V.N. Denisov

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.

Is C60 fullerite harder than diamond

Abstract Raman spectra of C60 fullerite at pressures up to 37 GPa with shear deformation are studied. We have found two states at high pressures, that persist after pressure release and have various transparencies in the near IR region. The nontransparent state is formed at 6–18GPa and has a Raman spectrum with broadened bands at frequencies close to those of the initial fullerite. The transparent state was obtained at pressures higher than 18GPa, and the Raman bands are broadened and overlapping in comparison with those of the nontransparent state. We suppose that C60 molecules persist in both states. The transparent state of fullerite shows a hardness higher than that of diamond.

POLYMERIZATION AND PHASE DIAGRAM OF SOLID C70 AFTER HIGH-PRESSURE-HIGH-TEMPERATURE TREATMENT

Abstract For the first time investigations of structure and physical properties of C 70 after treatment at 4 ≤ P ≤ 12.5 GPa and 300 ≤ T ≤ 1770 K were carried out by X-ray, Raman and infrared spectroscopy. The results are plotted in a ( P , T ) diagram. The new superhard 3D-polymerized tetragonal phase has been found at P ≥ 9.5 GPa. The superhard disordered state of samples produced from C 70 at 12.5 GPa with a hardness close to that of (100)-face diamond has also been found.

Hard diamond-like layers produced during DIII-D tokamak operations

Abstract Measurement of the hardness of carbon films, obtained by the exposure of a graphite sample to the tokamak divertor plasma and the deposition of the eroded material on Si substrates, has shown the unexpected result that their hardness was very high. The hardness was about 10–20 GPa, which is close to the hardness of sapphire. We have studied the structural and optical properties of these films by Raman and infrared spectroscopy. The Raman spectra are consistent with a high hardness. The graphite-like Raman band at 1515 cm −1 is down-shifted, which indicates the presence of both sp 2 and sp 3 bonds in the cross-linked structure of the films which is typical for hard carbon films. From the transmission spectra it is found that the band-gap absorption edge is not lower than that of Si. Infrared spectra show a triplet around 2900 cm −1 , which is assigned to the CH stretching modes.

Mechanical, structural, and spectroscopic properties of C70 fullerite phases produced under high pressure and shear

X-ray diffraction patterns, Raman spectra, and the hardness of C70 fullerite subjected to a high pressure with shear are investigated. It is shown that these conditions favor the phase transformation of molecular fullerite into the hard amorphous phase. The hardness of a specimen removed from a diamond anvil cell loaded up to 26 GPa under shear deformation applied is found to be equal to 30 GPa.