Doi:10.1016/j.cplett.2007.09.050

Available online at www.sciencedirect.com Chemical Physics Letters 447 (2007) 316–319 He intercalated C60 solid under high pressure Department of Materials Science and Engineering, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Received 26 July 2007; in final form 19 September 2007 In situ synchrotron X-ray diffraction measurement of solid C60 under high pressure up to about 26 GPa was performed using He gas as pressure transmitting medium. This is the first study to precisely investigate the structural property of the He intercalated C60 underhigh pressure in a few tens GPa range. It was found that the He intercalated C60 solid is much less compressible than the pure C60 solid.
The bulk modulus of the He intercalated C60 solid is determined to be B0 = 35.1 (6.0) GPa. The effect of hydrostaticity on the deforma-tions of unit cell and C60 molecule is also discussed.
Ó 2007 Published by Elsevier B.V.
est. Although many experimental investigations on the ful-lerene polymers have been performed, the formation Fullerenes have attracted much attention not only from mechanism of the fullerene polymers has not yet been clar- an academic point of view for their extraordinary molecu- lar structures, but also from an industrial point of view for In order to elucidate the formation mechanism of the their unique physical and chemical properties. For exam- fullerene polymers and the elastic property of the C60 mol- ple, the elastic property of the C60 molecule is quite inter- ecule, it is very important to investigate the structural esting because its bulk modulus is calculated to be change of fcc–C60 under high pressure. In fact, many exper- greater than 700 GPa which significantly exceeds that of iments concerning the high pressure behavior of C60 have already been done . However, since only restricted exper- High pressure and high temperature (HPHT) treatments imental techniques and a small sample space are allowed of solid C60 (fcc–C60), in which C60 molecules are bound for high pressure experiments, it has not been very easy together by weak van der Waals forces, produce a variety to get high resolution structural data under high pressure.
of polymerized C60 phases known as ‘fullerene polymers’ On the other hand, experimental techniques to investigate The polymerized fullerenes consists of hybrid net- fine structures under pressure have recently undergone sig- works of sp2 and sp3 carbon atoms and are classified as a nificant advances. For example, synchrotron radiation and new family of crystalline carbon Therefore, they are a 2-dimensional detector (e.g., imaging plate, CCD detec- expected to have different properties from those of the tors, etc.) make it possible to get high resolution diffraction other carbon phases such as graphite, diamond, fcc–C60, data of small powder sample. Using noble gases as the and carbon nanotubes. Recently, it was reported that C60 pressure transmitting media expands the upper pressure molecules in peapods also polymerize by compression at limit applicable for hydrostatic pressure experiments. Due room temperature and it has been attracting much inter- to the above mentioned experimental advances, it hasnow become possible to obtain high resolution diffractiondata up to a few tens GPa. However, in the case of C60, one must pay attention to the choice of the pressure trans- Corresponding author. Fax: +81 52 735 5221.
E-mail address: (S. Kawasaki).
mitting medium, because some noble gases with small 0009-2614/$ - see front matter Ó 2007 Published by Elsevier B.V.
doi:10.1016/j.cplett.2007.09.050 S. Kawasaki et al. / Chemical Physics Letters 447 (2007) 316–319 atomic sizes can penetrate in the lattice of C60 solid. Schir-ber et al. and Pintschovius et al. pointed out the penetration of He and Ne gases into the C60 lattice. They also investigated the effect of the penetration on the high pressure behavior of the solid C60. However, their experi- ments are limited to the low pressure range below 1 GPa and the lattice volume change due to the intercalation is quite small. Therefore, the structural property of the noble still not known. In this study, we report the results of the in situ synchrotron XRD measurements of solid C60 under high pressure up to about 26 GPa using He gas as the pres- The in situ XRD measurements of fcc–C60 (>99.99%) under high pressure were performed using a diamond anvil cell at the beam line BL-10XU of SPring-8. Two kinds ofexperiments were carried out: one used He gas as the pres-sure transmitting medium and the other used a 4:1 mixtureof methanol and ethanol. For convenience, the former and the latter experiments are abbreviated Exp-He and Exp- M + E, respectively. The pressure was determined by the ruby fluorescence method. The XRD patterns were observed using an imaging plate detector.
a and b show the changes in the XRD patterns of the fcc–C60 in Exp-M + E and Exp-He as a function of pressure, respectively. Comparing a and b, no extra diffraction peak can be observed by the penetration of Hegas and all the diffraction lines can be indexed by cubic lat- tice. Since the alcohols mixture of methanol:ethanol = 4:1 used in Exp-M + E (does not freeze up to about 10 GPa, it has been reported to be possible to perform a hydrostatic pressure experiment up to its solidification pressure using this mixture It was also observed in Fig. 1. Changes in XRD patterns of fcc–C60 (a) in alcohol mixture, and the present study (that the full width at half max- (b) in He gas pressure transmitting medium.
imum (FWHM) of the observed 2 2 0 diffraction peakabruptly increased from about 10 GPa ) probablydue to the nonhydrostatic effect. In the FWHM val- lecular bonding, the ratio a/RC changes with pressure and ues were determined by fitting the observed data with a the accidental absence of the 2 0 0 line is eliminated at high pseudo voight function. On the other hand, in the Exp- pressure. In other words, the relative intensity of the 2 0 0 He experiment, no remarkable change in the FWHM of peak at high pressure expresses the degree of the change of the a/RC ratio from ambient pressure. Therefore, the dif- Another important difference between and b is ference in the relative intensity of the 2 0 0 peak in Exp-He the relative intensity of the 2 0 0 diffraction peak at about and Exp-M + E indicates that the compression behaviors the same pressure. Although in both a and b, the of the C60 molecule and/or fcc lattice differs in the two 2 0 0 peak can be observed at high pressure, the peak in Exp-M + E appears at a pressure lower than that in the shows the lattice constant a determined by the Exp-He. The scattering factor of the 2 0 0 peak position, observed diffraction patterns of the two kinds of experi- which is determined by the ratio of the lattice constant a ments. The pressure dependence of the a values of Exp- and the molecular radius RC, is accidentally almost 0 at M + E is in good agreement with previous reports, especially atmospheric pressure. However, since the intramolecular with Duclos’s experiment in which the same alcohols bonds are significantly less compressible than the intermo- mixture pressure transmitting medium was used. By fitting S. Kawasaki et al. / Chemical Physics Letters 447 (2007) 316–319 penetration of He into the C60 lattice. By applying the sameanalysis method as in the case of Exp-M + E, the B0 and B0 values of the He intercalated C60 solid are determined to be35.1 (6.0) GPa and 4.1 (1.1), respectively.
The main reason why the 2 0 0 diffraction peak of Exp- M + E appeared at a lower pressure than that of Exp-Heis probably due to the difference in the lattice compressibil-ity mentioned above. If the pressure deformation of the C60 molecule differs in Exp-M + E and Exp-He, it would be an another possible explanation for the anomaly of the 2 0 0peak. The deformation can be analyzed using the relativediffraction intensities. In , the relative intensities ofthe 2 0 0 peak in the two experiments to the 1 1 1 and 2 2 0diffraction peak intensities are plotted as a function of the lattice constant a. The solid curves in are the curves calculated using the following conditions: the C60 molecular scattering factor can be approximated by the Fig. 2. FWHM of 2 2 0 peaks as a function of pressure. Solid circles and 0th order spherical Bessel function, and the radius of the open diamonds correspond to Exp-M + E and Exp-He, respectively.
molecule is fixed at 0.35 nm. Therefore, the solid lines indi-cate the relative intensities when the deformation of C60molecule does not occur and only the lattice shrinks bycompression.
Although the relative intensities I2 0 0/I1 1 1 and I2 0 0/I2 2 0 significantly differ in Exp-He and Exp-M + E as a functionof the pressure and b), those plotted as a function of a do not show any remarkable difference at least in the ˚ . In the case of Exp-He, since both the Fig. 3. Lattice constant a as a function of pressure. Solid circles and open diamonds correspond to Exp-M + E and Exp-He, respectively. Solid linesare fitting curves using Birch–Murnaghan’s equation of state.
the relative unit cell volume change (V =V 0 ¼ a3=a3, where V 0 are unit cell volume and lattice constant at ambient pressure, respectively) with pressure to the Birch–Murna- ghan’s equation of state (P = 1.5B0[(V/V0)À7/3 À (V/V0)À5/3][1 + 0.75(B0 À 4)((V/V and its first pressure derivative B0 are determined to be 15.8 (1.7) GPa and 6.4 (0.4), respectively. These values are B0 = 5.3 (0.6). It should be noted that the B Fig. 4. Relative intensities of I2 0 0 to (a) I2 2 0 and (b) I1 1 1 as a function of of the present Exp-M + E agree fairly well with Horikawa’s the lattice constant a. Solid circles and open diamonds correspond to Exp- M + E and Exp-He, respectively. The solid lines denote the lines simulated which was performed using a multianvil press with no liquid with a spherical shell model (see text in detail). The errors were estimatedfrom the fitting errors. Some error bars are within the data marks.
pressure transmitting medium. On the other hand, the pres- Pressures are: A (0.1 MPa), B (5.5 GPa), C (10.6 GPa), D (15.0 GPa), E sure change of the lattice constant a derived from Exp-He is (18.6 GPa), a (0.1 MPa), b (2.1 GPa), c (5.2 GPa), d (10.3 GPa), e more gentle than that of Exp-M + E probably because of the (14.4 GPa), f (18.0 GPa), g (22.0 GPa), h (25.9 GPa).
S. Kawasaki et al. / Chemical Physics Letters 447 (2007) 316–319 I2 0 0/I1 1 1 and I2 0 0/I2 2 0 data points (the data at 22.0 GPa the pure C60 solid (B0 = 15.8 GPa). Diffraction intensity and 25.0 GPa are missing in because the 1 1 1 dif- analysis revealed that the intensity ratio (e.g., I2 0 0/I2 2 0) fraction peak could not be detected at those pressures deviates from the isotropic lattice compression model at (b)) are in good agreement with the calculation, it about 10.6 GPa in the case of the experiment using the is indicated that the C60 molecules of Exp–He do not alcohol mixture as the pressure transmitting medium. It deform very much even at high pressure. Although this is indicates that the C60 molecular shape begins to deform probably due to the good hydrostaticity of the He pressure transmitting medium, it should be also considered that theintercalated He might protect C60 from deformation. Onthe other hand, the I 2 0 0/I1 1 1 and I2 0 0/I2 2 0 data of Exp- M + E disagree with the calculation curves on the left sides [1] R.S. Ruoff, A.L. Ruoff, Nature 350 (1991) 663.
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