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FULLERENES, NANOTUBES, AND CARBON NANOSTRUCTURES Vol. 12, Nos. 1 & 2, pp. 301 ­ 304, 2004

Raman Modes and Stability at High Pressure of the Two-Dimensional Rhombohedral Polymer of C60
K. P. Meletov*
Institute of Solid State Physics, RAS, Chernogolovka, Moscow Region, Russia

ABSTRACT
Phonon modes and stability of the planar rhombohedral polymer of C60 have been studied at pressure up to 30 GPa by means of in situ Raman scattering. At P 15 GPa the phonon frequencies show an irreversible transition to a new phase related with random covalent bonding between the molecules in adjacent polymeric sheets. Key Words: C60. Phonon modes; Raman scattering; Rhombohedral polymer;

The treatment of C60 under high pressure and high temperature leads to its polymerization related to the intermolecular covalent bonding via sp3-like

*Correspondence: K. P. Meletov, Institute of Solid State Physics, RAS, 142432 Chernogolovka, Moscow Region, Russia; E-mail: mele@issp.ac.ru. 301
DOI: 10.1081/FST-120027219 Copyright # 2004 by Marcel Dekker, Inc. 1536-383X (Print); 1536-4046 (Online) www.dekker.com


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coordinated carbon atoms. The crystal structures of polymers have been identified as one-dimensional orthorhombic (1D-O), two-dimensional tetragonal (2D-T) and rhombohedral (2D-R), and three-dimensional facecentered cubic.[1 ­ 3] The number of sp3-like coordinated atoms per C60 cage increases from 4 to 8 and to 12 for 1D-O, 2D-T, and 2D-R phases, respectively. Theoretical studies[4,5] have predicted that uniaxial compression of the 1D or 2D polymers leads to 3D polymerization with 24, 52, 56, and even 60 sp3-like coordinated atoms per C60 cage. Raman studies of the 2D-T polymer at high pressure have revealed irreversible transition near 20 GPa,[6] related to its polymerization in accordance with.[4] In this communication we report the pressure-induced transformations of the 2D-R polymer studied by means of in situ Raman scattering at pressure up to 30 GPa. The 2D-R polymer was obtained by subjecting 99.99% pure C60 powder to a pressure of P 5 GPa at a temperature of 773 K. The X-ray analysis ° Ї confirmed the rhombohedral crystal structure (space group: R3m, a ј 9.22 A ° ).[1] Raman measurements at high pressure were carried out and c ј 24.6 A using a diamond anvil cell of Mao-Bell type and a triple monochromator.

Figure 1. pressures.

Raman spectra of the 2D-R polymer at room temperature and various


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2D Rhombohedral Polymer of C

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The Raman spectra of the 2D-R polymer recorded under pressure increase from 0.9 to 30 GPa and then back to 0.5 GPa are shown in Fig. 1. As the pressure increases, the Raman peaks shift to higher energies and their bandwidth gradually increases. Irreversible changes take place at 15 GPa when the Raman spectrum becomes very diffuse. The transition is preceded by rapid decrease in the intensity of the Ag(2) "pentagonal pinch" (PP) mode. Figure 2 shows the pressure behavior of the Raman modes in the energy region 1370 ­ 1710 cm21. To reveal the contribution of imperfections in the broadening of bands we have annealed the recovered samples at T 600 K. The Raman spectra of the annealed sample show transformation of material to a mixture, containing monomers and dimers of C60 (insert in Fig. 2). The pre-transitional decrease of the PP-mode intensity is associated with destruction of double C5 C bonds and creation of covalent intermolecular 5 links. Even though pressure decreases preferentially the interplane distance of

Figure 2. The pressure dependence of the Raman modes of the 2D-R polymer. Circles (squares) represent the initial phase (high-pressure phase), open (solid) symbols show increasing (decreasing) pressure runs. Shaded area denotes the pressure range of the transformation. Insert: Raman spectra of the 2D-R polymer and of the recovered sample. (a) The initial 2D-R polymeric phase. (b) The "high-pressure" phase. (c) The "high-pressure" phase after being annealed.


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the 2D-R polymer, C60 molecules belonging to adjacent polymeric sheets do not have optimal relative orientations for the covalent bonding. So, we may assume that the new bonds are formed in a random way due to some distortion in the molecular orientations. As a result, the new high-pressure phase exhibits a degree of disorder related to a random out-of-plane polymerization. Finally, the quenching of PP-mode, the retention of molecular cage and the irreversibility of transition are the indications that a new high-pressure phase of the 2D-R polymer may be related with the 3D cross-linked polymerized C60. ACKNOWLEDGMENTS The support by the Russian Foundation for Fundamental Research, grant #03-02-16011, and the support by the Russian State Research Program "Physical properties of the carbon-based nanostructures and development of a new electronic devices" is gratefully acknowledged. REFERENCES 1. Iwasa, Y.; Arima, T.; Fleming, R.M.; Siegrist, T.; Zhou, O.; Haddon, R.C.; Rothberg, L.J.; Lyon, K.B.; Carter, H.L., Jr.; Hebard, A.F.; Tycko, R.; Dabbagh, G.; Krajewski, J.J.; Thomas, G.A.; Yagi, T. New phases of C60 synthesized at high pressure. Science 1994, 264, 1570 ­ 1572. 2. Nunez-Rugueiro, M.; Marques, L.; Hodeau, J.-L.; Bethoux, O.; Perroux, M. Polymerized fullerite structures. Phys. Rev. Letters 1995, 74, 278 ­ 281. 3. Brazhkin, V.V.; Lyapin, A.G.; Popova, S.V. Mechanism of threedimensional polymerization of fullerite C60 at high pressure. JETP Letters 1996, 64, 802 ­ 807. 4. Okada, S.; Saito, S.; Oshiyama, A. New metallic crystalline carbon: three dimensionally polymerized C60 fullerite. Phys. Rev. Letters 1999, 83, 1986 ­ 1989. 5. Burgos, E.; Halac, E.; Weht, R.; Bonadeo, H.; Artacho, E.; Ordejon, P. New superhard phases for three-dimensional C60-based fullerites. Phys. Rev. Letters 2000, 85, 2328 ­ 2331. 6. Meletov, K.P.; Assimopolous, S.; Tsilika, I.; Kourouklis, G.A.; Ё Arvanitidis, J.; Ves, S.; Sundqvist, B.; Waberg, T. High-pressure induced metastable phase in tetragonal 2D polymeric phase of C60. Chem. Phys. Letters 2001, 341, 435 ­ 441. Submitted July 4, 2003 Accepted August 10, 2003