Pressure-induced transformations in two-dimensional polymeric phases of C60 (original) (raw)
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Phase transformations in pressure polymerized C60
Chemical Physics Letters, 2003
Differential Scanning Calorimetry and infra-red spectroscopy have been used to study the depolymerization of the two pressure polymerized phases of C 60 , i.e., rhombohedral (R) and orthorhombic (O) into monomeric C 60 at p ¼ 1 atm. The depolymerization enthalpies obtained were 10.3 AE 0.5 and 20.1 AE 0.6 kJ/mol for R-, O-phases, respectively. The R-phase, prepared at 1025-1050 K and 6 GPa (Ômagnetic carbonÕ) demonstrated the depolymerization behaviour, similar to the typical R-phase.
Pressure-induced polymerization of fullerenes: A comparative study of C60 and C70
Physical Review B, 1996
To investigate the polymerization of fullerenes, solid C 60 and C 70 have been subjected simultaneously to high pressures and temperatures, with pressures up to 7.5 GPa and temperatures up to 800°C. X-raydiffraction measurements indicate that the fcc solid C 60 transforms to an orthorhombic structure consisting of a polymerized linear chain of C 60 molecules. The associated changes in the intramolecular vibrational modes have been probed through Raman and infrared measurements. In contrast to solid C 60 , under similar conditions, polymerization is not observed in the case of solid C 70 , although there is a structural transition from the initial hcp structure to a rhombohedral structure. This lack of polymerization in C 70 is discussed in terms of the structure of solid C 70 under high pressure and temperature conditions, coupled with the topochemical features of the C 70 molecule.
Structural and physical properties of pressure polymerized C60
Carbon, 1998
We discuss the structural and dynamic properties of C,, polymerized under low-P, low-T conditions, and suggest that the disordered mixed orthorhombic-tetragonal-rhombohedral phases produced under these conditions arise from nucleation of molecular chains in random directions because of the quasi-free molecular rotation under standard reaction conditions in the fee phase of C,. Polymerization in He aives results aualitativelv different from those obtained in other media. 0 1998 Elsevier Science Ltd. iI rights reserved.
New Superhard Phases for Three-Dimensional C60-based Fullerites
Physical Review Letters, 2000
We have explored new possible phases of 3D C60-based fullerites using semiempirical potentials and ab-initio density functional methods. We have found three closely related structures -two body centered orthorhombic and one body centered cubic -having 52, 56 and 60 tetracoordinated atoms per molecule. These 3D polymers result in semiconductors with bulk moduli near 300 GPa, and shear moduli around 240 GPa, which make them good candidates for new low density superhard materials.
Spectroscopic study of pressure-polymerized phases of C 60
Physical Review B, 2000
Comparative studies of the infrared ͑IR͒ and Raman spectra of the orthorhombic, tetragonal, and rhombohedral pressure-polymerized phases of C 60 as well as the pressure-dimerized state have been performed at ambient conditions. The tentative assignment of the vibrational spectra of different phases has shown that they could be perfectly described by the molecular symmetry of the C 60 polymers which are the structure-forming elements of crystalline polymerized phases. This allowed one to consider the IR and Raman spectra of the crystalline polymerized phases as characteristic of those of the appropriate one-and two-dimensional C 60 polymers and to use them for molecular fractional analysis of polymerization products.
Structural evolution of low-pressure polymerised C60 with polymerisation conditions
Journal of Physics and Chemistry of Solids, 1999
We show in this paper that characteristic features in the Raman spectra, especially the frequency of the pentagonal pinch mode, can give information about the polymeric structure of pressure polymerised C 60 . High-pressure treatment at 1 GPa below 510 K for 3 h results in the formation of a low fraction of dimers only, while treatment at the same pressure and time above 540 K affords a fully polymerised material. In the latter case, different relative fractions of dimers and polymer chains are obtained depending on whether the final reaction conditions were reached by isobaric or isothermal path. We suggest that this difference results from different reaction dynamics in the two cases. The polymerisation rate depends on T and p and on the rotational and orientational states of the molecules. At 1 GPa no polymerisation is observed in sc C 60 , while in "hexagon" oriented sc C 60 at 1.7 GPa dimers are already formed 175 K below the fcc-sc transition and a fully polymerised material is obtained just below the transition to the fcc phase. ᭧
Comparative Raman Study of the 1D and 2D Polymeric Phases of C60 under Pressure
physica status solidi (b), 1999
The effect of symmetry lowering on the phonon spectra as well as the pressure effects on the vibrational spectrum of polymerized C 60 were studied by Raman spectroscopy. Drastic changes related to the splitting of degenerate modes of the C 60 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 C 60 , 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.
Carbon, 2005
X-ray diffraction and Raman spectroscopy have been used to characterize the structures obtained when C 60 single crystals are treated at 2 GPa-700 K. Two different experimental procedures have been applied: the temperature is raised before the pressure is applied, or the opposite. The ''heating-then-pressing'' path leads to the tetragonal polymer structure (P4 2 /mmc) together with a minor fraction of rhombohedral structure, which confirms previous results. In contrast, the ''pressing-then-heating'' path leads to a different state presenting similarities with both the rhombohedral and the disordered dimer structures. The results are discussed in light of the orientational and dynamical aspects of the C 60 polymerization.