HIGH-RESOLUTION NEUTRON POWDER DIFFRACTION - A CASE-STUDY OF THE STRUCTURE OF C-60

High resolution time-of-flight neutron powder diffraction has been use d to determine the detailed structure of C60 as a function of temperat ure. Rapid data collection coupled with high resolution has enabled su btle aspects of the 86 K orientational glass transition and precursor effects of the 260 K order-disorder transition to be observed. This su rveying capability complements traditional single crystal methods. The power of the Rietveld method of profile refinement is demonstrated in the elucidation of the detailed crystal structure of the orientationa lly-ordered low temperature phase and in the evaluation of the departu re from isotropic scattering of the C60 molecule in the disordered hig h temperature phase. The counter-intuitive success in obtaining high-o rder cubic-harmonic coefficients, albeit to poorer precision than sing le crystal X-ray measurements, confirms the efficacy of the Rietveld p rofile refinement method. The collapse of three dimensions of diffract ion information on to the one dimension of a high resolution powder di ffraction pattern can still lead to an impressive amount of structural information that substantiates the assertion made by W. H. Bragg 'the second method [powder diffraction], first used independently by Debye and Hull, can be used when the crystal is in powder, and can, therefo re be employed when no single crystal can be obtained of sufficient si ze. All the spectra of the different planes are thrown together on the same diagram or photograph, and must be disentangled. This is not as difficult as it may seem...'.