ELSEVIER Synthetic Metals 86 (1997) 2327-2328 Single crystal x-ray diffuse scattering studies of the intermolecular interactions in solid C&o R. Moret, P. Launois, S. Ravy, M. Julier and J.M. Godard Laboratoire de Physique des Solides, URA CNRS 02, Universite’ Paris&d, 91405 Orsay, France Abstract Current microscopic models of the C60 intermolecular interactions fail to give an accurate description of the ordering phenomena in solid C60, where orientational correlations of the fullerene molecules play a leading role. At room temperature these orientational correlations produce x-ray diffuse scattering intensity modulations whose analysis is a good test for the microscopic models. Below the 259K transition, the remaining x-ray diffuse scattering was measured up to large wave vector values. Its analysis confirms that the low- temperature structure is characterised by two nearly degenerate and randomly distributed orientations of the C60 molecules. Keywords: Fullerenes and derivatives; order-disorder phase transitions; x-ray emission, diffraction and scattering, The understanding of intermolecular interactions in solid C60 is still a fundamental issue in the field of fullerene research in particular because orientational ordering phenomena are not completely clarified. Diffuse scattering (i.e. the modulations of the background intensity between the Bragg peaks) is worth analyzing as it contains information on the state of order and on the intermolecular correlations . At room temperature, orientational correlations produce both radial and azimuthal modulations of the single-crystal diffuse scattering intensity [l-3]. Intensity maxima are concentrated around X, L and I points of the Brillouin zone, although some strong “ extra-scattering” is not related to high-symmetry points. The X-point scattering alone could have been attributed to pretransitional effects of the To%2.59K transition from the room- temperature FmTm phase to the low-temperature Pa7 phase. However, the other types of diffuse scattering indicate the coexistence of several instabilities. We summarize here extensive calculations of the diffuse scattering intensity for various models of intermolecular interactions (using a mean-field theory) and their comparison to the experimental intensity distribution. While the general features of the diffuse scattering are reproduced a completely satisfactory agreement is not obtained. Below To, two orientations of the molecules prevail: near- neighbors adopt a configuration where pentagons (P) or hexagons (H) of one molecule face double bonds of its neighbors, the fraction of P-oriented molecules increasing as T is reduced [4]. We have measured and analyzed the 10K x-ray diffuse scattering intensity up to large wave vector values. Its modulations reflect the contributions of the C60 form factors for orientations very close to the P and H ones and negligible correlations. 1. Room-temperature diffuse scattering and intermolecular potentials Various models of intermolecular potentials have been developed in recent years [6-l 11. They are based on combinations of van der Waals-type and electrostatic interactions, with distributions of interaction centers and effective charges on the C atoms and the C-C single and double bonds. 0379-6779/97/$17.00 0 1997 Elsevier Science S.A All rights reserved PII SO379-6779(96)04855-2 The main steps of the diffuse scattering intensity calculation are as follows (details can be found in [2,6,12,13,14]). For a given model we evaluate the interaction terms using the formalism of the symmetry-adapted functions. From the Fourier transform of the generalized susceptibility, using the fluctuation- dissipation theorem, we get the orientation-orientation pair correlation functions and then the diffuse scattering intensity. A first set of results was presented in [13]. Other types of simulations and calculations can be found in [3,12b]. Fig. 1 shows a comparison of the experimental intensity with that calculated for different models of intermolecular interactions (see [ 141 for details). Fig.1 Calculated and experimental (x-rays, 295K [2]) diffuse scattering intensity from (4.5,4.5,0) to (8.5,0.5,0). Various models of intermolecular interactions are compared: vdW [7], SCKI and 2 [S], LLM 191, PC [II] and L.M [IO]. The temperatures were chosen to give the best fit to the data. Two main contributions to the diffuse scattering intensity can be identified [13,14]. First the average orientation of a single molecule produces a slowly-varying intensity distribution in reciprocal space. This term can be derived from the mean-field energy determined experimentally by analysis of the Bragg peak intensities. The second contribution is due to orientational correlations between the molecules. It varies more rapidly, i.e.