Nadejda R. Serebryanaya

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.

GeTe-phases under shear deformation and high pressure up to 56 GPa

Abstract An “in situ” X-ray diffraction study of the GeTe under shear deformation and nonhydrostatic pressure conditions has been performed. By applying the shear deformation, two phase transitions with structure changes from NaCl to GeS and further to CsCl types were first detected at 19.2 and 38 GPa respectively. The X-ray powder diffraction data and unit-cell parameters of both pressure-induced phases of GeTe are given. The shear-deformation dependence of the GeTe transition pressures is found.

Elastic properties of ultrahard fullerites

The elastic properties of C60 fullerite samples synthesized under pressure P=13.0 GPa at high temperatures were investigated using acoustic microscopy. The velocities of longitudinal (cL=17–26 km/s) and transverse (cT=7.2–9.6 km/s) elastic waves in the samples were measured. It was established that the longitudinal sound velocity of ultrahard fullerites is higher than that of any other known solid. The bulk modulus of these ultrahard samples is higher than that of diamond and reaches a value greater than 1 TPa. The high bulk modulus, the relatively large shear moduli, and the substantial Poisson ratio indicate that the structure of the ultrahard fullerites is fundamentally different from that of diamond.

DSC study of annealing and phase transformations of C60 and C70 polymerized under pressures in the range 9.5–13 GPa

C60 and C70 fullerenes polymerized under pressures between 9.5 and 13 GPa and temperatures between 670 and 1850 K were investigated by differential scanning calorimetry (DSC) in the range 240–640 K ...

Structure and electrical properties of Sb2Te3 and Bi0.4Sb1.6Te3 metastable phases obtained by HPHT treatment

We have obtained the metastable phase of the thermoelectric alloy Bi0.4Sb1.6Te3 with electron type conductivity for the first time using the method of quenching under pressure after treatment at P=4.0 GPa and T=400–850 °C. We have consequently performed comparative studies with the similar phase of Sb2Te3. The polycrystalline X-ray diffraction patterns of these phases are similar to the known monoclinic structure α-As2Te3 (C2/m) with less monoclinic distortion, β ≈ 92°. We have measured the electrical resistivity and the Hall coefficient in the temperature range of T=77−450 K and we have evaluated the Hall mobility and density of charge carriers. The negative Hall coefficient indicates the dominant electron type of carriers at temperatures up to 380 K in the metastable phase of Sb2Te3 and up to 440 K in the metastable state of Bi0.4Sb1.6Te3. Above these temperatures, the p-type conductivity proper to the initial phases dominates.

The Fourier Raman Spectra of HiPCO Single-Wall Carbon Nanotubes under High Pressure

The Raman spectra of single-wall carbon semiconducting nanotubes of the HiPCO type were investigated upon excitation at 1.06 µm at pressures up to 3 GPa. A method of purification of the initial material from iron is proposed and the diffraction patterns of purified nanotubes are considered. The diameters and chiralities of the nanotubes manifesting themselves in the spectra are determined. Pressure dependences of the frequencies are obtained and the Gruneisen parameters and their relationship with interatomic interactions in nanotubes are discussed.

Superhard superconductor composites obtained by sintering of diamond, c-BN and C60 powders with superconductors

Superhard superconducting samples with a critical temperature of TC = 10.5 - 12.6 K were obtained by high-pressure / high-temperature sintering of synthetic diamond powders coated with a niobium film and in 50% - 50% composition with superhard C60 fullerene. Superhard superconductors with TC = 9.3 K were obtained when diamond and molybdenum powders were sintered at a pressure of 7.7 GPa and a temperature of 2173 K. Superconducting samples with TC = 36.1 - 37.5 K have been obtained in the systems diamond-MgB2 and cubic boron nitride-MgB2.

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.