The absorption and desorption of hydrogen at high pressure and temperature by opal matrices formed by amorphous silica spheres with a diameter of 0.235 and 1.6 mm have been studied. The Raman spectra of hydrogen-saturated opal matrices measured at a temperature of 80 K and ambient pressure show that hydrogen molecules are adsorbed in two different ways, directly into silica spheres and mesopores between them. The kinetics of hydrogen desorption was studied in-situ from a change in the relative intensity of rotational modes in the Raman spectra under annealing at 163-213 K. The hydrogen content decreases exponentially under isothermal heating, while the exponential decay time constant T increases with a decreasing temperature showing the activation nature of desorption. The data for various temperatures are well described by the Arrhenius dependence t(T) = A x exp(EA/kBT) with the activation energy EA=(162 +/- 13) meV and time (c) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

This paper is aimed at studying the effect of Li cationson thethermal stability of concentrated solid solutions of molecular hydrogenin the Li2O & BULL;6SiO(2)-0.39H(2) andLi(2)O & BULL;6SiO(2)-0.25H(2) compoundssynthesized at pressures of 7.5 and 6.6 GPa and a temperature of 280 & DEG;C. The decomposition of the solutions was examined by Ramanscattering under isothermal annealing at 21-70 & DEG;C andby hot hydrogen desorption into a pre-evacuated volume at 0-97 & DEG;C. The activation energies E (a) =(0.419 & PLUSMN; 0.019) and (0.411 & PLUSMN; 0.06) eV/H-2 of hydrogendesorption from the near-surface layer of the samples with 0.39H(2) and 0.25H(2), respectively, determined by Ramanspectroscopy were higher than E (a) = (0.16 & PLUSMN; 0.02) eV/H-2 previously determined for hydrogenatedpure silica glass. This suggests the occurrence of a "Kubas"or similar interaction between the lithium cations and hydrogen molecules.The interaction leads to a large decay time constant of & tau; =3220 s at room temperature compared to & tau; = 3 s for the hydrogensolutions in pure silica glass. The hot desorption showed that thediffusion activation energy was higher than 0.51 eV and also had agreat impact on the stability of the hydrogen solutions in bulk lithiumsilicate glass, which additionally increased the decay time constants several times.

The stability of saturated solid solutions of molecular hydrogen in silica glass was examined in the temperature range 85 divided by 175 K by Raman spectroscopy. The kinetics of hydrogen desorption was studied in-situ by tracking time-dependent changes in the intensities of hydrogen rotational modes at an elevated temperature. The heating results in an exponential decrease in the hydrogen content with a temperature dependent time constant. The data at different temperatures are well described by the Arrhenius formula showing the activation character of desorption with the activation energy E-A = (0.16 +/- 0.01) eV and time constant A = (0.027 +/- 0.003) sec.

The Raman spectra of saturated solid solutions of hydrogen and deuterium in silica glass, SiO2-0.6H2 and SiO2-0.63D2, have been measured for the first time at liquid nitrogen temperature. The comparative analysis of H2 phonon modes in solid solution of hydrogen in silica glass and those of hydrogen gas shows a decrease in the frequency of rotational modes contrary to an increase in the H–H stretching vibration mode frequency. Hydrogen rotational modes, overlapped with the phonon modes of silica glass, strengthen at an elevated temperature, while the phonon modes of silica glass soften. The isotopic substitution of hydrogen by deuterium leads to a decrease in the stretching vibration mode frequency proportionally to the square root of D2/H2 mass ratio, whereas rotational mode frequencies decrease as a ratio of D2 to H2 inertia moments. The intensity of H2 phonon modes gradually decreases upon the heating of hydrogen-saturated silica glass along with the evolving of dissolved hydrogen.

The pressure-assisted photopolymerization in the fullerene complex with ferrocene C 60 · {Fe(C 5 H 5 ) 2 } 2 was studied for the first time by simultaneous in-situ Raman and X-ray diffraction (XRD) techniques at pressures up to 8 GPa. The irreversible splitting of the A g (2) mode in the Raman spectra, resembling pristine fullerene polymerization, was observed at 4.6 GPa. The structural analysis does not show the formation of covalent bonds between adjacent fullerene molecules at a pressure up to 5 GPa. The unit cell volume and interfullerene distances are smooth and monotonous functions of pressure, and the shortest interfullerene distance 9.4 Å at 5 GPa significantly exceeds 9.06 Å typical of fullerene polymers. The changes in the Raman spectra at 4.6 GPa grow with the laser power and exposure time, showing typical photopolymerization phenomena. The distance between ferrocene and fullerene molecules decreases linearly with pressure changing the slope at 2 GPa due to intermolecular interaction crossover. The length of a ferrocene molecule in the complex increases with pressure in contrast to its decrease in a pristine ferrocene crystal. This behavior is associated with a change in the charge state of an iron atom and indicates a charge transfer from ferrocene to fullerene molecules.

The Raman scattering spectra and crystalline structure of vanadyl IV phthalocyanine (VOPc) at normal and high pressures has been studied. According to the X-ray diffraction data, the initial microcrystalline powder represented a mixture of the triclinic α phase (79%) and the monoclinic β phase (21%) possessing P$$\bar {1}$$ and P21/c symmetry, respectively. Raman spectra of the two phases are similar, but the phonon modes of the β phase are shifted toward higher frequencies (energies). The pressure dependence of the spectra of the α phase has been determined and it is established that the interval of 2.3–3.4 GPa reveals reversible pressure-dependent variations: above 3 GPa, some phonon modes exhibit splitting and the coefficients of pressure-induced (baric) shift for almost all modes show a decrease. A high-pressure feature observed in the Raman spectra can be related to changes in intermolecular interactions in crystalline structure of the α phase. The pressure dependence of the α phase unit cell volume measured at pressures increasing up to 4 GPa is a smooth monotonic function that can be well described by the Murnaghan equation of state. The obtained data were used to calculate the Grüneisen parameters of VOPc phonon modes.

The crystal structure of the molecular complex C-60 center dot{Fe(C5H5)(2)}(2) was studied by single crystal X-ray diffraction (XRD) analysis at pressures up to 5 GPa using the diamond anvil cell (DAC) technique. The XRD data and subsequent structural analysis clearly show that there is no pressure-induced polymerization in fullerene layers in pressure range studied. The reciprocal unit cell volume V-0/V is a smooth and monotonous function of pressure and fits well to the Murnaghan equation of state (V-0/V) (B')= {1 + P center dot(B'/B-0)}, where V-0 is the volume at ambient pressure, B-0=8.7 GPa and B'=10.5 are the bulk modulus and its derivative, respectively. Pressure dependence of the shortest distance between the C5H5 ring of the ferrocene molecule and the center of the nearest fullerene molecule is linear, while the slope changes at 2.2 GPa indicating on the intermolecular interactions crossover. Other relevant parameter, the Fe-C bond lengths of ferrocene, sensitive to iron charge state, gradually increases. This peculiarity can a sign of pressure-induced partial charge transfer between the donor ferrocene and acceptor fullerene molecules.

A cold-loaded cryostat was designed to measure the optical spectra of thermally unstable samples. The temperature stabilization system with a resistive heater provides temperature control in the range of 80–270 K with an accuracy of ±0.3 K. The cryostat was used to measure the Raman spectra of molecular hydrides of silica glass at different temperatures.

The temperature dependence of the photopolymerization rate in pristine C-60 and fullerene molecular complexes {Pt(dbdtc)(2)}center dot C-60 and {Pt(nPr(2)dtc)(2)}center dot(C-60)(2) was studied. The Raman spectra measured in the temperature region 190-323 K exhibit an intensity increase of the dimer-related A(g)(2) mode of the C-60 molecule and a decrease of the monomer's one under long laser irradiation. The photopolymer content grows exponentially with the laser exposure time while the growth time constant decreases with the increase of temperature. Since the polymerization via the 2 + 2 cycloaddition reaction necessitates the parallel orientation of double C = C bonds of neighboring C-60 molecules, the acceleration of fullerene rotations at elevated temperatures affects the polymerization rate. The photopolymerization rate under fixed conditions of laser power, excitation wavelength and beam focusing exhibits an activation-type dependence on temperature. The activation energy E-A, obtained from the Arrhenius dependence of the polymer growth time constant on the temperature, increases from (0.13 +/- 0.01) eV and (0.18 +/- 0.01) eV for {Pt(nPr(2)dtc)(2)}center dot(C-60)(2) and {Pt(dbdtc)(2)}center dot C-60 molecular complexes to (0.24 +/- 0.04) eV for pristine C-60.

Based on X-ray diffraction data, a comparative analysis of the molecular geometry of eight phthalocyanine anions [TiIVOPc]n–, [VIVOPc]n– (n = 1, 2) and neutral phthalocyanines [TiIVOPc] and [VIVOPc] is carried out to study Jahn–Teller distortions of their molecular structure. It is experimentally shown that the occupation of a doubly degenerate lowest unoccupied molecular orbital of the [MIVOPc] molecule level by additional electrons leads to core tetragonal symmetry lowering from $${{C}_{{4{v}}}}$$ to $${{C}_{{2{v}}}}$$ with a significant change in the bond lengths distribution in the molecule. The effect manifests itself to a greater extent in imine bonds of the C–Nim–C fragment, in which a systemic alternation of bonds into short and long is observed. The degree of distortion is proportional to the charge of the metal phthalocyanine macrocycle of the organic part of the molecule, C32H16N8. The Jahn–Teller distortions of the macrocycle noticeably affect the position of frequencies characteristic of [MIVOPc] in the Raman spectra of single crystals of anionic phthalocyanine complexes in the range 1100–1700 cm–1. The experimental patterns agree fairly well with the results of DFT calculations.

We measured Raman spectra in crystals of molecular donor–acceptor fullerene complexes {Me(nPr2dtc)2} · (C60)2 (Me = Ni, Cu, Pt). In the spectra of the {Pt(nPr2dtc)2} · (C60)2 complex under prolonged irradiation with a laser with λ = 532 nm, characteristic changes in the photopolymerization of fullerene are observed, associated with the splitting of degenerate phonon Hg modes and softening of Ag modes of the C60 molecule. The kinetics of photopolymerization under conditions of weak irradiation at room temperature is studied. It was found that thermal destruction of the photopolymer with increasing temperature leads to a decrease in its concentration in the final photopolymerization product. The kinetics of thermal destruction is described by the Arrhenius equation, with the activation energy EA of (0.68 ± 0.03) eV; the dimers are destructed to a concentration of 1% within 15 min at ~114°C.

The decomposition kinetics of the fullerene dimers and photo-oligomers was studied at elevated temperature by Raman scattering. The polymeric content decreases exponentially with the thermal treatment time while the decay time constant decreases at higher temperatures. The activation-type behavior is well described by the Arrhenius law that gives the activation energy E-A = (1.71 +/- 0.06) eV/molecule for the dimers and E-A = (0.87 +/- 0.06) eV/molecule for the C-60 photopolymer.

The crystal structure of the molecular donor-acceptor complex [{Cd(Et(2)dtc)}(2)center dot DABCO]center dot C-60 center dot(DABCO)(2) (where dtc is dithiocarbamate, DABCO is diazabicyclooctane) was studied by X-ray diffraction (XRD) at high pressure using the diamond anvil cell (DAC) technique. The pressure dependence of lattice parameters is smooth and monotonous, the bulk modulus and its derivative B-0 = 7.94 GPa and B' = 9.65 are close to those of pristine C-60. Raman spectra of the complex measured at lambda(exc) = 532 nm showed a peculiarity in the pressure dependence of A(g) (1), A(g) (2), H-g (1) and H-g (7) modes of the C-60 molecule near 2 GPa. This peculiarity relates to pressure-assisted photopolymerization in the fullerene layers which is suppressed in Raman measurements at lambda(exc) = 7 8 5 nm showing smooth pressure behavior of phonon modes.

Samples of the clathrate Na x Si 136 were saturated with hydrogen to 100 atm at 25°C in a Sievertstype apparatus and at pressures of 6 and 28 kbar in lentil-type high-pressure apparatuses at 100 and 250°C. X-ray powder diffraction analysis and Raman spectroscopy of the samples quenched after the saturation with hydrogen showed that the phase composition of the clathrates did not change. Heating of the quenched samples to room temperature in a thermal desorption setup produced not hydrogen, but hydrogen-containing gases, as we assumed, silanes. Heating to 650°C leads to decomposition of these compounds to form hydrogen.

The photopolymer formation in the fullerene layers of the C60 complex with platinum dibenzyldithiocarbamate is reported for the first time. The photo-oligomer peaks appear in the Raman spectra near the Ag(2) mode of the C60 molecule upon sample illumination with various laser wavelengths. The photo-oligomers are unstable upon heating and revert back to the C60 monomeric state. The activation energy of the thermal decomposition, obtained from the Arrhenius dependence of the decay time constant on temperature, is (1.12 ± 0.11) eV and the photo-oligomers decompose at ∼130 °C, being more fragile than the crystalline polymers of C60.