This paper is a brief summary of the results of long-term experimental studies of C60Hx hydrofullerites with a hydrogen content of up to x ~ 60 or more, obtained by loading C60 fullerites with hydrogen at pressures up to 9 GPa and temperatures up to 500°C. Basically, this is an overview of already published data. Some results for hydrofullerites with compositions of x ~ 60 and x ~ 90 are presented for the first time.

A new hydrocarbon - hydrographite - with the composition close to CH is shown to form from graphite and gaseous hydrogen at pressures above 2 GPa and temperatures from 450 to 700 degrees C. Hydrographite is a black solid thermally stable under ambient conditions. If heated in vacuum, it decomposes into graphite and molecular hydrogen at temperatures from 500 to 650 degrees C. Powder X-ray diffraction characterizes hydrographite as a multilayer "graphane II" phase predicted by ab initio calculations [Wen X-D et al. PNAS 2011; 108: 6833] and consisting of graphane sheets in the chair conformation stacked along the hexagonal c axis in the - ABAB - sequence. The crystal structure of the synthesized phase belongs to the P6(3)mc space group. The unit cell parameters are a = 2.53(1) angstrom c = 9.54(1) angstrom and therefore exceed the corresponding parameters of graphite by 2.4(2)% and 42.0(3)%. Stretching vibrations of C-H groups on the surface of the hydrographite particles are examined by infrared spectroscopy. (C) 2015 Published by Elsevier Ltd.

The transmission and Raman spectra of C 60 H 42 samples synthesized at a high hydrogen pressure and stored under different conditions have been measured. It has been found that, upon interaction of the compound with air, a part of C 60 -H chemical bonds are replaced by C 60 -O-H bonds. It has been experimentally shown that the observed changes in the vibrational and electronic properties of C 60 H 42 are caused by the interaction of the compound with atmospheric oxygen and water vapor. The rate of oxidation of the studied samples is significantly less than the value previously published for the oxidation of the C 60 H 36 compound synthesized by the reduction of fullerene C 60 dissolved in different solvents with the use of Zn/HCl. This is explained by the fact that particles of the product of the hydrogenation at a high hydrogen pressure are polycrystals with relatively large sizes, unlike the fullerene hydrides synthesized by the reduction of C 60 with Zn/HCl.

The high-resolution near edge X-ray absorption fine structure spectroscopy and X-ray photoelectron spectroscopy were used to elucidate the nature of chemical bonding between carbon and hydrogen atoms on the surface and inside hydrogenated single-walled carbon nanotubes (H-SWNTs). The measured spectra showed formation of chemical bonding between the hydrogen and carbon atoms in H-SWNTs. In order to obtain a detailed understanding origin of the chemical bond between C and H atoms, density functional calculations and theoretical analysis of experimental NEXAFS spectra were carried out. (C) 2014 Elsevier B.V. All rights reserved.

High-pressure hydrogenation of the single-walled carbon nanotubes, graphite nanofibers and fullerenes C-60 developed. Produced samples have been studied by their combustion, gas thermodesorption, mass-spectroscopy, X-ray, IR and Raman scattering spectroscopes.Synthesized carbon nanotubes, graphite nanofibers with the hydrogen content corresponding to the chemical formula CH0.8 divided by 0.9 and fullerenes C60Hx with x from x = 36 to the unbelievable x = 60 were produced and elucidated. Physisorption takes place only for small percent of hydrogen in this case. Dominant amount of hydrogen forms the strong covalent C-H bonds thermally stable up to 600 degrees C. As a result of hydrogenation high-frequency conductivity of free carriers in nanotubes decreases by one order of magnitude and electron transitions between van Hove singularities in the density of electron states of nanotubes disappear.We have shown that C60H36 is a set of isomers in our case. Combination of the vibrational modes of C60H60 and, for example, C60H48 can explain the emission and absorption spectra of interstellar and circumstellar clouds: spectral positions of not only narrow lines, but broad backgrounds as well.Hydrogen interaction with the carbon nanostructural materials (nanotubes, nanofibers, fullerenes C-60 and C-70 has been intensively studied over the last years. A developed surface of nanotubes and nanofibers induced a considerable applied interest aimed at hydrogen storage and reduced consumption of organic fuel in modern industry. For the academic studies, of interest is the nature of the hydrogen interaction with the carbon nanomaterials.In view of practical application, the carbon nanotubes or nanofibers were saturated with molecular hydrogen under relatively mild conditions: the hydrogen pressure did not exceed 10 divided by 12 MPa at room or liquid nitrogen temperatures. The data of the application research were reviewed, e.g., by Dillon and Heben (2001).

The dependence of the temperature of the transition to the superconducting state T c on the pressure up to 60.8 GPa is measured for the TiV alloy. The dependence T c ( P ) is increasing except for an anomaly in the form of a local minimum near P = 10 GPa. At the maximum pressure of 60.8 GPa, the superconducting transition temperature T c reaches 18.2 K. The obtained curve T c ( P ) is compared with the known dependences for pure vanadium, for which T c ( P ) increases to 17.2 K at P = 120 GPa, and for pure niobium and the ZrNb alloy, for which the dependences T c ( P ) also have anomalies in the form of local maxima at pressures of 5–10 GPa.

The superconducting transition temperature T c of bcc Zr is measured at pressures to 64 GPa. The T c value gradually decreases as pressure is increased. For the Zr-Nb alloys, there are found anomalies in the T c (P) dependences at rather low pressures. The anomalies are discussed within the available theoretical models. We assume on the basis of the T c (P) experimental data for the Zr-Nb alloys that the T c (P) curve for bcc zirconium has a maximum in the metastability region.

The effect of pressure on the superconducting transition temperature T c of vanadium and V 94 Ti 6 , V 85 Ti 15 , V 67 Ti 33 , and V 48 Ti 52 (at %) bcc alloys has been studied. It has been found that the T c ( P ) dependence of pure vanadium is close to linear in the pressure ranges 0–14 and 23–32 GPa, whereas dT c / dP decreases to zero with a pressure increase in the 14–23 GPa range. The T c ( P ) curves for all alloys are nonmonotonic and have two features in the respective pressure ranges of 3–11 and (a peak-shaped feature) 15–25 GPa.

Phase separation in the hexagonal ω modification of the Ti-Zr system was observed. The ω → ω 1 + ω 2 decomposition in an equiatomic TiZr alloy after prolonged thermal treatment at P = 5.5 ± 0.6 GPa and T = 440 ± 30°C was revealed using x-ray diffraction. It is found that the concentration dependence of the specific volume of the ω phase of Ti-Zr alloys deviates from the Vegard law to higher values. An isobaric section of the equilibrium P - T - x phase diagram of the Ti-Zr system is shown to have the shape of an eutectoid diagram at pressures higher than 8 GPa.

Optical transmission spectra of C60Hx with x in the range 36–60 show an increase of the molecular symmetry at x = 60. Hydrogenated fullerenes C60 were synthesized at a hydrogen pressure of 50±5 kbar and temperatures up to 773 K. Combination of the vibration modes of C60H60 and, for example, C60H48 may explain the emission and absorption spectra of some interstellar and circumstellar clouds.

The phase decomposition phenomenon is found in the hexagonal ω-phase of the Ti—Zr system under high pressure. The ω → ω1 + ω2 decomposition of the equiatomic TiZr alloy occurred due to long thermobaric treatment at P = 5.5±0.6 GPa and T = 710±30 K. The chemical compositions of the ω1- and ω2-phases recovered to ambient conditions were estimated from the X-ray data to be around Ti20Zr80 and Ti83Zr17. The experimental data were used to calculate the mixing energy and the top of the decomposition curve in the isobaric T-C diagram of this system. We find that the equilibrium T-C phase diagram of the Ti-Zr system at pressures above ∼8 GPa is of the eutectoid type with the high-temperature β-phase and the low-temperature ω1- and ω2-phases.

The hydrogen-induced changes in the phonon spectra of the C60H36 and C60H60 fullerene hydrides prepared under high hydrogen pressure are studied by Raman spectroscopy. The isotopic substitution by deuterium results in large isotopic shift of the C-H stretching modes.

Single-walled carbon nanotubes containing 5.4 wt% H are prepared under a hydrogen pressure of 50 kbar at the temperature T = 500°C. Analysis of the optical transmission spectra has revealed that the hydrogenation of single-walled carbon nanotubes brings about suppression of high-frequency conduction provided by free charge carriers in the nanotubes, the disappearance of interband electronic transitions, and the appearance of an absorption line at 2845 cm−1 corresponding to stretching vibrations of the C-H bonds. The removal of hydrogen from hydrogenated single-walled carbon nanotubes owing to vacuum annealing at a temperature of 500°C is accompanied by a linear decrease in the intensity of this line as the hydrogen content in the system decreases. This phenomenon indicates that the greater part of the hydrogen atoms in single-walled carbon nanotubes are covalently bonded to the carbon atoms.

Single-walled carbon nanotubes (SWNT) were loaded with 5.2wt% hydrogen at a hydrogen pressure of 3GPa and T=620K, quenched to 80K and studied at ambient pressure and 15K by inelastic neutron scattering (INS) in the range of energy transfers 3–400meV. An analysis of the measured INS spectra showed that the quenched SWNT & H sample contained hydrogen in two different forms, as H atoms covalently bound to the carbon atoms (∼4.7wt%) and as H2 molecules (∼0.5wt%) exhibiting nearly free rotational behavior. Annealing the sample in vacuum at 332K removed about 65% of the H2 molecules and annealing at 623K removed all of them. This demonstrates that H2 molecules were kept in this sample more tightly than in earlier studied SWNT & H samples that were hydrogenated at lower pressures and temperatures and lost all molecular hydrogen on heating in vacuum to room temperature.

The superconducting transition temperature of the Ti metal is measured in dependence on pressure to 56.0 GPa. A linear T-c(P) increase is observed in the stability range of omega-Ti, like in the Zr and Hf omega-phases. This general behavior can be related to low electron density of states at the Fermi level at zero pressure and its pressure-induced increase due to the s-d electron transfer. (c) 2006 Elsevier B.V. All rights reserved.

The superconducting transition temperature of the equiatomic ZrNb alloy is measured in dependence on pressure to 56.4GPa. The Tc(P) dependence is found to achieve a maximum around 42GPa and to have an anomalous behavior with a shallow minimum in the pressure range 5–10GPa. This behavior of Tc(P) is similar to that earlier observed on pure metal Nb. A comparison with recent model calculations suggests that the features of the Tc(P) behavior both in pure Nb and in the ZrNb alloy are of the same nature and result from the high-pressure anomalies in their phonon spectra.

Single-wall carbon nanotubes (SWNT) hydrogenated at P≈5.0GPa and T≈500°C (SWNT-H) were investigated by micro-Raman spectroscopy. The main feature of the Raman spectrum of the as-prepared SWNT-H is a giant structureless hot luminescence background that screens all vibrational modes. This disorder-induced background, attributed to random binding of the hydrogen atoms, vanishes after annealing of the SWNT-H in vacuum or in air. The changes in the Raman spectra of the partially annealed samples reflect the gradual decrease of structural disorder, related to removal of randomly bonded hydrogen. Complete outgassing restores the characteristic Raman spectrum of the pristine SWNT.

Powder samples of YH3 and YD3 have been studied by neutron diffraction (ND) with a much higher statistical accuracy than obtained previously. The profile analysis of the obtained ND patterns confirmed the high-symmetry HoH3-type structure of YH3 and ruled out the 'broken symmetry' structures proposed recently to explain the insulating properties and lattice dynamics of this compound. At the same time, it was demonstrated that the HoH3-type structure is only the structure of the mean lattice of YH3. Large static displacements of H atoms from the symmetrical positions in this structure do occur, and ordering of these displacements on a short-range scale can settle the controversies between the crystal structure and physical properties of YH3.