were grown over the catalysts by decomposition of C₂H₂ at 800 °C for 30 min. It was found that 5 and 10 wt.% Co-Mo/MgO catalysts produced single-wall nanotubes (SWNTs), whereas 20, 30 and 40 wt.% Co-Mo/MgO catalysts produced multi-wall nanotubes (MWNTs). The catalyst Mo/MgO was inactive in growing CNTs. In Co-Mo/MgO catalysts, however Mo generated a favorable environment to grow SWNTs. The growth of SWNTs was strongly dependent on the formation of small clusters of cobalt, which may generate from the decomposition of CoMoO₄ species during the nanotube growth. MWNTs were produced over comparatively larger cobalt clusters generated from Co₃O₄ phase during the nanotube growth stage. The yields of SWNTs were about 6% and 27% over 5 and 10 wt.% Co-Mo/MgO catalysts, respectively. MWNTs yield (576%) was observed over 40 wt.% Co-Mo/MgO catalyst. Carbon yield (%) highly varied with acetylene concentration.     

NaV2O5 nanoplates: Hydrothermal synthesis,characterization and study of their optical and electrochemical properties

Plate-like nanocrystalline NaV₂O₅ has been synthesized hydrothermally via a simple and elegant route. The morphology, the structure, the crystallinity and the composition of the samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, nitrogen adsorption/desorption isotherms and photoluminescence. Electrochemical measurements have revealed reversible redox behavior with doping/dedoping process corresponding to reversible cation intercalation/deintercalation into the crystal lattice of the nanoplates. This process is easier in propylene carbonate than in aqueous solvent and is easier for the small Li⁺ cation than larger ones K⁺. This is attributed to probable presence of two different tunnel cavities in the NaV₂O₅ lattice.     

Dielectric properties of CuO-doped La2.98/3Ba0.01(Mg0.5Sn0.5)O3 ceramics at microwave frequency

The microwave dielectric properties of La_2.98/3Ba_0.01(Mg_0.5Sn_0.5)O₃ ceramics prepared by the conventional solid-state method were investigated for application in mobile communication. A 100 °C reduction of the sintering temperature was obtained by using CuO as a sintering aid. A dielectric constant of 20.0, a quality factor (Q × f) of 50,100 GHz and a temperature coefficient of resonant frequency τ_f of −78.3 ppm/°C were obtained when La_2.98/3Ba_0.01(Mg_0.5Sn_0.5)O₃ ceramics with 0.25 wt.% CuO were sintered at 1500 °C for 4 h.     

XRD and TEM study of high pressure treated single-walled carbon nanotubes and C60-peapods

In situ synchrotron X-ray diffraction measurements of single-walled carbon nanotube and C₆₀-peapod samples under high pressures up to 13 GPa and at high temperature were carried out. Anisotropical shrinkages of their bundle two-dimensional lattices by compression at room temperature were observed. It was found that the lattices recover original forms reversibly upon pressure release. It was also found that irreversible phase transformations occur by raising temperature at the highest pressure. The high pressure and high temperature treated samples were examined by X-ray diffraction, transmission electron microscope, and Raman measurements. It was indicated by transmission electron microscope observation that hexagonal diamond is able to be synthesized by high pressure and high temperature treatment of C₆₀-peapods.     

Soot-free synthesis of C60

A new synthetic medium for the production of C₆₀ has been found that does not produce soot. C₆₀ was produced in the liquid phase of an aerosol of precursor soot at 700 °C. The precursor soot aerosol, a high temperature stable form of hydrocarbon, was produced by pyrolysis of acetylene at atmospheric pressure in a flow tube reactor. At 700 °C, the effluent particles were found to contain PAHs, small hydrocarbons and fullerenes but no observable black material. However, when the reactor temperature was changed to 800 °C, soot was also produced in the effluent particles along with PAHs and other small hydrocarbons, and the fullerene product disappeared. These results show a clear competition between the production of fullerenes and other forms of carbon. The filter-collected effluent was shown to be completely soluble in conventional solvents suggesting the possibility of an efficient cyclic synthetic process. Fullerenes were only found in the particle phase implying the first observed liquid phase synthesis of C₆₀.     

Soot-free synthesis of C60

A new synthetic medium for the production of C₆₀ has been found that does not produce soot. C₆₀ was produced in the liquid phase of an aerosol of precursor soot at 700 °C. The precursor soot aerosol, a high temperature stable form of hydrocarbon, was produced by pyrolysis of acetylene at atmospheric pressure in a flow tube reactor. At 700 °C, the effluent particles were found to contain PAHs, small hydrocarbons and fullerenes but no observable black material. However, when the reactor temperature was changed to 800 °C, soot was also produced in the effluent particles along with PAHs and other small hydrocarbons, and the fullerene product disappeared. These results show a clear competition between the production of fullerenes and other forms of carbon. The filter-collected effluent was shown to be completely soluble in conventional solvents suggesting the possibility of an efficient cyclic synthetic process. Fullerenes were only found in the particle phase implying the first observed liquid phase synthesis of C₆₀.     

Structural anomaly and electrical relaxation in (Na2/3Pb1/3)(Mn1/2Nb1/2)O3 ceramics

The X-ray diffraction patterns of (Na_2/3Pb_1/3)(Mn_1/2Nb_1/2)O₃ ceramics were measured within 15–850 K temperature range. The anomaly in the thermal expansion temperature dependence occurred in 250–365 K range. The generalised Cole–Cole model was proposed to describe the measured effective electric permittivity influenced by high electric conduction and the coexistence of two contributions ?*(T,f) = ?*_lattice + ?*_carriers was considered. The analysis of the electric permittivity and conduction exhibited two relaxation processes. The electric conduction relaxation characteristic time values indicated the small polaron mechanism with τ₀ ≈ 10⁻¹³ s occurring in 240–345 K range and the ionic mechanism with τ₀ ≈ 10⁻¹¹ s involved in the other relaxation occurring in the 320–510 K range. The ionic relaxation process was ascribed to a subsystem of defects, which was weakly interrelated to the anomaly in thermal expansion of the (Na_2/3Pb_1/3)(Mn_1/2Nb_1/2)O₃ ceramics. The Gate model was proposed to describe the ionic relaxation mechanism.