Coherent quasielastic neutron scattering study of the rotational dynamics ofC60in the orientationally disordered phase

Abstract

Coherent quasielastic neutron scattering has been used to investigate the character of the rotational dynamics in the high-temperature solid phase of C6o. The observed scattering can be described by a model in which each molecule undergoes rotational diffusion which is uncorrelated with the motions of adjacent molecules. The rotational diffusion constant DR is (I.4±0.4)x I0'^ s~' at 260 K and (2.8 ±0.8)X 10'^ s~' at 520 K. The temperature dependence of DR is consistent with a thermally activated process having an activation energy of 35 ± 15 meV. 36.90.+f, 82.80.Jp C60 (buckminsterfullerene) forms a mostly orientationally ordered [1] simple cubic phase belonging to the space group Fa2> [2] below -255 K. Near this temperature it undergoes a transition [3] to a face-centered-cubic phase which is an excellent example of a broad class of molecular solids, sometimes termed "plastic crystals," which exhibit rapid molecular reorientations [4,5]. Quasielastic neutron scattering is capable of determining both the time scale of reorientations and, more importantly, the detailed nature of the reorientations [6]. Here we present such a study of the rotational dynamics of C6o in the orientationally disordered, high-temperature phase and show that the observed scattering is well described by a model in which there are no intermolecular correlations, and each molecule reorients completely randomly, as opposed to jump rotations between a specific set of sites.The sample consisted of -2.5 g of powdered Ceo of which 1.5 g were prepared at the University of Pennsylvania and 1.0 g at Exxon, using standard methods [7,8]. After the chromatographic separation of Ceo from the Ceo-Cyo extract, great care was taken to remove the solvent, since neutrons are -15 times more sensitive to hydrogen than to carbon. Prompt gamma activation analysis [9] showed a hydrogen content of 0.008 ± 0.002 atom per C atom for the Exxon sample and 0.0057 ±0.0011 atom per C atom for the Penn sample. Scattering from the H in the residual solvent was observed in our experiments, but does not affect our conclusions.The neutron scattering results were obtained using triple-axis spectrometers at the Neutron Beam Split-Core Reactor (NBSR) at NIST. For scattering vectors, g, below about 4.0 A~\ the neutrons were monochromated and analyzed using the (002) reflection of pyrolytic graphite. Collimators having angular spreads of 10'-20'-20'-20' were placed before and after both crystals, resulting in a measured energy resolution of 0.50 meV full width at half maximum (FWHM) at the elastic position. To reach larger values of g a shorter wavelength was necessary. In order to maintain a relatively good energy resolution, the Cu (220) reflection was chosen to monochromate the beam and the graphite (004) reflection was used to analyze the energy of the scattered neutrons. Combined with collimations of 40'-40'-40'-40' , this resulted in a measured resolution of 1.2 meV FWHM. For measurements in the fixed final energy configuration, the data were corrected for the separately m...

Raman depolarization ratio of vibrational modes in solid C60

“…In the high-temperature f.c.c. phase, C 60 molecules execute quasifree rotations and exhibit an almost complete orientational disorder [2,3]. The low-temperature s.c. phase is characterized by reducing the rotation to "rachet-like" orientational motions (librational modes) about the four specific (111) directions [4,5].…”

“…A recent theoretical study [23] predicts a dispersion bandwidth of Ϸ9 cm Ϫ1 for the A g (1) mode and less than 3 cm Ϫ1 for the A g (2) mode in solid C 60 . This is in reasonable agreement with the observed splitting of 5 cm Ϫ1 for the A g (1) mode and no splitting for the A g (2) mode, considering that the Davidov splitting should be within the dispersion width of the phonon branch. It is worth noting that even if the Davidov splitting is resolvable, sometimes it is difficult to distinguish from other small splitting effects as for instance the isotope effects [10].…”

Phonons et vibrations dans les fullerènes, les nanotubes de carbone et leurs composés

“…les molécules de C 60 sont assimilables à des sphères homogènes. Ces mouvements de réorientations rapides quasi-isotropes dans la phase désordonnée (T > 260 K) ont été mis en évidence par des techniques expérimentales telles que la résonance magnétique nucléaire [6], la diffusion diffuse des rayons X [7] et la diffusion quasi-élastique des neutrons [8]. Si l'on abaisse la température en dessous de T c 260 K, le cristal de C 60 subit une transition dynamique vers une phase dite "ordonnée", dans laquelle les C 60 restent sur les mêmes sites cristallographiques que dans la phase désordonnée, mais ne présentent plus de mouvement de rotation.…”

“…On voit apparaître des lobes intenses situés à l'énergie E 2.5 meV et aux vecteurs de diffusion Q = 3.3 et 5.0 Å −1 . Ils correspondent aux maxima du facteur de forme associé aux mouvements de rotation de la molécule de C 60 [4,8,15,17]. Par la suite, on appellera souvent ce pic situé à environ 2.5 meV, "la" libration du C 60 .…”

Lattice dynamics of a rotor-stator molecular crystal: Fullerene-cubaneC60⋅C8H8

“…The Q dependency of the integrated intensity of this quasielastic signal shows a maximum at 3.4 Å −1 which is characteristic of C 60 rotations, analogous to what is observed in their pure crystalline phase. 19 It is well known that this signal, dynamic in nature, is at the origin of the diffuse scattering that is observed as a halo in the diffraction images and discussed in the previous section. FIG.…”