W0055

Low Temperature Study of High Fullerene C84 (300-50 K). Duc TranQui¹, Olivier Isnard¹, Robert Almirac², Jean Lapasset², Jacques Moret² and Jean Pierre Lauriat³, ¹Lab. de Cristallographie-CNRS, F38042 Grenoble, BP 166, ²Groupe de Dynamique des Phases Condensées, F34095 Montpellier, ³LURE F91405 Orsay Cedex

Like other fullerenes, C84 consists of a skeleton of carbon atoms distributed on a distorted spherical surface. The scattering power of such an object in free rotation state is modulated by a spherical Jo(QR) function. Inspection of synchrotron single-crystal and powder data at room-T of C84 shows that all allowed face-centered-cubic (fcc) [h00] structure factors or equivalents - which are close to the zero node of Jo(QR) - are reduced nearly to zero. Intensities of others reflections are found factorized by a still-undetermined-oscillating-periodic factor, which cannot be solely ascribed to Jo(QR) modulation effect.

Thus, modeling solid C84 at room-T as thin charge-density shells closely packed together forming a fcc lattice, a= 15.90(4) Å, should be considered as a first approximation. To satisfy the fcc requirement, C84 molecules "have" to average their local symmetry to spherical by random rotation around their centers. However, hindered by their large size and their non-spherical shape, free rotation for C84 is likely less favorable energetically than for C60. Consequently, non-cubic symmetry of solid C84 should not be excluded at room-T. At low-T, in spite of no clear signals of transition onsets from DSC data, existence of two transitions at 181 and 235 K is supported by following findings: Enlargement of reflection profile: formation domain structure. Angular distance between [hk0] and [-k,h,0] reflections suggests that the low-T phases might have lower symmetry. Formation of domain structure and possible transitions of C84 from pseudo fcc to non-cubic phases will be discussed.