An X-ray diffraction study of the crystal structures of hexamethylethane and its phase transitions

Abstract

We have examined the Im3m α-phase of hexamethylethane and low temperature phases β and δ, by Bragg and diffuse X-ray scattering. The Im3m phase fits well to a four thermal-parameter model at 188 and 295 K (R W = 12 per cent). The data suggest that significant improvement may come only from inclusion of intermolecular correlation between centre-of-molecular-mass and the orientation of neighbouring molecules' ethane axes. This results in a ‘static’ centre-of-mass positional disorder and a distribution in potential barriers to rotation about the molecular ethane axes. The β and metastable δ phases are tentatively identified as P21/c and P42/mnm, each derived from the Im3m structure by ordering of the molecular ethane axes, together with small distortions. The effect of crystal strain in stabilizing the δ-phase is discussed. Atom-atom intermolecular potentials are used to calculate the stabilities of various phases and the important correlations in the disordered Im3m phase.