Proposed model for disorder in the warmest crystalline phase of C₄F₈

Abstract

A representation of the molecular disorder in the plastically crystalline phase of perfluorocyclobutane existing between 216 K and 233 K is formulated to account for the intensities of the four X-ray and four neutron powder diffraction lines observed in recent experiments. A previous interpretation of the X-ray pattern in terms of a body-centred cubic lattice with two molecules per cell is retained and possible models distributing puckered, cyclic molecules among partially occupied cubic sites are examined. Among such models only one, orienting the carbon ring diagonals parallel to the cell axes, space group Imm, with six molecular orientations per site, satisfied both the X-ray and neutron intensities. The proposed model has all nonbonded contacts at or exceeding the normal van der Waals distances. Debye-Waller factors and diffuse scattering indicate large molecular motions, compatible with the known facile self-diffusion reported in an N.M.R. study. The model suggests that flexible C₄F₈ molecules are puckered by the same amount in the solid and gas phases.