Gennadii A. Dubitsky

Ultrahard and superhard carbon phases produced from C60 by heating at high pressure: structural and Raman studies

Investigations of X-ray diffraction patterns, Raman spectra, hardness and other physical properties were carried out for fullerite after a heat treatment up to 1830 K at 9.5 and 13 GPa nonhydrostatic pressure of bulk samples. Both superhard and ultrahard forms of carbon have been obtained from C60 at 620–1830 K. The hardness of the materials exceeded that of diamond. The X-ray and Raman measurements have shown that both crystal structures and fullerene molecules were retained in superhard and ultrahard states, but a gradual increase of disorder occurred and a random network of linked molecules at high temperatures was formed.

Phase transformations in solid C60 at high-pressure-high-temperature treatment and the structure of 3D polymerized fullerites

New data concerning the solid C60 phase transformations in the pressure-temperature range P = 6.5–13 GPa, T = 300–2100 K and nonhydrostatic conditions are presented and plotted in the P-T coordinates. The structure of superhard and ultrahard carbon phases obtained with these conditions is investigated by X-ray powder diffractometry and Raman scattering. New crystal structures of distorted bcc type and transient to diamond states are revealed in the synthesized samples. Fragments of different lengths of polymerized fullerene cages are found from resonance Raman spectra of the samples obtained at P = 13 GPa and T < 1200 K.

Structures and physical properties of superhard and ultrahard 3D polymerized fullerites created from solid C60 by high pressure high temperature treatment

Abstract Superhard and ultrahard phases of C60 were synthesized by quenching at high pressures up to 13 GPa and high temperatures in the 300–2100 K range. The structures of the samples are discussed on the basis of X-ray and Raman spectra and electron microscopy data. The following physical properties of hard samples were studied: specific gravity; specific heat in the range 400–600 K; sound velocities; elastic moduli; electrical properties; resistance to uniaxial stress; stability against oxidation. These properties are different from those of diamond and other carbon forms. The hardness of ultrahard fullerites exceeds the hardness of diamond.

Cluster structure and elastic properties of superhard and ultrahard fullerites

Velocities of the longitudinal and shear sound waves are measured in ultrahard fullerites created by static high-pressure-high-temperature treatment under P=13 GPa and T=1670–1870 K. The highest value of 26.0 km/s for the longitudinal waves is measured, that is about 40% more than in diamond. Bulk modulus of different ultrahard fullerites covers the range of about 600–1700 GPa. The highest hardness is about 30×103 kg/mm3. We ascribe these unique properties to formation of 20–30 atoms clusters by the walls of adjacent molecules under process of 3D-cross-linking. Most distinctly these clusters declare themselves in the cubic structure with the lattice parameter about 6 A and 32 atoms per unit cell.

Superhard Superconductive Composite Materials Obtained by High-Pressure-High-Temperature Sintering

Superhard superconducting materials are of considerable interest for the creation of high pressure devices for investigating electrical and superconducting properties of various materials. The superconducting composites consisting of superconductors and superhard materials that are in thermal and electrical contacts may satisfy very conflicting requirements imposed on superconducting materials for special research cryogenic technique, wear-resistive parts of superconductor devices, superconducting micro-electromechanical systems (MEMS), etc. The design of materials combining such properties as superconductivity, superhardness, and high strength is an interesting task for both scientific and applied reasons. Superconducting composites may be used for the production of large superconducting magnetic systems (Gurevich et al., 1987). The discovery of superconductivity in heavily boron-doped diamonds (Ekimov et al., 2004; Sidorov et al., 2005) has attracted much attention. Superconducting diamonds are the hardest known superconductors. The potential applications of superconducting diamonds are broad, ranging from anvils in research high-pressure apparatus to supecronducting MEMS. However, the highest value of the superconductivity onset temperature in borondoped diamonds was found just about 7 K in thin CVD-grown films (Takano et al., 2004) and at about 4 K in bulk diamonds grown at high-pressure and high-temperature (Ekimov et al., 2004; Sidorov et al., 2005). In these pioneering works bulk polycrystalline diamonds with micron grainsize have been synthesized from graphite and B4C composition (Ekimov et al., 2004) and graphite with 4 wt% amorphous boron (Sidorov et al., 2005). The synthesis have been carried out at 8-9 GPa pressure and 2500-2800 K temperature in both cases. Later Dubrovinskaya et al., 2006, carried out synthesis of graphite with B4C composition at much higher pressure value 20 GPa but the same temperature of 2700K and found the superconducting state transition at lower temperature 2.4 1.4 K in the obtained doped polycrystalline diamonds. Due to the sharpening of the temperature interval of the superconductivity transition in magnetic field they suggested that superconductivity could arise from filaments of zero-resistant material. An alternative method for the creation of composite diamond superconductors was suggested by one of the authors of the present

Transformation of solid C70 into nanotubes

Transformations of C70 fullerite materials under high pressure (up to 15 GPa) and temperature (up to 1720 K) influence are studied. The crystal structures of polymeric hard forms of C70 material have been studied by X‐ray powder diffraction. We have supposed new polymer phases of C70 during this treatment: precursor phase (under low pressure up to 9 GPa, temperature up to 600 K), chains (up to 12 GPa, 570K), phase of mixture of 3D polymerized C70 and nanotube fragments transformed from chains (high pressure up to 15 GPa, high temperature up to 1720 K).

Dissociation energy of 3D-polymeric C60: Calorimetry study and structural analysis

Annealing of 2D- and 3D-polymeric C[sub60] fullerene obtained under pressures of 9.5 and 12.5 GPa and temperatures of 670 and 770 K has been investigated by DSC in the range 240–640 K. An endotherm ...

Pressure effect on electrical properties and photoluminescence spectra of solid C60 and C70 fullerenes

Electrical resistivity of crystalline and disordered fullerite samples obtained by static high-pressure-high-temperature treatment of C60 and C70 at P = 12.5 GPa and T = 820-1500 K was investigated in the temperature range of 2.4-300 K. Room-temperature activation energy of charge carriers was found to be in the range 40-200 meV. T3/2 and T4 dependencies of conductivity versus temperature were revealed both in crystalline and disordered structures. Photoluminescence spectra of C60 samples treated at P = 13 GPa, T = 770-1470 K show 50 nm short-wavelength shift of characteristic 750 nm PL band.

Electric resistivity and magnetoresistance of some superhard and ultrahard fullerites in the range 300-2K

Electric resistivity and magnetoresistance were measured on samples with disordered structure synthesized from pure C60 and C70 at pressure in the range 8–12.5 GPa and temperature 900–1500 K. Different types of behavior were observed: semimetallic and semiconducting, depending on the particular short-range order of the structure.

Microphotoluminescence of C60 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 Ts = 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 ≤ Ts ≤ 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.