Structure and dynamics of crystalline C-60

Much of our discussion in this chapter has concentrated on the intermolecular potential in crystalline C60 as deduced through the combination of scattering methods and statistical mechanics. Molecular rotator functions and their thermodynamic averages [formula omitted], provide the necessary parameters for describing the orientational dynamics and equations of state, and while the formalism is complex, it is correspondingly rich in possibilities. Both neutron and x-ray measurements, together with a variety of related techniques that probe the underlying orientational dynamics, have provided us with a matching richness of data, which must finally be understood from a consistent theoretical viewpoint. It has been our ambition here to assemble this in a critical fashion to highlight both the successes and failures of current theoretical understanding. For example, in Section 9, Fig. 8 we present a portion of the single particle potential [formula omitted], extracted from single crystal Bragg intensities, which places some constraints on orientational potential calculations. Section 17 develops this theme further to illustrate how the theoretical state of affairs, as of this writing, wants further consideration. It is quite interesting that the C60, molecule, while seemingly coupled rather weakly to its neighbors, nonetheless experiences at room temperature a potential barrier height to free rotation of approximately 600K or 50 meV. This is an appreciable activation barrier, and it does not permit a linearized treatment of the orientational distribution function fi(wi). Below the orientational order-disorder transition, this barrier increases to ~250meV and the rotations are further restricted to jump reorientation among selected orientational minima. The nature of the phas transition and its description via a Landau treatment reveals a complex behavior due to rotational localization. Fortunately, this can be sorted out in scattering experiments, since the order parameters [formula omitted] appear more or less separately at selected reciprocal lattice points. Much more structurally related work on this material remains to be done and includes additional studies of the orientational pair potential via x-ray scattering and single crystal dynamical studies. While an enormous amount of static and dynamical information has been obtained using orientationally averaged powder samples, the more detailed neutron determination of S(Q, w) awaits larger crystals. In addition to obtaining information on the orientational pair potential and dynamic orientational correlations, this would allow an examination of the possible roles that rotational-translational coupling and librational mode softening play in the sc-fcc phase transformation. Currently, the only evidence that these effects are important is rather indirect, coming both from the jump in lattice constant at the orientational order-disorder transformation and the presence of a co-existence regime and from the precursor effects observed below the transformation. Also, the nature of the �glassy� low temperature behavior awaits further clarification. Finally, we look forward to the development of a similar body of information on the higher fullerenes, in particular crystalline C70 which exists in a plastic crystal phase (dynamically disordered slightly above room temperature). © 1994 Academic Press, Inc.