STRUCTURAL STUDIES OF C-60 USING HIGH-RESOLUTION NEUTRON POWDER DIFFRACTION

This paper presents the results of neutron powder diffraction measurem ents of C-60 using the high resolution neutron powder diffractometer, HRPD, at ISIS. The high-temperature plastic-crystal phase is closely m odelled in terms of a cubic-closed packed molecular structure of essen tially spherical shells of scattering: deviations from isotropy are sh own to be of the order of only a few percent and are analysed using ku bic harmonic functions. In the low temperature phase below 260 K, the structure is primitive cubic. Although ordered, rapid reorientation st ill occurs between two similar but distinct molecular orientations dow n to temperatures of around 100 K. Below 86 K, however, molecular reor ientation rates become prohibitively long and do not occur during typi cal experimental time-scales, An orientational glass transition thus o ccurs. The effects of these transitions are discussed.