K. P. Meletov

High-pressure induced metastable phase in tetragonal 2D polymeric C60.

The structural stability of the tetragonal two-dimensional (2D) polymeric phase of C60 has been studied under pressure up to 24 GPa and room temperature by means of in situ Raman scattering. An irr ...

Isotopic and isomeric effects in high-pressure hydrogenated fullerenes studied by Raman spectroscopy

Raman spectra of hydro- and deuterofullerenes, C60H36 and C60D36, prepared under hydrogen or deuterium pressure of 3.0 GPa at diAerent conditions of temperature and hydrogenation time, have been measured at room temperature and normal pressure. Spatially resolved micro-Raman study shows that the homogeneity and optical properties of the samples depend primarily on the synthesis temperature. The Raman spectrum of hydrofullerene contains a large number of prominent peaks originating from the various isomers. The comparison of the experimental data with results of molecular dynamics calculations shows that various isomers are present in the samples, with the most abundant of them being the ones with symmetries S6, T and D3d. The Raman spectrum of the deuterofullerene is similar in structure to that of the hydrofullerene, however, significant diAerences occur in the frequency positions of the C‐H and C‐D stretching and bending modes which are expected from the isotopic eAect. In particular, the isotopic shift ratio for the stretching mode, XH=XD 1:32, is smaller than the expected one from the mass ratio for pure C‐H and C‐D stretching modes. ” 2001 Elsevier Science B.V. All rights reserved.

On the nature of the laser irradiation induced reversible softening of phonon modes in C60 single crystals

Abstract The average temperature rise in the laser excitation spot of the C60 single crystal has been determined using the Stokes to anti-Stokes integrated peak intensity ratio for the Hg(1) phonon mode. The reversible softening of the Ag(2) pentagon pinch mode was found to be due to the heating of the sample caused by the laser irradiation, in agreement with experimental results obtained for uniformly heated samples. These findings are in quantitative agreement with results obtained by numerical calculations of the local temperature rise, which indicates the highly non-uniform temperature distribution in the laser excitation spot associated with the small thermal conductivity of solid C60.

High pressure optical properties of fullerene and fullerene derivatives

Abstract High-pressure Raman spectroscopy is a useful tool to investigate various pressure-induced effects on fullerene and its chemical derivatives. In the case of pristine C60 the pressure-induced freezing of molecular rotations causes the softening, while the lowering of symmetry causes the splitting of several phonon modes. In the neutral state donor–acceptor molecular complex C60∗TMTSF∗2(CS2) high pressure leads to an irreversible charge-transfer phase transition, associated with the transfer of one electron from TMTSF donor to C60 acceptor molecule. In the case of azafullerene (C59N)2, the pressure dependencies of the phonon frequencies exhibit reversible changes in their slopes, associated with the attainment of the ideal HCP structure (originally c/a=1.623), as well as with the starting of the intradimer bridge shortening.

Structural stability of the rhombohedral 2D polymeric phase of C60 studied by in situ Raman scattering at pressures up to 30 GPa

Abstract The structural stability of the rhombohedral two-dimensional (2D) polymeric phase of C 60 has been studied as a function of pressure up to ∼30 GPa at room temperature by means of in situ Raman scattering. An irreversible transformation to a new disordered phase was observed at a pressure of ∼15 GPa. The intensity of the A g (2) pentagonal pinch (PP) mode rapidly decreases in the pre-transitional pressure range while the Raman spectrum of the transformed material becomes very diffuse. The high-pressure phase recovered to normal conditions is metastable and transforms under heating to a mixture of pristine and dimerized C 60 as can be seen by their Raman spectra. The retention of the fullerene molecular cage at high pressure and quenching of the PP-mode are the indications that the high-pressure phase may be associated with a random creation of new polymeric bonds between the molecules in adjacent polymeric planes of the 2D-rhombohedral phase of C 60 .

Raman spectra of MgB2 at high pressure and topological electronic transition

Raman spectra of MgB2 ceramic samples were measured as a function of pressure up to 32 GPa at room temperature. The spectrum at normal conditions contains a very broad peak at ∼590 cm−1 related to the E2g phonon mode. The frequency of this mode exhibits a strong linear dependence in the pressure region from 5 to 18 GPa, whereas, beyond this region, the slope of the pressure-induced frequency shift is reduced by about a factor of two. The pressure dependence of the phonon mode up to ∼5 GPa exhibits a change in the slope, as well as a “hysteresis” effect in the frequency vs. pressure behavior. These singularities in the E2g mode behavior under pressure support the suggestion that MgB2 may undergo a pressure-induced topological electronic transition.

Pressure-induced charge transfer phase transition in crystalline C60*C10H12Se4*2(CS2) molecular complex studied by Raman spectroscopy

Abstract The Raman spectra of the molecular neutral state complex C 60 *C 10 H 12 Se 4 *2(CS 2 ) [C 10 H 12 Se 4 , tetramethyl-tetra-selenafulvalene (TMTSF)] have been measured as a function of pressure up to 9.5 GPa at room temperature. The pressure dependence of the frequencies of almost all intramolecular phonon modes exhibits irreversible changes at 5.0±0.5 GPa. These changes include splitting and softening of various modes, the most characteristic being the irreversible softening by 9 cm −1 of the A g (2) pentagon-pinch (PP) mode, observed upon total release of pressure. This softening is the same as in the case of the potassium-doped fullerene KC 60 . These observations are attributed to a pressure-induced irreversible phase transition, associated with the transfer of one electron from TMTSF donor to C 60 acceptor molecule.

Pressure- and temperature-induced transformations in crystalline polymers of C60

The great advantage of the C60 molecule is its potential for polymerization, due to which the molecule can be the building block of new all carbon materials. In addition, it contains, both sp2 and sp3 hybridized carbon atoms, which allows synthesizing new carbon materials with desired physicochemical properties using both types of carbon bonding. The one- and two-dimensional polymeric phases of C60 are prototype materials of this sort. Their properties, especially polymerization under pressure and room temperature via covalent bonding between molecules belonging to adjacent polymeric chains or polymeric layers, can be used for further development of new materials. The present review focuses on the study of the pressure-induced polymerization and thermodynamic stability of these materials and their recovered new phases by in-situ high-pressure Raman and X-ray diffraction studies. The phonon spectra show that the fullerene molecular cage in the high-pressure phases is preserved, while these polymers decompose under heat treatment into the initial fullerene C60 monomer.

Comparative Raman Study of the 1D and 2D Polymeric Phases of C60 under Pressure

The effect of symmetry lowering on the phonon spectra as well as the pressure effects on the vibrational spectrum of polymerized C60 were studied by Raman spectroscopy. Drastic changes related to the splitting of degenerate modes of the C60 molecule were observed together with selected softening of some of them. In spite of many similarities in the Raman spectra of the one- (1D) and two-dimensional (2D) polymeric forms of C60, some salient differences in the peak intensities and the appearance of complementary modes are evident. In the Raman spectrum of the 2D polymer under high pressure, new modes, which may be related to the deformations of molecular cages, appear. The observed pressure effects are reversible and the material remains stable for pressures up to 8.8 GPa.

Pressure-induced transformations and optical properties of the two-dimensional tetragonal polymer of C60 at pressures up to 30 GPa

The Raman spectra of the two-dimensional tetragonal (2D(T)) polymeric phase of C60 have been studied in situ at pressures up to 30 GPa and room temperature. The pressure dependence of the phonon modes shows an irreversible transformation of the material near 20 GPa into a new phase, most probably associated with the covalent bonding between the 2D polymeric sheets. The Raman spectrum of the high-pressure phase is intense and well resolved, and the majority of modes are related to the fullerene molecular cage. The sample recovered at ambient conditions is in a metastable phase and transforms violently under laser irradiation: the transformed material contains mainly dimers and monomers of C60 and small inclusions of the diamond-like carbon phase. The photoluminescence spectra of the 2D(T) polymer of C60 were measured at room temperature and pressure up to 4 GPa. The intensity distribution and the pressure-induced shift of the photoluminescence spectrum drastically differ from those of the C60 monomer. The deformation potential and the Grüneisen parameters of the 2D(T) polymeric phase of C60 have been determined and compared with those of the pristine material.