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High Pressure Research, 20M). Vol. 18, pp. 145-151 Reprints available directly from the publisher Photocopying permitted by license only

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2000 OPA (Overseas Publishers Assonation) N.V. Published by license under the Gordon and Breach Science Publishers imprint. Printed in Malaysia.

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HIGH PRESSURE STUDY OF THE 2D POLYMERIC PHASE OF C60 BY MEANS OF RAMAN SPECTROSCOPY
J. ARVANITIDIS", K. P. MELETOV b,c, G. A. KOUROUKLIS ', K. PAPAGELIS a, A. SOLDATOV K. PRASSIDESd and S. VESax *
aPhysics Department, Aristotle University of Thessaloniki, GR-540 06 Thessaloniki, Greece; bZnstitute of the Solid State Physics RAS, Chernogolovka, Moscow region, 142432, Russia; 'Physics Division, School of Technology, Aristotle University of Thessaloniki, GR-540 06 Thessaloniki, Greece; dSchool of Chemistry, Physics and Environmental Science, University of Sussex, Brighton BNI 9QJ. UK; eDepartment of Experimental Physics, Umea University, S-901 87, Umea, Sweden
(Received injnal form 9 September 1999)

The effect of high hydrostatic pressure, up to 12GPa, on the intramolecular phonon frequencies and the material stability of the two-dimensional tetragonal Cm polymer has been studied by means of Raman spectroscopy in the spectral range of the radial intramolecular modes (200-800cn-'). A number of new Raman modes appear in the spectrum for pressures 1.4 and 5.0GPa. The pressure coefficients for the majority of the phonon modes exhibit changes to lower values at P= 4.0 GPa, which may be related to a structural modification of the 2D polymer to a more isotropic phase. The peculiarities observed in the Raman spectra are reversible and the material is stable in the pressure region investigated.
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Keywords: Fullerenes; 2D tetragonal polymer; Raman spectroscopy; high pressure

1. INTRODUCTION The polymerized C60materials have attracted considerable attention during the last few years due to the variety of their structures and their
*Corresponding author. Tel.: ves@ccf.auth.gr

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interesting optical and mechanical properties [l]. C60 has been found to polymerize under light illumination [2] and upon alkali metal doping [3,4]. In addition, high-pressure treatment of C60 at high temperatures leads to polymerization of the pristine material [5]. The polymerized forms of C60comprise of molecular units linked by C-C covalent bonds [6]. The polymerization process involves either [2 21 cycloadditions between double bonds of neighboring c60 molecules or the formation of single C-C bond links. The Raman studies of fullerenes under external perturbations, like temperature and pressure, or doping have yielded a wealth of information on phase transitions and irreversible chemical transformations induced by them [7- 91. In this work we present a high pressure Raman study of the two-dimensional (2D) tetragonal c60 polymer in the frequency range 200-800cm-I. The pressure response of the phonon spectrum allows us to investigate the stability of the material and the role of the anisotropic intermolecular interactions upon pressure application.

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2. EXPERIMENTS

The samples were prepared from sublimed 99.99% pure C60 powder pressurized in a piston and cylinder device. The pristine material was subjected to pressures in the range 2.3-2.5 GPa at a temperature of about 820K to obtain 2D polymerized c60. According to X-ray analysis of the samples after the high-pressure treatment the crystal structure of the polymer is tetragonal [lo]. Raman spectra were recorded using a triple monochromator equipped with a CCD liquid-nitrogen cooled de(DILOR XY-500) tector system. The spectral width of the system was w 5 cm-'. The 514.5nm line of an Art laser was used for excitation. The laser power kept lower than 4mW, measured directly before the cell, in order to avoid de-polymerization caused by laser heating effects and related changes in the phonon spectrum and the crystal structure [6,11]. Measurements of the Raman spectra at high pressures were carried out using the diamond anvil cell (DAC) of Mao-Bell type [12]. The 4: 1 methanol-ethanol mixture as well as pure glycerol were used as pressure transmitting media and the well-known ruby fluorescence technique was used for pressure calibration [13].


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3. RESULTS AND DISCUSSION
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The spectrum of the polymer at normal conditions is more complicated than that of the pristine c60 because of the splitting of the Raman active fivefold degenerate Hg modes. In addition, the lowering of the molecular symmetry may result in the appearance of new peaks in the spectra of the polymer which are not Raman active in pristine c60. The phonon frequencies of the 2D polymer, at normal conditions, obtained by using the same experimental setup, were reported earlier [14]. In this work, we follow the assignment given in Ref. [14], in which for simplicity the irreducible representations H, and A, of the c60 molecular vibrations are used for characterization of the appropriate modes of the polymer. It must be emphasized that the Raman peaks of the 2D polymer under study are very narrow, a feature which is quite different from other polymeric fullerenes. This fact is related, in our opinion, to the higher homogeneity of the 2D polymerization along with the high quality of the samples used. It is also important to note that the Raman peaks remain narrow for pressures up to 12 GPa showing the high hardness of the 2D polymer, in comparison with the pristine c60 [7]. The pressure dependence of the observed phonon mode frequencies is shown in Figures 1 and 2. The open (solid) symbols denote data taken for increasing (decreasing) pressure cycles. The peak positions were determined by fitting of Voigtian line shapes to the experimental data. A number of new modes appear in the spectrum under high pressure. Three of them at frequencies 363,418 and 455cm-' appear for pressures higher than 1.4 GPa and their intensities continuously increase with pressure. It is important to note that even at ambient pressure the first peak, and possibly the others, have been observed by Davydov et al. [15]. The mode at 363cm-' may be related to that at 344cm-' observed in the 1D polymer, which has been proposed to be the Raman signature of the linear chains constructed from covalently bonded c60 molecules [16]. The appearance of the new modes in the 2D polymer and the enhancement of their intensities under pressure may be associated with deformations in the c60 molecular cages 1141. Another reason for appearance of new modes is the pressure-induced enhancement of the initially small mode splitting, resulting from symmetry lowering. For pressures higher than 5 GPa two additional modes appear at frequencies 755 and 768cm-'. It is difficult to say if


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2D polymer

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300

250

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Pressure (GPa)
FIGURE 1 The pressure dependence of the intramolecular Raman modes of the 2D polymer in the frequency region 250-530cn-'. The open (solid) symbols denote data taken for increasing (decreasing) pressure runs. The dashed Iine denotes the pressure where new peaks appear while the shaded area at P=4.0f0.5GPa denotes the change in the slope of the pressure dependence.

this further splitting is due to molecular cage deformation or to an enhancement in the splitting of the Hg(4) mode. All the above changes in the Raman spectrum of the 2D polymer under high pressure are


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750

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550 0

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Pressure (GPa)
FIGURE 2 The pressure dependence of the intramolecular Raman modes of the 2D polymer in the frequency region 530-8OOcm-'. The open (solid) symbols denote data taken for increasing (decreasing)pressure runs. Shaded area at P = 4.0 f 0.5 GPa denotes the change in the slope of the pressure dependence.

reversible upon pressure release. This means that the intermolecular bonds are stable, at least for pressures up to 12GPa, and the appearance of new modes cannot be related to polymeric bond breaking.


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The modes A,(l) and Hg(l)-Hg(4), according to Refs. [14] and [16], are related to the intramolecular modes of pristine c60 whereas other modes are associated with molecular symmetry lowering and splitting of degenerate modes. We have been able to follow the pressure evolution of all the individual components because they exhibit very narrow line-shapes. With increasing pressure, most of them exhibit positive pressure slopes except those in the frequency region 700-800cm-', which soften as in the case of the pristine c60 [7]. The pressure response of the two components belonging to the H,(1) mode is very impressive. These components show continuous intensity enhancement and their crossing at PNN GPa, is an indi7.0 cation that they belong to different symmetry species in the polymer. In addition, the modes at 666, 683 and 703crn-', which possibly show result from the splitting of the H,(3) mode of the pristine 0, different sign in their pressure response compared to their parent mode. It is known from X-ray diffraction measurements that in the 2D polymer the molecular dimensions decrease by 5% in the direction perpendicular to the polymerization planes [ 171. The above observations concerning the different pressure behavior of the split components show that this 5% deviation from the initially quasispherical shape of the pristine C60 molecule can affect considerably the intramolecular modes. As can be seen from Figures 1 and 2, the majority of the Raman modes show, at 4.0GPa, reversible changes in the pressure coefficients to lower values. The two-dimensional polymerization of CS0 results in different nature of the in-plane (covalent) and out-of-plane (van der Waals) intermolecular bonds. In the low-pressure limit the out-of-plane intermolecular distances will decrease faster than the in-plane ones, which remain essentially unchanged. This implies that the compressibility of the 2D polymer is rather anisotropic up to some pressure range, above this range, where the out of plane and the in plane intermolecular distances become comparable, the material becomes more isotropic. Similar behavior was observed earlier in the case of the linear polymerized cSc60 [18]. The reversible changes in the pressure coefficients of the phonon modes in the vicinity of 4.0GPa may be attributed to the fact the material is approaching a more isotropic phase as far as its compressibility is concerned.

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Acknowledgements
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Support by NATO Grant (HTECH.CRG 97-2317) is gratefully acknowledged. K. P. M. acknowledges the support by the General Secretariat for Research and Technology, Greece, and the Russian Foundation for Fundamental Research (Grant No. 99-02-17555). A. S. acknowledges the award of a Royal Society/NATO fellowship.
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