Structural Phase Transitions in the Fullerene C ₆₀

Abstract

High-resolution powder neutron diffraction has been used to study the crystal structure of the fullerene Cso in the temperature range 5 K to 320 K. Solid C , adopts a cubic structure at all temperatures. The experimental data provide clear evidence of a continuous phase transition at ca. 90 K and confirm the existence of a first-order phase transition at 260 K. In the hightemperature face-centred-cubic phase (T > 260 K), the Cs0 molecules are completely orientationally disordered, undergoing continuous reorientation. Below 260 K, interpretation of the diffraction data is consistent with uniaxial jump reorientation principally about a single (111) direction.In the lowest-temperature phase (T < 90 K), rotational motion is frozen although a small amount of static disorder still persists. 1. Introduction. c 6 0 buckminsterfullerene [l-31 is the most stable member of the whole family of closed carbon cage molecules-the fullerenes [4,5]. Its extraction and purification from arc-processed carbon [2,31 have not only enabled the original structural proposal [ll to be confirmed [2,3,61 but have also led to numerous experiments that are revealing many novel physical and chemical properties [7]. At room temperature, crystalline c& adopts a face-centred cubic crystal structure in which each of the c 6 0 molecules is orientationally disordered [8,9]. The structure may be regarded as cubic-closed-packed in which rotations and orientations of individual c 6 0 molecules are uncorrelated with their neighbours [9]. 13C NMR [lo-121 and quasi-elastic neutron scattering [13] measurements confirm this rapid isotropic reorientation at room temperature that results in time-averaging of the truncated icosahedron to spherical symmetry. Perhaps surprisingly in view of the almost spherical symmetry, this orientational disorder does not persist to low temperatures. Differential scanning calorimetry (DSC) [14] and X-ray diffraction [9] measurements established the existence of a first-order phase transition near 250 K. More recent work [9,15,16] has confirmed an ordered simple cubic crystal structure for Cw at low temperatures. The reason for the orientational order has been discussed in terms of van der Waals bonding and electrostatic repulsion that results in the facing of the

Dynamics of CO Molecules in SolidC60as a Function of Cavity Size

“…These two orientations correspond to an electron-rich double bond on a C 60 molecule facing an electron-poor region on each of its neighbors, either a pentagonal face (p orientation) or a hexagonal face (h orientation). Under ambient pressure the p orientation is energetically preferred, and gains in occupation relative to the h orientation when the temperature is decreased [13]. When pressure is applied to a C 60 sample, the transition temperature of the first order phase transition increases, and the orientationally ordered sc phase exists at room temperature for pressures above 3-4 kbar [14,15].…”

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

“…Gauche : Évolution de la proportion de C 60 en orientation P (notée (a)) par rapport à l'orientation H avec la température -Droite : Évolution du paramètre de maille du cristal de C 60 avec la température. D'après la référence [19].…”

Vibrational spectroscopy of C60

“…However, a second source of disorder in low-temperature C 60 crystals does alter the local symmetry and may activate silent modes not allowed by the Pa3 symmetry. The origin of this disorder is the existence of a secondary energy minimum for a rotation about a 111 axis, which brings a double bond in one molecule close to a hexagonal face in a neighboring molecule (instead of a pentagonal face, as in the ground state) [23,24,30,38]. Below the phase transition at 260 K, the molecules librate about their equilibrium positions and occasionally jump between these two energy minima.…”

“…Below the phase transition at 260 K, the molecules librate about their equilibrium positions and occasionally jump between these two energy minima. At about 90 K the jumps are frozen on a laboratory time scale, and the crystal exhibits a residual disorder, whereby 84% of the molecules occupy the double-bond/pentagon configuration and the remaining 16% occupy the double-bond/hexagon configuration [38]. The existence of this disorder destroys the cubic and inversion symmetry and may activate all silent modes.…”

“…Combining (36), (38), and (39) we can write the mode polar-izability derivatives in the convenient form [22] …”