Structural studies of 60C using high resolution neutron powder diffraction

This paper presents the results of neutron powder diffraction measurements of 60C using the high resolution neutron powder diffractometer, HRPD, at ISIS. The high-temperature plastic-crystal phase is closely modelled in terms of a cubic-closed packed molecular structure of essentially spherical shells of scattering: deviations from isotropy are shown to be of the order of only a few percent and are analysed using kubic harmonic functions. In the low temperature phase below 260 K, the structure is primitive cubic. Although ordered, rapid reorientation still occurs between two similar but distinct molecular orientations down to temperatures of around 100 K. Below 86 K, however, molecular reorientation rates become prohibitively long and do not occur during typical experimental time-scales. An orientational glass transition thus occurs. The effects of these transitions are discussed.