V. Agafonov

Thermal studies of C60 transformed by temperature and pressure treatments

Raman spectroscopy, X-ray and electron diffractions have been applied to study samples of fullerene C60 after they have undergone pressure and temperature treatments up to 8 GPa and 1073 K. It is confirmed that mixtures of rhombohedral and tetragonal structures are formed in the 2–4 GPa and 673–1073 K pressure-temperature domain. Traces of a hexagonal phase are also observed in the same range. Only the rhombohedral one is obtained in the 4–8 GPa range in the same temperature interval. The material obtained at temperatures between 473 and 673 K in the whole pressure range which has previously been described in terms of a cubic structure can be understood in terms of an orthorhombic one. It may also contain mixtures of different phases including a cubic one and even a cubic superstructure. The widened poorly resolved X-ray profiles indicate the existence of disorder within the materials formed and the increasing complexity of Raman spectra as temperature increases at fixed pressures (from 2 to 8 GPa) suggests that this disorder is related to phase mixtures in almost all samples. A rhombohedral sample formed at 6 GPa-873 K reverted to a mixture of rhombohedral and tetragonal phases at 2.5 GPa-873 K. Thus thermodynamic equilibrium between these two kinds of systems could exist.

Production of nano- and microdiamonds with Si-V and N-V luminescent centers at high pressures in systems based on mixtures of hydrocarbon and fluorocarbon compounds

A new method has been proposed for the synthesis of nano- and microdiamonds with various contents of luminescent silicon-vacancy (Si-V) and nitrogen-vacancy (N-V) centers at high static pressures in growth systems based on mixtures of hydrocarbon, fluorocarbon, and organic silicon compounds without catalyst metals. The high efficiency of the proposed method of the doping of diamonds at nano- and microlevels has been demonstrated by analyzing the spectra of photoluminescence and absorption of the diamonds obtained.

Pressure-induced polycondensation of C60 fullerene

Modifications of carbon which are formed from C60 fullerite at pressures up to 10.0 GPa and temperatures up to 1900 K are studied by x-ray diffraction, Raman spectroscopy, and atomic force microscopy methods. The pressures p and temperatures T at which atomic, molecular, and polymolecular structures form under conditions of quasihydrostatic compression are determined. It is shown that, together with polymerization, another type of chemical interaction of the molecules, called polycondensation, which leads to the formation of polymolecular structures with a shortest intermolecular distance of 0.65 nm, is possible in the system. Three-dimensional polycondensation of C60 fullerene is explained by the special properties of the new carbon states.

Identification of the polymerized orthorhombic phase of C60 fullerene

It is established by x-ray diffraction and Raman scattering that the polymerization of C60 fullerene at 1.5 GPa and 723 K leads to the formation of an orthorhombic phase that is different from the previously identified high-pressure orthorhombic phase. It is determined by a calculation of the optimal packing of linear C60 polymers by the method of atom-atom potentials that the energetically favorable structure of the orthorhombic phase belongs to the space group P n n m and not the previously proposed group I m m m. The computed value of the rotation angle of the polymer chains that corresponds to the minimum packing energy was equal to 61°. The mechanisms leading to the formation of the polymerized phases are discussed on the basis of the results obtained.

Synthesis of a new cubic phase in the B-C-N system

Phase formation in the B-C-N system has been studied at pressures from 6 to 15 GPa and temperatures from 1000 to 1600°C using mixtures of carbon nitride (C3N4) nanospheres and boron. A new ternary phase with the structure of cubic boron nitride in which some of the nitrogen and boron atoms are replaced by carbon has been obtained at p ≃ 8 GPa and t ≃ 1500°C. According to the Rietveld refinement results, its composition is BC0.47N0.85.

Pressure-induced dimerization of C60 fullerene

The dimerization of C60 fullerene under conditions of quasi-hydrostatic compression at temperatures above 293 K is investigated by IR spectroscopy, Raman scattering (RS) spectroscopy, and x-ray diffraction. The measured dimer (C60)2 content in the products of the polymerization of fullerite as a function of the pressure, temperature, and treatment time shows that dimerization occurs even at room temperature in the entire pressure range above ∼1.0 GPa. However, at least at temperatures above 400 K dimerization does not result in the formation of a dimer phase as a stable modification of the system, since the dimer is an intermediate product of the transformation. It is shown that increasing the holding time at 423 K decreases the content of the dimer fraction in the samples and results in the formation of linear (at 1.5 GPa) and two-dimensional (at 6.0 GPa) polymers, which are structure-forming elements of the orthorhombic and rhombohedral polymerized phases.

Size-dependent nanodiamond-graphite phase transition at 8 GPa

The size-dependent diamond-graphite phase transition was detected in the course of a study of the growth of nanosized diamond particles under conditions of thermal treatment at 8 GPa. It was found that a critical size of diamond nanoparticles, on reaching which they transformed into graphite, was reached at 1623 K and a treatment time of 60 s, and it was equal to ∼18 nm. The activation energy of the solid-phase growth of nanosized diamond particles at 8 GPa was determined to be 112±8 kJ/mol.

Tetragonal polymerized phase of C60: experimental artifact or reality?

Abstract The tetragonal polymerized phase (T) of C60 previously considered as metastable is shown experimentally to be thermodynamically stable. Three different pressure-temperature paths were followed to prepare it. It has been found that the increase of treatment time entails progressive disappearance of impurities to the benefit of the T phase in the p,T-range of its stability. The experimental paths leading to almost pure T phase were established. Crystallographic, IR and Raman data related to the T phase are presented.

High‐Resolution X‐Ray Powder Diffraction Structure Determination of C60F48

Abstract Whereas previously reported X‐ray powder diffraction experiments could be interpreted using an isotropic spherical two‐shell model, our recently obtained data cannot. Rather, they were Rietveld‐analyzed using three distinct anisotropic models for the C60F48 molecule featuring the S6, D3 and Th molecular symmetries, respectively. The best fit was obtained for the D3 model, which is consistent with the 19 F NMR characterization of our sample. The χ2 dependence upon the Euler angles of the C60F48 molecule is found to be substantial. The unit cell is very nearly (if not) tetragonal and P 21/n turns out to be a satisfactory space group.

Carbon-Encapsulated Iron Carbide Nanoparticles in the Thermal Conversions of Ferrocene at High Pressures

Pressure-temperature-induced transformations of ferrocene at 8 GPa and different temperatures up to 1000°C have been studied by XRD, SEM and TEM techniques. It was shown that the thermal destruction of ferrocene at 480–600°C is attended with the formation of quasi-spherical nanosized (10–30 nm) particles containing the iron carbide (Fe7C3) core and external carbon envelope from the fragments of graphene layers. Magnetic measurements indicate that the nanoparticles obtained at 8 GPa and 480–560°C demonstrate a superparamagnetic behaviour at room temperature. Above 600°C, the Fe7C3 nanoparticles decompose into Fe3C and graphite.