On the evaluation of Bragg intensities in the low-temperature phase of C-60

The temperature dependence of Bragg intensities in C-60 is discussed for th e simple cubic Pa (3) over bar phase occurring below 260 K. It is shown tha t, generally, the intensities of Bragg reflections may be surprisingly stro ngly affected even by seemingly minor variations of certain parameters. Amo ng these the most important are the respective molecular orientational angl es of the so-called pentagon and hexagon orientations, the fractions of mol ecules adopting these orientations and the lattice parameter. Correct deter mination of the Debye-Waller factor is found to be unusually difficult beca use of this behaviour. Numerical calculations are used to provide explicit examples and lead to the conclusion that the evaluation of Bragg intensitie s in this system requires considerable care.